,,visual ,,illusions ,,_! ,,causes1 ,,*>act]i/ics ,,& ,,applications ,,by ,m4 ,,luckie% ,,director ,,( ,,appli$ ,,sci5ce1 ,,nela ,,rese>* ,,laboratories1 ,,national ,,lamp ,,works ,,( ,,g5]al ,,electric ,,co4 ,,au?or ,,( ,,color ,,& ,,its ,,applications1 ,,light ,,& ,,%ade ,,& ,,_! ,,applications1 ,,! ,,light+ ,,art1 ,,! ,,language ,,( ,,color1 ,,artificial ,,light--,,its ,,influ5ce ,,^u ,,civilization1 ,,light+ ,,! ,,home1 ,,etc4 #ajj ,,illu/rations ,,new ,,york ,d4 ,,van ,,no/r& ,,company ,,eight ,,w>r5 ,,/reet #aibb ,,copyright1 #aibb1 ,,by ,d4 ,,van ,,no/r& ,,company ,,preface ,ev5tually "o ( ! results ( application to ! analysis & measurem5t ( ! ph5om5a ( light1 color1 light+1 & vision is a firmly entr5*$ conviction ( ! inadequacy ( physical measurem5ts z a means = repres5t+ :at is p]ceiv$4 ,physical measurem5ts h suppli$ mu* ( ! f\ndation ( k & x is n a reflection ^u _! great usefulness to /ate t (t5 !y diff] f ! results ( intellectual appraisal ?r\gh ! visual s5se4 ,9 o!r words1 !re are numb]less s-call$ visual illusions :i* mu/ be tak5 into acc\nt4 ,all are ( int]e/2 _m c be utiliz$2 & "s mu/ be suppress$4 ,sci5tific lit]ature yields a great _m valuable discussions f !oretical & exp]im5tal viewpo9ts b mu* ( ! mat]ial is controv]sial4 ,! practical aspects ( visual illusions h be5 q g5]ally pass$ by &1 inasmu* z !re does n appe> to be a volume available :i* treats ! subject 9 a cond5s$ mann] b ) a broad scope1 ?is small volume is contribut$ t[>d fill+ ! gap4 ,! extreme complexity ( ! subject is recogniz$1 b an attempt t[>d simplicity ( treatm5t has be5 made by conf9+ ! discussion *iefly to /atic visual illusions1 by suppress+ m9or details1 & by subord9at+ !ory4 ,9 o!r words1 ! int5t has be5 to emphasize exp]im5tal facts4 ,ev5 ^! are s num]\s t only ! m]e/ glimpses ( v>i\s aspects c be giv5 9 ord] to limit ! text to a small volume4 ,"s !oretical aspects ( ! subject are /ill extremely controv]sial1 s !y are introduc$ only occasionally & !n *iefly = ! purpose ( illu/rat+ ! complexities & ! tr5ds ( attempt$ explanations4 ,space does n ev5 admit _m qualifications :i* may be necess>y 9 ord] to escape criticism entirely4 ,! visual illusions discuss$ are *iefly ( ! /atic type1 al?\gh a few o!rs h be5 introduc$4 ,"s ( ! latt] bord] ^u motion1 o!rs ^u halluc9ations1 & /ill o!rs produc$ by ext]nal optical m$ia are illusions only by ext5sion ( ! t]m4 ,^! exceptions are includ$ = ! purpose ( provid+ glimpses into ! bord]l&s4 ,x is hop$ t ?is cond5s$ discussion1 :i* is ambiti\s only 9 scope1 w be ( int]e/ to ! g5]al read]1 to pa9t]s1 decorators1 & ar*itects1 to light+ exp]ts1 & to all int]e/$ 9 light1 color1 & vision4 ,x is an ess5tial supplem5t to c]ta9 previ\s works4 ,m4 ,,luckie% ,novemb]1 #aibj4 ,,cont5ts ,,*apt] ,,page ,i4 ,introduction #a ,,ii4 ,! eye #ac ,,iii4 ,vision #bi ,,iv4 ,"s types ( geometrical illusions #dd ,v4 ,equivocal figures #fd ,,vi4 ,! influ5ce ( angles #gf ,,vii4 ,illusions ( dep? & ( di/ance #ajb ,,viii4 ,irradiation & brightness-contra/ #aad ,,ix4 ,color #abd ,x4 ,light+ #add ,,xi4 ,nature #afd ,,xii4 ,pa9t+ & decoration #agi ,,xiii4 ,ar*itecture #aie ,,xiv4 ,mirror ,magic #bje ,,xv4 ,cam\flage #baj ,,li/ ,,( ,,illu/rations ,,figure ,,page #a4 ,pr9cipal p>ts ( ! eye #ad #b4 ,/]eoscopic pictures = comb9+ by conv]g+ or div]g+ ! optical axes #da #c4 ,/]eoscopic pictures #da #d4 ,! v]tical l9e appe>s long] ?an ! equal horizontal l9e 9 ea* case #df #e4 ,! v]tical dim5sion is equal to ! horizontal "o1 b ! =m] appe>s great] #dg #f4 ,! divid$ or fill$ space on ! left appe>s long] ?an ! equal space on ! "r #di #g4 ,! ?ree l9es are ( equal l5g? #ej #h4 ,! di/ance betwe5 ! two circles on ! left is equal to ! di/ance betwe5 ! outside edges ( ! two circles on ! "r #ej #i4 ,?ree squ>es ( equal dim5sions :i* appe> diff]5t 9 area & dim5sion #ea #aj4 ,! v]tical di/ance betwe5 ! upp] circle & ! left-h& "o ( ! gr\p is equal to ! ov]all l5g? ( ! gr\p ( ?ree circles #eb #aa4 ,two equal semi-circles #ec #ab4 ,arcs ( ! same circle #ec #ac4 ,?ree incomplete b equal squ>es #ec #ad4 ,middle sections ( ! two l9es are equal #ed #ae4 ,an effect ( contra/+ areas 7,baldw9's figure7 #ed #af4 ,an illusion ( contra/ #ee #ag4 ,equal circles :i* appe> unequal due to contra/ 7,ebb+haus' figure7 #ef #ah4 ,equal circles appe>+ unequal ow+ to contra/+ conc5tric circles #ef #ai4 ,circles influ5c$ by position )9 an angle #eg #bj4 ,contra/+ angles #eg #ba4 ,ow+ to p]spective ! "r angles appe> oblique & vice v]sa #eh #bb4 ,two equal diagonals :i* appe> unequal #eh #bc4 ,app>5t v>iations 9 ! di/ance betwe5 two p>allel l9es #ei #bd4 ,a /rik+ illusion ( p]spective #fj #be4 ,di/ortion ( a squ>e due to sup]pos$ l9es #fa #bf4 ,di/ortion ( a circle due to sup]pos$ l9es #fb #bg4 ,illu/rat+ fluctuation ( att5tion #fe #bh4 ,! gr\p+ ( ! circles fluctuates #ff #bi4 ,cross$ l9es :i* may be int]pret$ 9 two ways #fg #cj4 ,rev]sible cubes #fh #ca4 ,! rev]sible op5 book 7aft] ,ma*7 #fi #cb4 ,a rev]sible tetrah$ron #fi #cc4 ,rev]sible p]spective ( a gr\p ( r+s or ( a tube #gj #cd4 ,s*ro$]'s rev]sible /aircase #gj #ce4 ,?i]y's figure #ga #cf4 ,illu/rat+ c]ta9 influ5ces ^u ! app>5t direction ( vision4 ,by cov]+ all b ! eyes ! latt] appe> to be drawn alike 9 bo? sket*es #gc #cg4 ,zoelln]'s illusion ( direction #gg #ch4 ,p>allel l9es :i* d n appe> s #gi #ci4 ,wundt's illusion ( direction #gi #dj4 ,h]+'s illusion ( direction #hj #da4 ,simple effect ( angles #ha #db4 ,! effect ( two angles 9 tilt+ ! horizontal l9es #hc #dc4 ,! effect ( cross$ l9es ^u _! respective app>5t directions #hc #dd4 ,ano!r /ep t[>d ! ,zoelln] illusion #hd #de4 ,! two diagonals w\ld meet on ! left v]tical l9e #he #df4 ,pogg5dorff's illusion4 ,:i* oblique l9e on ! "r is ! prolongation ( ! oblique l9e on ! left8 #he #dg4 ,a /raight l9e appe>s to sag #hf #dh4 ,di/ortions ( cont\r due to contact ) o!r cont\rs #hg #di4 ,an illusion ( direction #hh #ej4 ,twi/$-cord illusion4 ,^! are /raight cords #hi #ea4 ,twi/$-cord illusion4 ,^! are conc5tric circles #hi #eb4 ,a spiral :5 rotat$ appe>s to exp& or contract1 dep5d+ ^u direction ( rotation #ij #ec4 ,angles affect ! app>5t l5g? ( l9es #ia #ed4 ,! horizontal l9e appe>s to tilt d[nw>d t[>d ! ends #ib #ee4 ,! horizontal l9e appe>s to sag 9 ! middle #ib #ef4 ,! ,muell]-,ly] illusion #ic #eg4 ,comb9$ influ5ce ( angles & contra/+ l5g?s #ie #eh4 ,two equal oblique l9es appe> unequal because ( _! diff]5t positions #ie #ei4 ,an illusion ( area #if #fj4 ,five equal areas %[+ ! influ5ce ( cont\r ^u judgm5t ( area #ig #fa4 ,%[+ ! effect ( direct+ ! att5tion #ih #fb4 ,simple app>atus = demon/rat+ ! rem>kable effects ( contra/s 9 brightness & color #aae #fc4 ,illu/rat+ brightness-contra/ #aag #fd4 ,an effect ( brightness-contra/4 ,note ! d>k5+ ( ! int]sections ( ! :ite /rips #aah #fe4 ,! ph5om5on ( irradiation #aba #ff4 ,an excell5t patt]n = demon/rat+ color-contra/ #abf #fg4 ,by rotat+ ?is ,mason 7black & :ite7 disk color-s5sations are produc$ #acc #fh4 ,= demon/rat+ retir+ & advanc+ colors #acg #fi4 ,by comb9+ ^! /]eoscopically ! effect ( metallic lu/re 7simil> to graphite 9 ?is case7 is obta9$ #ada #gj4 ,a bas-relief light$ f above #adf #ga4 ,an intaglio light$ f above #adg #gb4 ,a bas-relief light$ f ! left #adh #gc4 ,an intaglio light$ f ! left #adi #gd;a4 ,a disk 7above7 & a sph]e 7bel[7 light$ f ov]head #ade b4 ,a disk & a sph]e light$ by p]fectly diffus$ light #ade #ge4 ,a concave hemisph]ical cup on ! left & a convex hemisph]e on ! "r light$ by a light-s\rce ( l>ge angle su* z a w9d[ #aej #gf4 ,! same z ,fig4 #ge1 b light$ by a v small light-s\rce #aea #gg4 ,app>5t end+ ( a se>*light beam #afa #gh4 ,an accurate trac+ f a photograph 7cont9ual exposure7 ( ! moon ris+ #aga #gi4 ,accurate trac+s f a photograph 7%ort exposures at int]vals7 ( ! sun sett+ #agb #hj4 ,explanation (f]$ by ,smi? ( ! app>5t enl>gem5t ( heav5ly bodies ne> ! horizon #agd #ha4 ,explanation ( a common mirage #agf #hb4 ,illu/rat+ ! app>5t di/ortion ( a picture frame 9 :i* ! gra9 ( ! wood is visible #aij #hc4 ,ano!r example simil> to ,fig4 #hb #aia #hd4 ,f actual photographs ( ! end-gra9 ( a bo>d #aib #he4 ,exagg]at$ illusions 9 ar*itecture #aih #hf4 ,illu/rat+ ! influ5ce ( visual angle ^u app>5t v]tical height #aii #hg4 ,irradiation 9 ar*itecture #bjj #hh4 ,"s simple geometrical-optical illusions 9 ar*itecture #bja #hi4 ,by decreas+ ! expos$ l5g? ( %+les t[>d ! top a great] app>5t expanse is obta9$ #bjb #ij4 ,an example ( a mirror illusion #bjg #ia4 ,ano!r example ( mirror magic #bjh #ib4 ,a prim>y /age 9 ! evolution ( ! use ( geometrical-optical illusions on %ips #bbf #ic & #id4 ,attempts at di/ortion ( outl9e :i* prec$$ ! adoption ( geometrical-optical illusions #bbh #ie & #if4 ,illu/rat+ ! use ( models by ! ,navy ,dep>tm5t 9 develop+ ! geometrical-optical illusion = %ips #bbi #ig & #ih4 ,examples ( ! geometrical-optical illusion z f9ally appli$ #bca #ii4 ,repres5tative e>? backgr\nds = an airplane 7uncam\flag$7 z view$ f above #bce #ajj4 ,illu/rat+ ! /udy ( patt]n = airplanes4 ,! photograph 0 tak5 f an altitude ( #aj1jjj feet4 ,! ins]t %[s ! relative l5g?s 7v]tical scale7 ( an airplane ( #ej-;foot spread at v>i\s di/ances bel[ ! obs]v] #bci ,,visual ,,illusions ,i ,,introduction ,see+ is deceiv+4 ,?us a famili> epigram may be *all5g$ 9 ord] to indicate ! tr5d ( ?is book :i* aims to treat c]ta9 phases ( visual illusions4 ,9 g5]al1 we d n see ?+s z !y are or z !y are relat$ to ea* o!r2 t is1 ! intellect does n correctly int]pret ! deliv]ances ( ! visual s5se1 al?\gh sometimes ! optical me*anism ( ! eyes is directly responsible = ! illusion4 ,9 o!r words1 none ( our conceptions & p]ceptions are q adequate1 b =tunately mo/ ( !m are satisfactory = practical purposes4 ,only a "p ( :at is p]ceiv$ comes ?r\gh ! s5ses f ! object2 ! rema9d] always comes f )94 ,9 fact1 x is ! visual s5se or ! intellect :i* is responsible = illusions ( ! v>i\s types to be discuss$ 9 ! foll[+ *apt]s4 ,our pa/ exp]i5ces1 associations1 desires1 dem&s1 imag9+s1 & o!r m or less obscure influ5ces create illusions4 ,an illusion does n g5]ally exi/ physically b x is difficult 9 "s cases to expla9 ! cause4 ,c]ta9ly !re are _m cases ( errors ( judgm5t4 ,a mi/ak5 e/imate ( ! di/ance ( a m\nta9 is due to an error ( judgm5t b ! p]ception ( a piece ( :ite pap] z p9k on a gre5 backgr\nd is an error ( s5se4 ,x is realiz$ t ! =ego+ comp>ison leads directly to "o ( ! mo/ controv]sial que/ions 9 psy*ology1 b !re is no int5tion on ! au?or's "p to cl+ dogmatically to ! op9ions express$4 ,9 fact1 discussions ( ! psy*ological judgm5t involv$ 9 ! pres5tations ( ! visual s5se are n introduc$ ) ! hope ( /at+ ! f9al ^w b to give ! read] an idea ( ! inn] process ( p]ception4 ,! f9al ^w w be left to ! psy*ologi/s b x appe>s possible t x may nev] be =mulat$4 ,9 g5]al1 a tree appe>s ( great] l5g? :5 /&+ ?an :5 ly+ ^u ! gr\nd4 ,l9es1 areas1 & masses are n p]ceiv$ 9 _! actual physical relations4 ,! appe>ance ( a color$ object v>ies consid]ably ) its environm5t4 ,! sky is n p]ceiv$ z inf9ite space nor z a hemisph]ical dome1 b z a flatt5$ vault4 ,! moon app>5tly dim9i%es 9 size z x rises t[>d ! z5i?4 ,a bright object appe>s l>g] ?an a d>k object ( ! same physical dim5sions4 ,flat areas may appe> to h a ?ird dim5sion ( dep?4 ,& s on4 ,illusions are s num]\s & v>i$ t !y h long *all5g$ ! int]e/ ( ! sci5ti/4 ,!y may be s useful or ev5 s disa/r\s t !y h be5 utiliz$ or c\nt]act$ by ! skill$ arti/ or artisan4 ,! ar*itect & pa9t] h us$ or avoid$ !m4 ,! /age-arti/ employs !m to c>ry ! audi5ce 9 its imag9ation to o!r environm5ts or to f> c\ntries4 ,! magician has employ$ !m 9 8 ent]ta9m5ts & ! cam\fleur us$ !m to advantage 9 ! practice ( deception dur+ ! rec5t w>4 ,!y are va/ly ent]ta9+1 useful1 deceiv+1 or disa/r\s1 dep5d+ ^u ! viewpo9t4 ,incid5tally1 a few s-call$ illusions w be discuss$ :i* are n due /rictly to errors ( ! visual s5se or ( ! intellect4 ,examples ( ^! are ! mirage & c]ta9 optical effects employ$ by ! magician4 ,9 su* cases nei!r ! visual s5se nor ! intellect errs4 ,9 ! case ( ! mirage rays ( light com+ f ! object to ! eye are b5t f _! usual /raight-l9e c\rse & ! object appe>s to be :]e x really is n4 ,h[ev]1 ) ^! few exceptions1 :i* are introduc$ = _! specific int]e/ & = ! emphasis !y give to ! true illusion1 x w be und]/ood t illusions 9 g5]al z h]e9aft] discuss$ w mean ^? due to ! visual me*anism or to errors ( judgm5t or intellect4 ,= ! sake ( brevity we might say t !y are ^? due to errors ( visual p]ception4 ,fur!rmore1 only ^? ( a /atic type w be consid]$2 t is1 ! va/ complexities due to motion are n ( int]e/ f ! viewpo9t ( ! aims ( ?is book4 ,!re are two well-kn[n types ( mislead+ p]ceptions1 namely illusions & halluc9ations4 ,if1 = example1 two l9es appe> ( equal l5g? & are n1 ! error 9 judgm5t is responsible = :at is t]m$ an illusion4 ,if ! p]ceptual consci\sness ( an object appe>s al?\gh ! object is n pres5t1 ! result is t]m$ an halluc9ation4 ,= example1 if some?+ is se5 :i* does n exi/1 ! ess5tial factors are suppli$ by ! imag9ation4 ,%ad[s are (t5 wr\ght by ! imag9ation into animals & ev5 human be+s b5t ^u evil purpose4 ,gho/s are creat$ 9 ?is mann]4 ,halluc9ations dep5d l>gely ^u ! rec5cy1 frequ5cy1 & vividness ( pa/ exp]i5ce4 ,a consid]ation ( ?is type ( mislead+ p]ception does n advance ! aims ( ?is book & !re=e w be omitt$4 ,! connection betwe5 ! mat]ial & m5tal 9 vision is incompreh5sible & app>5tly mu/ ev] rema9 s4 ,objects emit or reflect light & ! optical me*anism kn[n z ! eye focuses images ( ! objects ^u ! ret9a4 ,messages are !n c>ri$ to ! bra9 :]e c]ta9 molecul> vibrations take place4 ,! physiologi/ records c]ta9 physical & *emical effects 9 ! muscles1 n]ves1 & bra9 & behold6 !re appe>s consci\sness1 s5sations1 ?\ghts1 desires1 & volitions4 ,h[8 &1 ,:y8 are que/ions :i* may nev] be answ]$4 ,x is dang]\s to use ! ^w nev]1 b ! ultimate answ]s to ^? que/ions appe> to be s remote t x disc\rages "o f proce$+ f> ov] ! hazy c\rse :i* leads t[>d !m4 ,9 fact1 x does n appreciably fur!r ! aims ( ?is book to devote mu* space to ef=ts t[>d explanation4 ,9 cov]+ ?is va/ & complex field !re are multitudes ( facts1 _m hypo!ses1 & num]\s !ories f :i* to *oose4 ,judgm5t dictates t ( ! limit$ space mo/ ( x be giv5 to ! pres5tation ( repres5tative facts4 ,?is is ! reason+ :i* l$ to ! =mulation ( ! outl9e ( *apt]s4 ,ow+ to ! va/ complex beyond ! physical ph5om5a1 physical measurem5ts ^u objects & space :i* h done s mu* t[>d build+ a solid f\ndation = sci5tific k fail ultimately to provide an exact ma!matical picture ( t :i* is p]ceiv$4 ,mu* ( ! au?or's previ\s "w has be5 devot$ to ! physical realities b ! ev]-pres5t diff]5ces betwe5 physical & p]ceptive realities h emphasiz$ ! ne$ = consid]+ ! latt] z well4 ,illusions are legion4 ,!y greet ! c>eful obs]v] on e h&4 ,!y play a prom95t "p 9 our appreciation ( ! physical _w4 ,sometimes !y mu/ be avoid$1 b (t5 !y may be put to "w 9 v>i\s arts4 ,_! widespread exi/5ce & _! =cefulness make visual p]ception ! f9al judge 9 decoration1 9 pa9t+1 9 ar*itecture1 9 l&scap+1 9 light+1 & 9 o!r activities4 ,! ultimate limitation ( measurem5ts ) physical in/rum5ts leaves ?is responsibility to ! intellect4 ,! m5tal be+ is impress$ ) ?+s z p]ceiv$1 n ) ?+s z !y are4 ,x is believ$ t ?is intellectual or judici>y phase :i* plays su* a "p 9 visual p]ception w be be/ br\ght out by examples ( v>i\s types ( /atic illusions c\pl$ ) c]ta9 facts p]ta9+ to ! eye & to ! visual process z a :ole4 ,9 special simple cases x is n difficult to det]m9e :5 or h[ ne>ly a p]ception is true b 9 g5]al1 agreem5t among normal p]sons is necess>y ow+ to ! abs5ce ( any def9ite measur+ device :i* w span ! gap betwe5 ! p]ception & ! objective reality4 ,illusions are sometimes call$ errors ( s5se & "s ( !m are su*1 b (t5 !y are errors ( ! intellect4 ,! s5ses may deliv] correctly b error may arise f imag9ation1 inexp]i5ce1 false assumptions1 & incorrect associations1 & ! rec5cy1 frequ5cy1 & vividness ( pa/ exp]i5ce4 ,! gifts ( sight are augm5t$ by ! m9d ) judgm5ts bas$ ^u exp]i5ce ) ^! gifts4 ,! direct data deliv]$ by ! visual s5se are light1 int5sity1 color1 direction4 ,^! may be consid]$ z simple or elem5tal s5sations because !y _c be fur!r simplifi$ or analyz$4 ,at ?is po9t x is hop$ t no controv]sy ) ! psy*ologi/ w be provok$4 ,9 ! space available x appe>s unfruitful to introduce ! _m qualifications necess>y to satisfy !1 z yet unc]ta9 or at lea/ conflict+1 def9itions & !ories und]ly+ ! sci5ce ( psy*ology4 ,if x is necess>y to add d>kness to ! =ego+ gr\p ( elem5tal visual s5sations1 ?is w gladly be agre$ to4 ,! p]ceptions ( outl9e-=m & surface-cont5ts p]haps rank next 9 simplicity2 h[ev]1 !y may be analyz$ into directions4 ,! p]ception ( ^! is s direct & s c]ta9 t x may be consid]$ to be imm$iate4 ,a r+ ( po9ts is app>5tly v simple & x might be consid]$ a direct s5se-p]ception1 b x consi/s ( a numb] ( elem5tal directions4 ,! p]ception ( solid-=m is f> m complex ?an outl9e-=m & !re=e m liable to error4 ,x is judg$ p>tially by b9ocul> vision or p]spective & p>tly by ! di/ribution ( light & %ade4 ,colors may help to mold =m & ev5 to give dep? to flat surfaces4 ,= example1 x is well kn[n t "s colors are advanc+ & o!rs are retir+4 ,p]haps ( /ill great] complexity are ! judgm5ts ( size & ( di/ance4 ,_m comp>isons ent] su* judgm5ts4 ,! unconsci\s acts ( ! muscles ( ! eye & v>i\s ext]nal conditions su* z ! cle>ness ( ! atmosph]e play prom95t p>ts 9 influ5c+ judgm5t4 ,^u ^! are sup]pos$ ! num]\s psy*o-physiological ph5om5a ( color1 irradiation1 etc4 ,9 vision judgm5ts are quickly made & ! process app>5tly is l>gely outside ( consci\sness4 ,high] & m complex visual judgm5ts pass into /ill high] & m complex intellectual judgm5ts4 ,all ^! may appe> to be prim>y1 imm$iate1 innate1 or in/9ctive & !re=e1 c]ta91 b ! fruits ( /udies ( ! psy*ology ( vision h %[n t ^! visual judgm5ts may be analyz$ into simpl] elem5ts4 ,!re=e1 !y are liable to error4 ,t ! anci5ts s5s$ ! exi/5ce or possibility ( illusions is evid5c$ by ! fact t !y tri$ to draw & to pa9t al?\gh _! inability to obs]ve c>efully is indicat$ by ! abs5ce ( true %ad+4 ,! ar*itecture ( anci5t ,greece reveals a k ( c]ta9 illusions 9 ! ef=ts to ov]come !m4 ,h[ev]1 ! /udy ( illusions did n engage ! att5tion ( sci5ti/s until a comp>atively rec5t p]iod4 ,not)/&+ ?is belat$ att5tion !re is a va/ sci5tific lit]ature p]ta9+ to ! multitud9\s phases ( ! subject2 h[ev]1 mo/ ( x is fragm5t>y & mu* ( x is controv]sial4 ,"s ( x deals ) !ory = a p>ticul> & (t5 a v simple case4 ,9 life complex illusions are met b at pres5t x w\ld be futile to attempt to expla9 !m 9 detail4 ,fur!rmore1 !re h be5 few attempts to g5]alize & to gr\p examples ( typical ph5om5a 9 su* a mann] z to enable a g5]al read] to see ! complex fabric z a :ole4 ,f9ally1 ! occurr5ce & application ( illusions 9 v>i\s arts & ! prom95ce ( illusions on e h& h n be5 especially treat$4 ,x is ! hope t ?is w be realiz$ 9 ! foll[+ *apt]s 9 s f> z brevity ( treatm5t makes ?is possible4 ,d\btless ?\ghtful obs]v]s ages ago notic$ visual illusions1 especially ^? f\nd 9 nature & 9 ar*itecture4 ,:5 x is consid]$ t geometrical figures are v commonly ( an illusory "* x appe>s improbable t optical illusions c\ld h escap$ ! ke5ness ( ,euclid4 ,! app>5t enl>gem5t ( ! moon ne> ! horizon & ! app>5t flatt5$ vault ( ! sky 7 notic$ at lea/ a ?\s& ye>s ago & lit]ature yields sev]al hundr$ memoirs on ^! subjects4 ,"o ( ! olde/ diss]tations ^u ! app>5t =m ( ! sky 0 publi%$ by ,alhaz51 an ,arab a/ronom] ( ! t5? c5tury4 ,kepl] 9 #afah wrote ^u ! subject4 ,philosoph]s ( ! pa/ c5turies prep>$ ! way t[>d an und]/&+ ( _m complexities ( today4 ,!y mold$ ?\ght into correct =m & e/abli%$ fundam5tal concepts & pr9ciples4 ,_! *ief tool 0 philosophy1 ! exp]im5tal attack be+ left to ! sci5ti/s ( ! mod]n age4 ,h[ev]1 !y e/abli%$ philosophically su* pr9ciples z space & "t are n realities ( ! ph5om5al _w b ! modes "u :i* we see ?+s ap>t4 ,z sci5ce became organiz$ dur+ ! pres5t exp]im5tal era1 measurem5ts 7 appli$ & !re began to appe> analytical discussions ( v>i\s subjects includ+ optical illusions4 ,"o ( ! e>lie/ inve/igations ( ! mod]n type 0 made by ,oppel1 an acc\nt ( :i* appe>$ 9 #ahed4 ,s9ce t "t sci5tific lit]ature has receiv$ ?\s&s ( wor?y contributions deal+ ) visual illusions4 ,!re are _m facts affect+ vision reg>d+ :i* no !ory is necess>y4 ,!y speak = !mselves4 ,!re are _m equally obvi\s facts :i* are n satisfactorily expla9$ b ! lack ( explanation does n prev5t _! recognition4 ,9 fact1 only ! sci5ti/ ne$s to worry ov] sy/ematic explanations & !oretical g5]alizations4 ,he ne$s ^! 9 ord] to invade & to explore ! o!r unkn[ns :]e he w add to 8 /oreh\se ( k4 ,a long /ep t[>d und]/&+ is made by becom+ acqua9t$ ) c]ta9 physical1 physiological1 & psy*ological facts ( light1 color1 & light+4 ,fur!rmore1 acqua9tance ) ! visual process & ) ! /ructure ( ! eye aids mat]ially4 ,= ?is reason ! next two *apt]s h be5 add$ ev5 at ! risk ( disc\rag+ "s read]s4 ,9 a broad s5se1 any visual p]ception :i* does n h>monize ) physical measurem5ts may be t]m$ an illusion4 ,!re=e1 ! t]m c\ld include ^? physical illusions obta9$ by means ( prisms1 l5ses1 & mirrors & su* illusions z ! mirage4 ,x c\ld also include ! physiological illusions ( light & color su* z aft]-images1 irradiation1 & contra/1 & ! psy*o-physiological illusions ( space & ! "* ( objects4 ,9 fact1 ! scope ( ! foll[+ *apt]s is arbitr>ily ext5d$ to include all ^! aspects1 b conf9es consid]ation only to /atic illusions4 ,9 a m common s5se att5tion is usually re/rict$ to ! la/ gr\p2 t is1 to ! psy*o-physiological illusions att5d+ ! p]ception ( space & ! "* ( objects al?\gh motion is (t5 includ$4 ,x %\ld be obvi\s t no simple or ev5 s+le !ory c cov] ! va/ range ( illusions consid]$ 9 ! broad s5se because !re are s _m diff]5t k9ds ( factors involv$4 ,= ?is reason explanations w be pres5t$ :]ev] feasible 9 connection ) specific illusions4 ,h[ev]1 9 clos+ ?is *apt] x appe>s ( int]e/ to t\* ^u ! m g5]ally exploit$ !ories ( illusions ( ! type consid]$ 9 ! =ego+ re/rict$ s5se4 ,hypo!ses p]ta9+ to illusions are g5]ally lack+ 9 agreem5t1 b = ! special case ( :at might be m safely t]m$ geometrical-optical illusions two diff]5t !ories1 by ,lipps & by ,wundt respectively1 are conspicu\s4 ,9 fact1 mo/ !ories are v>iants ( ^! two sy/ematic explanations ( illusions 79 ! re/rict$ s5se74 ,lipps propos$ ! pr9ciple ( me*anical-e/hetic unity1 accord+ to :i* we unconsci\sly give to e space-=m a liv+ p]sonality & unconsci\sly consid] c]ta9 me*anical =ces act+4 ,our judgm5ts are !re=e modifi$ by ?is an?ropomorphic attitude4 ,= example1 we reg>d ! circle z be+ ! result ( ! action ( tang5tial & radial =ces 9 :i* ! latt] appe> to triumph4 ,accord+ to ,lipps' !ory ! circle has a c5tripetal "* & ^! radial =ces t[>d ! c5t]1 :i* app>5tly h ov]come ! tang5tial =ces dur+ ! process ( creat+ ! circle1 lead to und]e/imation ( its size z comp>$ ) a squ>e ( ! same height & bread?4 ,by draw+ a circle & squ>e side by side1 ) ! diamet] ( ! =m] equal to ! l5g? ( a side ( ! latt]1 ?is illusion is readily demon/rat$4 ,( c\rse1 ! squ>e has a great] area ?an ! circle & x is difficult to det]m9e ! effect ( ?is disp>ity 9 area4 ,figure #fj :]e ! areas ( ! circle & squ>e are equal & consequ5tly ! height ( ! =m] is consid]ably great] ?an ! latt]1 is ( int]e/ 9 ?is connection4 ,by exp]im5t+ ) a s]ies ( pairs consi/+ ( a circle & a squ>e1 v>y+ 9 dim5sions f equal heights to equal areas1 an idea ( ! %r9k+ "* ( ! circle becomes q app>5t4 ,wundt does n attribute ! illusion to a deception or error ( judgm5t b to direct p]ception4 ,accord+ to 8 explanation1 ! laws ( ret9al image 7fixation7 & eye-movem5t are responsible4 ,= example1 v]tical di/ances appe> great] ?an horizontal ones because ! ef=t or exp5diture ( en]gy is great] 9 rais+ ! eyes ?an 9 turn+ !m ?r\gh an equal angle 9 a horizontal plane4 ,unconsci\s or involunt>y eye-movem5ts also appe> to play a "p 9 _m l9e> or m accurately1 angul> illusions1 b c]ta9ly ,wundt's explanation does n suffice = all illusions al?\gh x may expla9 _m geometrical illusions4 ,x may be said to be ( ! p]ceptive class & ,lipps' !ory to be ( ! judgm5t or high]-process class4 ,z already /at$1 mo/ ( ! o!r propos$ explanations ( geometrical illusions may be reg>d$ z be+ relat$ to "o ( ^! two !ories4 ,!re is ! indi/9ct vision !ory ( ,e9?ov52 ! p]spective !ory ( ,h]+1 ,guye1 ,?i]y1 & o!rs2 ! contra/ !ory ( ,helmholtz1 ,loeb1 & ,heyman2 & ! contra/-confluxion !ory ( ,muell]-,ly]4 ,9 ord] n to disc\rage ! read] at ! outset1 !ories z su* w be pass$ by ) ?is brief glimpse4 ,h[ev]1 m or less qualifi$ explanations are pres5t$ occasionally 9 "s ( ! *apt]s :i* foll[ 9 ord] to indicate or to sugge/ a tra9 ( ?\ght %\ld ! read] desire to attempt to und]/& "s ( ! num]\s int]e/+ illusions4 ,,ii ,,! ,,eye ,helmholtz1 :o contribut$ s mu* t[>d our k ( ! visual process1 9 ref]r+ to ! eye1 once /at$ t he c\ld make a mu* bett] optical in/rum5t b n a bett] eye4 ,9 o!r words1 ! eye is f> f an ideal optical in/rum5t b z an eye x is wond]ful4 ,its range ( s5sitiv5ess & its adaptability to ! extreme v>iety ( dem&s ^u x are truly m>vel\s :5 comp>$ ) in/rum5ts devis$ by mank9d4 ,obvi\sly1 ! eye is ! connect+ l9k betwe5 objective reality & visual p]ception &1 !re=e1 x plays an important "p 9 illusions4 ,9 fact1 sometimes x is solely responsible = ! illusion4 ,! process ( vision may be divid$ into sev]al /eps su* z 7#a7 ! light+1 color1 "*1 & disposition ( objects2 7#b7 ! me*anism by :i* ! image is =m$ ^u ! ret9a2 7#c7 v>i\s optical defects ( ?is me*anism2 7#d7 ! s5sitiv5ess ( ! p>ts ( ! ret9a to light & color2 7#e7 ! /ructure ( ! ret9a2 7#f7 ! p>ts play$ by monocul> & b9ocul> vision2 & 7#g7 ! v>i\s ev5ts :i* foll[ ! =mation ( ! image ^u ! ret9a4 ,! me*anism ( ! eye makes x possible to see n only light b objects4 ,elem5t>y eyes ( ! l[e/ animals p]ceive light b _c see objects4 ,^! eyes are m]ely specializ$ n]ves4 ,9 ! human eye ! optic n]ve spreads to =m ! ret9a & ! latt] is a specializ$ n]ve4 ,nature has accompani$ ?is evolution by develop+ an in/rum5t !--eye--= int5sify+ & def9+ & ! :ole is ! visual s5se-organ4 ,! latt] conta9s ! mo/ highly specializ$ n]ve & ! mo/ ref9$ physiological me*anism1 ! result be+ ! highe/ s5se-organ4 ,illu/ration3 ,fig4 #a4--,pr9cipal p>ts ( ! eye4 ,a1 ,conjunctiva2 ,b1 ,ret9a2 ,c1 ,*oroid2 ,d1 ,scl]a2 ,e1 ,fovea2 ,f1 ,bl9d ,spot2 ,g1 ,optic ,n]ve2 ,h1 ,cili>y ,muscle2 ,i1 ,iris2 ,j1 ,cornea2 ,k1 ,ligam5t4 ,! eye is approximately a sph]ical %ell transp>5t at ! front portion & opaque 7or ne>ly s7 ov] ! rema9+ eighty p] c5t ( its surface4 ,! optical pa? consi/s ( a s]ies ( transp>5t liquids & solids4 ,! *ief details ( ! /ructure ( ! eye are repres5t$ 9 ,fig4 #a4 ,beg9n+ ) ! ext]ior & proce$+ t[>d ! ret9a we f9d 9 succession ! cornea1 ! ant]ior *amb] conta9+ ! aque\s humor1 ! iris1 ! l5s1 ! l>ge *amb] conta9+ ! vitre\s humor1 & f9ally ! ret9a4 ,c]ta9 muscles alt] ! position ( ! eye & consequ5tly ! optical axis1 & focus+ 7accommodation7 is accompli%$ by alt]+ ! ?ickness & %ape1 & consequ5tly ! focal l5g?1 ( ! l5s4 ,! iris is a %utt] :i* automatically controls to "s degree ! am\nt ( light rea*+ ! ret9a1 !reby t5d+ to protect ! latt] f too mu* light4 ,x also has "s influ5ce ^u ! def9ition ( ! image2 t is1 ^u :at is t]m$ visual acuity or ! ability to di/+ui% f9e detail4 ,x is int]e/+ to comp>e ! eye ) ! cam]a4 ,9 ! case ( ! cam]a & ! photographic process1 we h 7#a7 an inv]t$ light-image1 a facsimile ( ! object usually dim9i%$ 9 size2 7#b7 an invisible image 9 ! photographic emulsion consi/+ ( molecul> *anges due to light2 & 7#c7 a visible image develop$ on ! plate4 ,9 ! case ( ! eye & ! visual process we h 7#a7 an inv]t$ light-image1 a facsimile ( ! object dim9i%$ 9 size2 7#b7 ! invisible image 9 ! ret9al sub/ances probably consi/+ ( molecul> *anges due to light2 & 7#c7 an ext]nal visible image4 ,x w be not$ t 9 ! case ( vision ! f9al image is project$ outw>d--x is ext]nal4 ,! m we ?9k ( ?is outw>d projection ! m int]e/+ & m>vel\s vision becomes4 ,= example1 x appe>s c]ta9 t if a photographic plate c\ld see or feel1 x w\ld see or feel ! silv] image ^u itself b n out 9 space4 ,h[ev]1 ?is po9t is discuss$ fur!r 9 ! next *apt]4 ,9 ! cam]a & photographic process we trace me*anism1 physics1 & *emi/ry ?r\gh\t4 ,9 ! eye & visual process we are able to trace ^! factors only to a c]ta9 po9t1 :]e we enc\nt] ! sup]-physical & sup]-*emical4 ,"h molecul> *ange is replac$ by s5sation1 p]ception1 ?\ght1 & emotion4 ,our exploration takes u f ! physical _w into ano!r1 :olly diff]5t1 :]e !re reigns ano!r ord] ( ph5om5a4 ,we h pass$ f ! mat]ial into ! m5tal _w4 ,! eye z an optical me*anism is r$ucible to a s+le l5s & !re=e ! image focus$ ^u ! ret9a is inv]t$4 ,h[ev]1 !re is no way = ! obs]v] to be consci\s ( ?is & !re=e ! inv]t$ image causes no difficulty 9 see+4 ,! images ( objects 9 ! "r half ( ! field ( view are focus$ ^u ! left half ( ! ret9a4 ,simil>ly1 ! left half ( ! field ( view corresponds to ! "r half ( ! ret9a2 ! upp] half ( ! =m] to ! l[] half ( ! latt]2 & s on4 ,:5 a ray ( light f an object /rikes ! ret9a ! impression is ref]r$ back along ! ray-l9e into ! orig9al place 9 space4 ,?is is int]e/+ly demon/rat$ 9 a simple mann]4 ,pun* a p9-hole 9 a c>d & hold x ab\t f\r in*es f ! eye & at ! same "t hold a p9-head z close to ! cornea z possible4 ,! backgr\nd = ! p9-hole %\ld be ! sky or o!r bright surface4 ,aft] a brief trial an inv]t$ image ( ! p9-head is se5 9 ! hole4 ,pun* sev]al holes 9 ! c>d & 9 ea* w be se5 an inv]t$ image ( ! p9-head4 ,! explanation ( ! =ego+ is n difficult4 ,! p9-head is s close to ! eye t ! image _c be focus$ ^u ! ret9a2 h[ev]1 x is 9 a v favorable position to ca/ a %ad[ ^u ! ret9a1 ! light-s\rce be+ ! p9-hole ) a bright backgr\nd4 ,light /ream+ ?r\gh ! p9-hole into ! eye ca/s an erect %ad[ ( ! p9-head ^u ! ret9a1 & ?is erect image is project$ into space & inv]t$ 9 ! process by ! effect ( ! l5s4 ,! latt] is n op]ative dur+ ! ca/+ ( ! %ad[ because ! p9-head is too close to ! l5s1 z already /at$4 ,x is fur!r prov$ to be outw>d projection ( ! ret9al image 7! %ad[7 because by multiply+ ! numb] ( p9-holes 7! light-s\rces7 !re are also a correspond+ numb] ( %ad[s4 ,! =ego+ n only illu/rates ! inv]sion ( ! image b aga9 emphasizes ! fact t we d n see ret9al images4 ,ev5 ! />s :i* we see on press+ ! eye-lid or on receiv+ a bl[ on ! eye are project$ into space4 ,! motes :i* we see 9 ! visual field :ile gaz+ at ! sky are defects 9 ! eye-m$ia1 & ^! images are project$ into space4 ,we d n see any?+ 9 ! eye4 ,! ret9al image impresses ! ret9a 9 "s def9ite mann] & ! impression is c>ri$ to ! bra9 by ! optic n]ve4 ,! intellect !n ref]s or projects ?is impression outw>d into space z an ext]nal image4 ,! latt] w\ld be a facsimile ( ! physical object if !re 7 no illusions b ! fact t !re are illusions indicates t errors are introduc$ some:]e along ! pa? f & to ! object4 ,x is int]e/+ to speculate :e!r ! fir/ visual impression ( a new-born babe is project$ outw>d or is p]ceiv$ z 9 ! eye4 ,x is equally futile to conjecture 9 ?is mann] because !re is no indication t ! "t w come :5 ! baby c answ] u imm$iately ^u exp]i5c+ its fir/ visual impression4 ,! p]iod ( infancy increases ) progress up ! scale ( animal life & ?is l5g!n+ is d\btless responsible & p]haps necess>y = ! developm5t ( highly specializ$ s5se-organs4 ,incid5tally1 suppose a bl9d p]son to be absolutely un$ucat$ by transf]r$ exp]i5ce & t he sudd5ly became a normal adult & able to see4 ,:at w\ld he say ab\t 8 fir/ visual impression8 ,app>5tly su* a subject is unobta9able4 ,! ne>e/ t su* a case _h be5 approa*$ is ! case ( a p]son born bl9d1 ^: sight has be5 re/or$4 ,?is p]son has acquir$ mu* exp]i5ce ) ! ext]nal _w ?r\gh o!r s5ses4 ,x has be5 record$ t su* a p]son1 aft] sight 0 re/or$1 appe>$ to ?9k t ext]nal objects t\*$ ! eyes4 ,only ?r\gh visual exp]i5ce is ?is error 9 judgm5t rectifi$4 ,man /udies 8 k9d too mu* ap>t f o!r animals & p]haps ei!r und]e/imates or ov]e/imates ! am\nt ( inh]it$1 innate1 in/9ctive qualities4 ,a new-born *ick 9 a few m9utes w walk /raight to an object & seize x4 ,app>5tly ?is implies p]ception ( di/ance & direction & a coo]d9ation ( muscles = walk+ & mov+ ! eyes4 ,x appe>s reasonable to conclude t a c]ta9 am\nt ( ! weal? ( capacities possess$ by ! individual is p>tly inh]it$1 & 9 man ! acquir$ pr$om9ates4 ,b all capacities are acquir$1 = ev5 ! inh]it$ 0 acquir$ 9 ance/ral exp]i5ce4 ,ev5 in/9ct 7:atev] t may be7 mu/ involve inh]it$ exp]i5ce4 ,^! glimpses ( ! dep?s to :i* "o mu/ dig if he is to une>? ! complete explanations ( visual p]ception--& consequ5tly ( illusions--indicate ! futility ( treat+ ! !ories 9 ! available space )\t encroa*+ unduly ^u ! aims ( ?is volume4 ,c]ta9 defects ( ! optical sy/em ( ! eye mu/ contribute t[>d caus+ illusions4 ,any p]fect l5s ( homog5e\s mat]ial has at lea/ two defects1 kn[n z sph]ical & *romatic ab]ration4 ,! =m] manife/s itself by ! b5d+ ( /raight l9es & is usually demon/rat$ by =m+ an image ( an object su* z a wire me% or *eck]bo>d2 ! out] l9es ( ! image are f\nd to be v mu* b5t4 ,?is defect 9 ! eye-l5s is some:at c\nt]act$ by a v>iable optical d5sity1 increas+ f ! out] to ! c5tral portion4 ,?is results 9 an increase 9 refractive-index z ! c5t] ( ! l5s is approa*$ & t5ds to dim9i% its sph]ical ab]ration4 ,! eye commonly possesses abnormalities su* z a/igmatism & ecc5tricity ( ! optical elem5ts4 ,all ^! contribute t[>d ! creation ( illusions4 ,:ite light consi/s ( rays ( light ( v>i\s colors & ^! are sep>at$ by means ( a prism because ! refractive-index ( ! prism diff]s = lights ( diff]5t color or wave-l5g?4 ,?is causes ! blue rays1 = example1 to be b5t m ?an ! r$ rays :5 trav]s+ a prism4 ,x is 9 ?is mann] t ! spectrum ( light may be obta9$4 ,a l5s may be consid]$ to be a prism ( revolution & x ?us becomes evid5t t ! blue rays w be br\ght to a focus at a less] di/ance ?an ! r$ rays2 t is1 ! =m] are b5t m f _! orig9al pa? ?an ! latt]4 ,?is defect ( l5ses is kn[n z *romatic ab]ration & is q obvi\s 9 ! eye4 ,x may be demon/rat$ by any simple l5s1 = ! image ( ! sun1 = example1 w appe> to h a color$ fr+e4 ,a purple filt] :i* transmits only ! violet & r$ rays is useful = ?is demon/ration4 ,by look+ at a lamp-filam5t or c&le-flame "s di/ance away ! object w appe> to h a violet halo1 b ! color ( ! fr+e w v>y ) accommodation4 ,on look+ ?r\gh a p9-hole at ! edge ( an object silh\ett$ aga9/ ! bright sky ! edge w appe> r$ if ! light f ! p9-hole ent]s ! pupil ne> its p]iph]y4 ,?is optical defect ( ! eye makes objects appe> m %>ply def9$ :5 view$ 9 mono*romatic light4 ,9 fact1 ?is is q obvi\s :5 us+ yell[ glasses4 ,! defect is also demon/rat$ by view+ a l9e-spectrum focus$ on a gr\nd glass4 ,! blue & r$ l9es _c be se5 di/9ctly at ! same di/ance4 ,! blue l9es c be focus$ at a mu* less di/ance ?an ! r$ l9es4 ,*romatic ab]ration c acc\nt = su* an illusion z ! famili> advanc+ & retir+ colors & d\btless x plays a "p 9 _m illusions4 ,! /ructure ( ! ret9a plays a v important "p 9 vision & acc\nts = v>i\s illusions & _m int]e/+ visual ph5om5a4 ,! optic n]ve spreads out to =m ! ret9a :i* con/itutes ! inn] portion ( ! sph]ical %ell ( ! eye ) ! exception ( ! front "p4 ,ref]r+ aga9 to ,fig4 #a1 ! out] coat+ ( ! %ell is call$ ! scl]otic4 ,?is consi/s ( d5se fibr\s tissue kn[n z ! :ite ( ! eye4 ,inside ?is coat+ is a lay] ( black pigm5t cells t]m$ ! *oroid4 ,next is ! bacill>y lay] :i* l9es ab\t five-six?s ( ! int]ior surface ( ! eye4 ,?is is =m$ by closely pack$ rods & cones1 :i* play a dom9ant role 9 ! visual process4 ,a light-s5sitive liquid 7visual purple7 & cellul> & fibr\s lay]s complete ! ret9al /ructure4 ,! place :]e ! optic n]ve ent]s ! eye-ball & beg9s to spread out is bl9d4 ,objects ^: images fall on ?is spot are invisible4 ,?is bl9d-spot is n p>ticul>ly ( int]e/ "h1 b x may be ( int]e/ to note its effect4 ,?is is easily done by clos+ "o eye & look+ directly at "o ( two small black circles ab\t two in*es ap>t on :ite pap] at a di/ance ( ab\t a foot f ! eye4 ,by mov+ ! objects ab\t until ! image ( ! circle n directly look$ at falls ^u ! bl9d-spot1 ?is circle w disappe>4 ,a ?ree-foot circle at a di/ance ( #cf feet w completely disappe> if its image falls directly ^u ! bl9d-spot4 ,at a di/ance ( #db in*es ! invisible area is ab\t #ab in*es f ! po9t ( sight & ab\t #c to #d in*es 9 diamet]4 ,at #cjj feet ! area is ab\t #h feet 9 diamet]4 ,! actual size ( ! ret9al bl9d-spot is ab\t #j4je in* 9 diamet] or ne>ly #e degrees4 ,b9ocul> vision ov]comes any annoyance due to ! bl9d-spots because !y d n ov]lap 9 ! visual field4 ,a "o-ey$ p]son is really totally bl9d = ?is portion ( ! ret9a or ( ! visual field4 ,! bacill>y lay] consi/s ( s-call$ rods & cones4 ,only ! rods function "u v l[ int5sities ( illum9ation ( ! ord] ( moonlight4 ,! cones are s5sitive to color & function only at int5sities great] ?an :at may be t]m$ twilight int5sities4 ,^! elem5ts are v small b ! fact t !y appe> to be connect+ l9ks betwe5 ! ret9al image & visual p]ception1 acuity or discrim9ation ( f9e detail is limit$ inasmu* z ! elem5ts are ( f9ite dim5sions4 ,! smalle/ image :i* w produce a visual impression is ! size ( ! end ( a cone4 ,! smalle/ di/ance betwe5 two po9ts :i* is visible at five in*es is ab\t #j4jja in*4 ,two cones mu/ be /imulat$ 9 su* a case4 ,f9e l9es may appe> crook$ because ( ! irregul> disposition ( ^! elem5tal light-s5sitive po9ts4 ,?is app>5t crook$ness ( l9es is an illusion :i* is directly due to ! limitations ( ret9al elem5ts ( f9ite size4 ,! di/ribution ( rods & cones ov] ! ret9a is v important4 ,9 ! fovea c5tralis--! po9t ( ! ret9a on ! optical axis ( ! eye--is a slight depression mu* ?9n] ?an ! rema9d] ( ! ret9a & ?is is inhabit$ *iefly by cones4 ,x is ?is spot :i* provides visual acut5ess4 ,x is easily demon/rat$ t f9e detail _c be se5 well def9$ outside ?is c5tral portion ( ! visual field4 ,:5 we desire to see an object di/9ctly we habitually turn ! head s t ! image ( ! object falls ^u ! fovea ( ea* eye4 ,helmholtz has comp>$ ! foveal & lat]al images ) a f9i%$ draw+ & a r\gh sket* respectively4 ,! fovea also conta9s a yell[ pigm5tation :i* makes ?is area ( ! ret9a selective z to color-vision4 ,on view+ c]ta9 colors a diff]5ce 9 color ( ?is c5tral portion ( ! field is (t5 v evid5t4 ,9 ! outly+ regions ( ! ret9a1 rods pr$om9ate & 9 ! int]m$iate zone bo? rods & cones are f\nd4 ,inasmu* z rods are n s5sitive to color & cones d n function at l[ int5sities ( illum9ation x is obvi\s t visual impressions %\ld v>y1 dep5d+ ^u ! area ( ! ret9a /imulat$4 ,9 fact1 _m int]e/+ illusions are acc\nt$ = 9 ?is mann]1 "s ( :i* are discuss$ lat]4 ,x is well kn[n t a fa9t /> is se5 be/ by av]t$ vision4 ,x may be q invisible :5 ! eye is direct$ t[>d x1 t is1 :5 its image falls ^u ! rod-free fovea4 ,h[ev]1 by av]t+ ! l9e ( sight slightly1 ! image is caus$ to fall on a ret9al area conta9+ rods 7s5sitive to feeble light7 & ! /> may be readily recogniz$4 ,! fovea is ! po9t ( di/9ct focus4 ,x is necess>y = fix$ ?\ghtful att5tion4 ,x exi/s 9 ! ret9a ( man & ( high] monkeys b x quickly disappe>s z we pass d[n ! scale ( animal life4 ,x may be necess>y = ! safety ( ! l[] animals t !y see equally well ov] a l>ge field2 h[ev]1 x appe>s advantage\s t man give fix$ & undivid$ att5tion to ! object look$ at4 ,man does n ne$ to tru/ solely to 8 s5ses to protect himself f dang]s4 ,he uses 8 intellect to inv5t & to con/ruct artificial def5ses4 ,)\t ! highly specializ$ fovea we might see equally well ov] ! :ole ret9a b c\ld n look att5tively at any?+1 & !re=e c\ld n obs]ve ?\ghtfully4 ,:5 an image ( a bright object exi/s ^u ! ret9a = a "t !re results a p>tial exhau/ion or fatigue ( ! ret9al processes ) a result t an aft]-image is se54 ,?is aft]-image may be bright = a "t ow+ to ! fact t x takes "t = ! ret9al process to die out4 ,!n !re comes a reaction :i* is app>5t :5 ! eye is direct$ t[>d illum9at$ surfaces4 ,! "p ( ! ret9a :i* has be5 fatigu$ does n respond z fully z ! fre%] areas1 ) ! result t ! fatigu$ area contributes a d>k] area 9 ! visual field4 ,?is is kn[n z an aft]-image & !re are _m int]e/+ v>iations4 ,! aft]-image usually und]goes a s]ies ( *anges 9 color z well z 9 brightness z ! ret9al process readju/s itself4 ,an aft]-image ( a color$ object may (t5 appe> ( a color complem5t>y to ! color ( ! object4 ,?is is g5]ally acc\nt$ = by fatigue ( ! ret9al process4 ,!re are _m conflict+ !ories ( color-vision b !y are n z conflict+ 9 respect to ! aspect ( fatigue z 9 "s o!r aspects4 ,if ! eye is direct$ t[>d a gre5 surface = a "t & !n turn$ t[>d a :ite surface1 ! fatigue to gre5 light dim9i%es ! ext5t ( response to ! gre5 rays 9 ! light reflect$ by ! :ite surface4 ,! result is ! p]ception ( a c]ta9 area ( ! :ite surface 7correspond+ to ! portion ( ! field fatigu$ by gre5 light7 z ( a color equal to :ite m9us "s gre5--! result ( :i* is p9k or purple4 ,?is is easily und]/ood by ref]r+ to ! pr9ciples ( color-mixture4 ,r$1 gre51 & blue 7or violet7 mix$ 9 prop] proportions w produce any color or t9t & ev5 :ite4 ,?us ^! may be consid]$ to be ! compon5ts ( :ite light4 ,h5ce if ! ret9a ?r\gh fatigue is unable to respond fully to ! gre5 compon5t1 ! result may be express$ ma!matically z r$ plus blue plus r$uc$ gre51 or syn!tically a purpli% :ite or p9k4 ,:5 fatigu$ to r$ light ! aft]-image on a :ite surface is blue-gre54 ,:5 fatigu$ to blue light x is yell[i%4 ,fur!r mixtures may be obta9$ by direct+ ! aft]-image ^u color$ surfaces4 ,9 ?is mann] _m ( ! int]e/+ visual ph5om5a & illusions associat$ ) ! view+ ( colors are acc\nt$ =4 ,! influ5ce ( a color$ environm5t ^u a color$ object is really v great4 ,?is is kn[n z simultane\s contra/4 ,! influ5ce ( ! imm$iately previ\s hi/ory ( ! ret9a ^u ! p]ception ( color$ surfaces is also v /rik+4 ,?is is call$ successive contra/4 ,x is int]e/+ to note t an aft]-image produc$ by look+ at a bright light-s\rce1 = example1 is project$ into space ev5 ) ! eyes clos$4 ,x is in/ructive to /udy aft]-images & ?is may be done at any mom5t4 ,on gaz+ at ! sun = an in/ant & !n look+ away1 an aft]-image is se5 :i* passes 9 color f gre51 blue1 purple1 etc41 & f9ally fades4 ,= a "t x is bright] ?an ! backgr\nd :i* may conv5i5tly be ! sky4 ,on clos+ ! eyes & plac+ ! h&s ov] !m ! backgr\nd n[ is d>k & ! appe>ance ( ! aft]-image *anges m>k$ly4 ,!re are _m k9ds1 effects1 & v>iations ( aft]-images1 "s ( :i* are discuss$ 9 o!r *apt]s4 ,z ! int5sity ( illum9ation ( a l&scape1 = example1 decreases t[>d twilight1 ! ret9a dim9i%es 9 s5sibility to ! rays ( long] wave-l5g?s su* z yell[1 orange1 & r$4 ,!re=e1 x becomes relatively m s5sitive to ! rays ( %ort] wave-l5g? su* z gre51 blue1 & violet4 ,! effects ( ?is ,purk9je ph5om5on 7nam$ aft] ! discov]]7 may be add$ to ! class ( illusions treat$ 9 ?is book4 ,x is int]e/+ to note 9 ?is connection t moonlight is repres5t$ on "s pa9t+s & especially on ! /age z gre5i% blue 9 color1 not)/&+ t physical measurem5ts %[ x to be approximately ! color ( sunlight4 ,9 fact1 x is sunlight reflect$ by dead1 frigid1 & practically colorless matt]4 ,"s illusions may be directly trac$ to ! /ructure ( ! eye "u unusual light+ conditions4 ,= example1 9 a d>k room hold a lamp obliquely outw>d b ne> "o eye 7! o!r be+ clos$ & %ield$7 & =w>d suffici5tly = ! ret9a to be /rongly illum9at$4 ,move ! lamp g5tly :ile gaz+ at a pla9 d>k surface su* z ! wall4 ,f9ally ! visual field appe>s d>k1 due to ! int5se illum9ation ( ! ret9a & !re w appe>1 app>5tly project$ ^u ! wall1 an image resembl+ a bran*+ leafless tree4 ,^! are really %ad[s ( ! blood vessels 9 ! ret9a4 ,! exp]im5t is m successful if an image ( a bright light-s\rce is focus$ on ! scl]otic ne> ! cornea4 ,if ?is image ( ! light-s\rce is mov$1 ! tree-l image se5 9 ! visual field w also move4 ,! rate ( gr[? & decay ( v>i\s color-s5sations v>ies consid]ably4 ,by tak+ advantage ( ?is fact _m illusions c be produc$4 ,9 fact1 ! c>eful obs]v] w enc\nt] _m illusions :i* may be readily acc\nt$ = 9 ?is mann]4 ,x may be said t 9 g5]al ! eyes are nev] at re/4 ,involunt>y eye-movem5ts are tak+ place all ! "t1 at lea/ dur+ consci\sness4 ,"s h giv5 ?is re/lessness a major "p 9 ! process ( vision b aside f ! correctness ( !ories involv+ eye-movem5ts1 x is a fact t !y are responsible = c]ta9 illusions4 ,on a />-lit night if "o lies d[n & looks up at a /> ! latt] w be se5 to appe> to be swimm+ ab\t m or less j]kily4 ,on view+ a rapidly revolv+ :eel ( an automobile z x proce$s d[n ! /reet1 occasionally x w be se5 to cease revolv+ mom5t>ily4 ,^! app>5tly are acc\nt$ = by involunt>y eye-movem5ts :i* take place reg>dless ( ! ef=t made to fixate vision4 ,if ! eyelids are almo/ clos$1 /ream]s appe> to radiate 9 v>i\s directions f a light-s\rce4 ,movem5ts ( ! eyelids :5 ne>ly clos$ sometimes cause objects to appe> to move4 ,^! may be acc\nt$ = p]haps by ! di/ortion ( ! moi/ film :i* cov]s ! cornea4 ,! =ego+ are only a few ( ! _m visual ph5om5a due l>gely to ! /ructure ( ! eye4 ,! effects ( ^! & _m o!rs ent] into visual illusions1 z w be se5 "h & !re ?r\gh\t ! *apt]s :i* foll[4 ,,iii ,,vision ,a description ( ! eye by no means suffices to cl>ify ! visual process4 ,ev5 ! descriptions ( v>i\s ph5om5a 9 ! prec$+ *apt] accompli% little m ?an to acqua9t ! read] ) ! op]ation ( a me*anism1 al?\gh !y sugge/ ! tr5d ( ! explanations ( _m illusions4 ,at be/ only monocul> vision has be5 treat$1 & x does n exi/ normally = human be+s4 ,a p]son capable only ( monocul> vision w\ld be l ,cyclops ,polyphemus4 ,we might h two eyes1 or ev51 l ,argus1 possess a hundr$ eyes & /ill n exp]i5ce ! wond]ful advantages ( b9ocul> vision1 = ea* eye might see indep5d5tly4 ,! ph5om5a ( b9ocul> vision are f> less physical ?an ^? ( monocul> vision4 ,!y are mu* m obscure1 illusory1 & p]plex+ because !y are m complexly int]wov5 ) or alli$ to psy*ological ph5om5a4 ,! s5se ( sight diff]s consid]ably f ! o!r s5ses4 ,! s5se ( t\* requires solid contact 7usually72 ta/e involves liquid contact2 smell1 gase\s contact2 & he>+ dep5ds ^u a relay ( vibrations f an object ?r\gh ano!r m$ium 7usually air71 result+ f9ally 9 contact4 ,h[ev]1 we p]ceive ?+s at a di/ance ?r\gh vibration 7electromagnetic waves call$ light7 convey$ by a subtle1 intangible1 univ]sal m$ium :i* is unrecognizable except+ z a hypo!tically necess>y be>] ( light-waves or1 m g5]ally1 radiant en]gy4 ,x also is int]e/+ to comp>e ! subjectiv5ess & objectiv5ess ( s5sations4 ,! s5sation ( ta/e is subjective2 x is 9 u1 n 9 ! body ta/$4 ,9 smell we p]ceive ! s5sation 9 ! nose & by exp]i5ce ref] x to an object at a di/ance4 ,! s5sation ( he>+ is objective2 t is1 we ref] ! cause to an object s completely t !re is practically no consci\sness ( s5sation 9 ! e>4 ,9 sight ! impression is s completely project$ outw>d into space & !re is s little consci\sness ( any occurr5ce 9 ! eye t x is extremely difficult to conv9ce ourselves t x is ess5tially a subjective s5sation4 ,! =ego+ ord] repres5ts ! s5se-organs 9 increas+ specialization & ref9em5t4 ,9 ! two high] s5ses--sight & he>+--!re is no direct contact ) ! object & an intricate me*anism is plac$ 9 front ( ! specializ$ n]ve to def9e & to int5sify ! impression4 ,9 ! case ( vision ?is highly develop$ in/rum5t makes x possible to see n only light b objects4 ,z we g up ! scale ( v]tebrate animals we f9d t !re is a gradual *ange ( ! position ( ! eyes f ! sides to ! front ( ! head & a *ange ( ! incl9ation ( ! optical axes ( ! two eyes f #ahj degrees to p>allel4 ,!re is also evid5t a gradual increase 9 ! f95ess ( ! bacill>y lay] ( ! ret9a f ! m>g9s t[>d ! c5t]1 &1 !re=e1 an increas+ accuracy 9 ! p]ception ( =m4 ,?is f9ally results 9 a highly organiz$ c5tral spot or fovea :i* is possess$ only by man & ! high] monkeys4 ,proce$+ up ! scale we also f9d an increas+ ability to conv]ge ! optic axes on a ne> po9t s t ! images ( ! po9t may co9cide ) ! c5tral spots ( bo? ret9as4 ,^! *anges & o!rs are closely associat$ ) ea* o!r & especially ) ! developm5t ( ! high] faculties ( ! m9d4 ,b9ocul> vision 9 man & 9 ! high] animals is ! la/ result ( ! gradual improvem5t ( ! mo/ ref9$ s5se-organ1 adapt+ x to meet ! requirem5ts ( highly complex organisms4 ,x _c exi/ 9 "s animals1 su* z birds & fi%es1 because !y _c conv]ge _! two optical axes ^u a ne> po9t4 ,:5 a *ick5 wi%es to look int5tly at an object x turns its head & looks ) "o eye4 ,su* an animal sees ) two eyes indep5d5tly & possibly moves !m indep5d5tly4 ,! normal position ( ! axes ( human eyes is conv]g5t or p>allel b x is possible to div]ge ! axes4 ,9 fact1 ) practice x is possible to div]ge ! axes suffici5tly to look at a po9t ne> ! back ( ! head1 al?\gh1 ( c\rse1 we d n see ! po9t4 ,! movem5t ( ! eyes is r complex4 ,:5 !y move toge!r to "o side or ! o!r or up & d[n 9 a v]tical plane !re is no rotation ( ! optical axes2 t is1 no torsion4 ,:5 ! visual plane is elevat$ & ! eyes move to ! "r !y rotate to ! "r2 :5 !y move to ! left !y rotate to ! left4 ,:5 ! visual plane is depress$ & ! eyes move to ! "r !y rotate to ! left2 :5 !y move to ! left !y rotate to ! "r4 ,?r\gh exp]i5ce we unconsci\sly evaluate ! muscul> /resses1 ef=ts1 & movem5ts accompany+ ! motion ( ! eyes & !reby int]pret mu* ?r\gh visual p]ception 9 reg>d to su* aspects ( ! ext]nal _w z size1 %ape1 & di/ance ( objects4 ,ev5 ?is brief glimpse ( ! pr9cipal movem5ts ( ! eyes indicates a complexity :i* sugge/s ! intricacy ( ! explanations ( c]ta9 visual ph5om5a4 ,at ?is po9t x appe>s advantage\s to set d[n ! pr9cipal modes by :i* we p]ceive ! ?ird dim5sion ( space & ( objects & o!r aspects ( ! ext]nal _w4 ,!y are z foll[s3 7#a7 ext5t2 7#b7 cle>ness ( brightness & color z affect$ by di/ance2 7#c7 int]f]5ce ( ne> objects ) ^? m di/ant2 7#d7 elevation ( objects2 7#e7 v>iation ( light & %ade on objects2 7#f7 ca/ %ad[s2 7#g7 p]spective2 7#h7 v>iation ( ! visor angle 9 proportion to di/ance2 7#i7 muscul> ef=t att5d+ accommodation ( ! eye2 7#aj7 /]eoscopic vision2 7#aa7 muscul> ef=t att5d+ conv]g5ce ( ! axes ( ! eyes4 ,x w be recogniz$ t only ! la/ two are necess>ily conc]n$ ) b9ocul> vision4 ,^! v>ieties ( exp]i5ces may be comb9$ 9 almo/ an inf9ite v>iety ( proportions4 ,wundt 9 8 attempt to expla9 visual p]ception consid]$ *iefly ?ree factors3 7#a7 ! ret9al image ( ! eye at re/2 7#b7 ! influ5ce ( ! movem5ts ( "o eye2 &1 7#c7 ! additional data furni%$ by ! two eyes function+ toge!r4 ,!re are ?ree fields ( vision correspond+ to ! =ego+4 ,^! are ! ret9al field ( vision1 ! monocul> field1 & ! b9ocul> field4 ,! ret9al field ( vision is t ( an eye at re/ z comp>$ ) ! monocul> field1 :i* is all t c be se5 ) "o eye 9 its entire range ( movem5t & !re=e ( exp]i5ce4 ,! ret9al field has no cle>ly def9$ b\nd>ies because x f9ally fades at its indef9ite p]iph]y into a region :]e s5sation ceases4 ,x might be tiresome to foll[ detail$ analyses ( ! _m modes by :i* visual p]ception is atta9$1 s only a few g5]alizations w be pres5t$4 ,= e volunt>y act ( sight !re are two adju/m5ts ( ! eyes1 namely1 focal & axial4 ,9 ! =m] case ! cili>y muscle adju/s ! l5s 9 ord] to produce a def9$ image ^u ! ret9a4 ,9 axial adju/m5ts ! two eyes are turn$ by c]ta9 muscles s t _! axes meet on ! object look$ at & ! images ( ! object fall on ! c5tral-spots ( ! ret9a4 ,^! take place toge!r )\t di/9ct volition = ea* b by ! s+le volunt>y act ( look+4 ,?r\gh exp]i5ce ! intellect has acquir$ a wond]ful capacity to int]pret su* factors z size1 =m1 & di/ance 9 t]ms ( ! muscul> movem5ts 9 g5]al )\t ! obs]v] be+ consci\s ( su* int]pretations4 ,b9ocul> vision is easily recogniz$ by hold+ a f+] be=e ! eyes & look+ at a po9t beyond x4 ,! result is two app>5tly transp>5t f+]s4 ,an object is se5 s+le :5 ! two ret9al images fall on correspond+ po9ts4 ,direction is a prim>y datum ( s5se4 ,! prop]ty ( correspond+ po9ts ( ! two ret9as 7b9ocul> vision7 & consequ5tly ( id5tical spatial po9ts 9 ! two visual fields is n s simple4 ,x is /ill a "q :e!r correspond+ po9ts 7t is1 ! exi/5ce ( a correspond+ po9t 9 "o ret9a = ea* po9t 9 ! o!r ret9a7 are innate1 in/9ctive1 & are antec$5t ( exp]i5ce or are pair$ z ! result ( exp]i5ce4 ,! "o view results 9 ! nativi/ic1 ! o!r 9 ! empiri/ic !ory4 ,inasmu* z "s sci5ti/s are array$ on "o side & "s on ! o!r1 x appe>s futile to dwell fur!r ^u ?is aspect4 ,x mu/ suffice to /ate t b9ocul> vision1 :i* consi/s ( two ret9as & consequ5tly two fields ( view absolutely coo]d9at$ 9 "s mann] 9 ! bra91 yields ext5sive in=mation conc]n+ space & its cont5ts4 ,aft] not+ aft]-images1 motes float+ 9 ! field ( view 7caus$ by defects 9 ! eye-m$ia7 & v>i\s o!r ?+s1 x is evid5t t :at we call ! field ( view is ! ext]nal projection into space ( ret9al /ates4 ,all ! v>iations ( ! latt]1 su* z images & %ad[s :i* are produc$ 9 ! ext]nal field ( "o eye1 are fai?fully reproduc$ 9 ! ext]nal field ( ! o!r eye4 ,?is s5se ( an ext]nal visual field is in]adicable4 ,ev5 :5 ! eyes are clos$ ! ext]nal field is /ill !re2 ! imag9ation or intellect projects x outw>d4 ,objects at diff]5t di/ances _c be se5 di/9ctly at ! same "t b by int]pret+ ! eye-movem5ts z ! po9t ( sight is run backw>d & =w>d 7v>y+ conv]g5ce ( ! axes7 ! intellect practically automatically appraises ! size1 =m1 & di/ance ( ea* object4 ,obvi\sly1 exp]i5ce is a prom95t factor4 ,! p]ception ( ! ?ird dim5sion1 dep? or relative di/ance1 :e!r 9 a s+le object or a gr\p ( objects1 is ! result ( ! successive comb9ation ( ! diff]5t p>ts ( two dissimil> images ( ! object or gr\p4 ,z already /at$1 ! p]ception ( di/ance1 size1 & =m is bas$ p>tly ^u monocul> & p>tly ^u b9ocul> vision1 & ! simple elem5ts ^u :i* judgm5ts ( ^! are bas$ are light1 %ade1 color1 int5sity1 & direction4 ,al?\gh ! int]pretation ( muscul> adju/m5ts plays a prom95t "p 9 ! =mation ( judgm5ts1 ! influ5ces ( ma!matical p]spective1 light1 %ade1 color1 & int5sity are m direct4 ,judgm5ts bas$ ^u focal adju/m5t 7monocul>7 are fairly accurate at di/ances f five in*es to sev]al y>ds4 ,^? f\nd$ ^u axial adju/m5t 7conv]g5ce ( ! two axes 9 b9ocul> vision7 are less 9 error ?an ! prec$+ ones4 ,!y are reliable to a di/ance ( ab\t #ajjj feet4 ,judgm5ts involv+ ma!matical p]spective are ( relatively great accuracy )\t limits4 ,^? arriv$ at by int]pret+ a]ial p]spective 7haz9ess ( atmosph]e1 r$uction 9 color due to atmosph]ic absorption1 etc47 are m]ely e/imates liable to l>ge errors1 ! accuracy dep5d+ l>gely ^u exp]i5ce ) local conditions4 ,! measur+ p[] ( ! eye is m liable to error :5 ! di/ances or ! objects comp>$ lie 9 diff]5t directions4 ,a special case is ! comp>ison ( a v]tical di/ance ) a horizontal "o4 ,x is n uncommon to e/imate a v]tical di/ance z mu* z #be p] c5t great] ?an an actually equal horizontal di/ance4 ,9 g5]al1 e/imates ( direction & di/ance are comp>atively inaccurate :5 only "o eye is us$ al?\gh a "o-ey$ p]son acquires unusual ability ?r\gh a ke5] exp]i5ce :ett$ by necessity4 ,a v]tical l9e drawn p]p5dicul> to a horizontal "o is likely to appe> b5t :5 view$ ) "o eye4 ,its app>5t incl9ation is v>iable b has be5 f\nd to v>y f "o to ?ree degrees4 ,monocul> vision is likely to cause /raight l9es to appe> crook$1 al?\gh ! crook$ness may seem to be m or less un/able4 ,! error 9 ! e/imate ( size is 9 reality an error 9 ! e/imation ( di/ance except 9 ^? cases :]e ! e/imate is bas$ directly ^u a comp>ison ) an object ( suppos$ly kn[n size4 ,an amus+ incid5t is told ( an old negro :o 0 hunt+ = squirrels4 ,he %ot sev]al times at :at he suppos$ to be a squirrel ^u a tree-trunk & 8 failure to make a kill 0 beg9n+ to weak5 8 r ample op9ion ( 8 skill z a m>ksman4 ,a complete %att]+ ( 8 fai? 9 8 skill 0 only escap$ by ! discov]y t ! squirrel 0 a l\se ^u 8 eyebr[4 ,simil>ly1 a gnat 9 ! air might appe> to be an airplane "u c]ta9 favorable circum/ances4 ,x is int]e/+ to note t ! e/imat$ size ( ! disk ( ! sun or moon v>ies f ! size ( a sauc] to t ( ! end ( a b>rel1 al?\gh a p9e tree at ! horizon-l9e may be e/imat$ z #be feet across despite ! fact t x may be entirely includ$ 9 ! disk ( ! sun sett+ beh9d x4 ,d\ble images play an important "p 9 ! comp>ison ( di/ances ( objects4 ,! d\bl+ ( objects is only equal to ! int]ocul> di/ance4 ,suppose two horizontal wires or clo!sl9es ab\t fifty feet away & "o a few feet beyond ! o!r4 ,on look+ at ^! no d\ble images are visible & x is difficult or ev5 impossible to see :i* is ! ne>] :5 ! po9ts ( atta*m5t ( ! ends are scre5$ f view4 ,h[ev]1 if ! head is turn$ to "o side & d[nw>d 7#ij degrees7 s t ! int]ocul> l9e is n[ at "r angles 7v]tical7 to ! horizontal l9es1 ! relative di/ances ( ! latt] are br\ght out di/9ctly4 ,d\ble images become visible 9 ! latt] case4 ,accord+ to ,bruecke's !ory ! eyes are cont9u\sly 9 motion & ! obs]v] by alt]nately increas+ or decreas+ ! conv]g5ce ( ! axes ( ! eyes1 comb9es successively ! diff]5t p>ts ( ! two sc5es z se5 by ! two eyes & by runn+ ! po9t ( sight back & =? by trial obta9s a di/9ct p]ception ( b9ocul> p]spective or relief or dep? ( space4 ,x may be assum$ t exp]i5ce has made ! obs]v] pr(ici5t 9 ?is appraisal :i* he arrives at almo/ unconsci\sly1 al?\gh x may be j z easy to accept ,:eat/one's explanation4 ,9 fact1 "s exp]i5ces ) ! /]eoscope appe> to support ! latt] !ory4 ,:eat/one discov]$ t ! dissimil> pictures ( an object or sc5e1 :5 unit$ by means ( optical sy/ems1 produce a visual effect simil> to t produc$ by ! actual solid object or sc5e provid$ ! dissimil>ity is ! same z t betwe5 two ret9al images ( ! solid object or sc5e4 ,?is is ! pr9ciple ^u :i* ! famili> /]eoscope is f\nd$4 ,:eat/one =mulat$ a !ory :i* may be briefly /at$ z foll[s3 ,9 view+ a solid object or a sc5e two slightly dissimil> ret9al images are =m$ 9 ! two eyes respectively1 b ! m9d completely fuses !m into "o m5tal image4 ,:5 ?is m5tal fusion ( ! two really dissimil> ret9al images is complete 9 ?is way1 x is obvi\s t !re _c exi/ a ma!matical co9cid5ce4 ,! result is a p]ception ( dep? ( space1 ( solidity1 ( relief4 ,9 fact ! ?ird dim5sion is p]ceiv$4 ,a /]eoscope accompli%es ?is 9 ess5tially ! same mann]1 = two pictures1 tak5 f two diff]5t positions respectively correspond+ to ! positions ( ! eyes1 are comb9$ by means ( optical sy/ems into "o image4 ,lack ( correct size & position ( ! individual elem5ts ( /]eoscopic pictures are easily detect$ on comb9+ !m4 ,t is1 _! dissimil>ity mu/ exactly correspond to t betwe5 two views ( an object or sc5e f ! positions ( ! two eyes respectively 7,fig4 #b74 ,?is fact has be5 made use ( 9 detect+ c\nt]feit notes4 ,if two notes made f ! same plate are view$ 9 a /]eoscope & ! id5tical figures are comb9$1 ! comb9ation is p]fect & ! plane ( ! comb9$ images is p]fectly flat4 ,if ! notes are n made f ! same plate b "o ( !m is c\nt]feit1 slight v>iations 9 ! latt] are unavoidable4 ,su* v>iations w %[ !mselves 9 a wavy surface4 ,! unwill+ness ( ! visual s5se to comb9e ! two ret9al images1 if !y are dissimil> to ! ext5t ( belong+ to two diff]5t objects1 is emphasiz$ by means ( colors4 ,= example1 if a gre5 glass is plac$ ov] "o eye & a r$ glass ov] ! o!r1 ! colors are n mix$ by ! visual s5se4 ,! addition ( ^! two colors results normally 9 yell[1 ) little or no sugge/ion ( ! compon5ts--r$ & gre54 ,b 9 ! =ego+ case ! visual field does n appe> ( a uni=m yell[4 ,x appe>s alt]nately r$ & gre51 z ?\gh ! colors 7 rival+ ea* o!r = complete ma/]y4 ,9 fact1 ?is ph5om5on has be5 t]m$ ret9al rivalry4 ,! l5ses ( ! /]eoscope supplem5t eye-l5ses & project on ! ret9a two p]fect images ( a ne> object1 al?\gh ! eyes are look+ at a di/ant object & are !re=e n accommodat$ = ! ne> "o 7! photographs74 ,! l5ses enl>ge ! images simil> to ! action ( a p]spective glass4 ,?is completes ! illusion ( an object or ( a sc5e4 ,!re is a rem>kable di/9ctness ( ! p]ception ( dep? ( space & !re=e a wond]ful resemblance to ! actual object or sc5e4 ,x is int]e/+ to note ! effect ( tak+ ! two orig9al photographs f di/ances sep>at$ by sev]al feet4 ,! effect is app>5tly to magnify dep?4 ,x is notewor?y t two pictures tak5 f an airplane at po9ts fifty feet or s ap>t1 :5 comb9$ 9 ! /]eoscope1 s magnify ! dep? t c]ta9 enemy-works c be m advantage\sly detect$ ?an f ord9>y photographs4 ,/]eoscopic images su* z repres5t$ 9 ,fig4 #b may be comb9$ )\t ! aid ( ! /]eoscope if ! optical axes ( ! eye c be suffici5tly conv]g$ or div]g$4 ,su* images or pictures are usually ^u a c>d & are int5d$ to be comb9$ beyond ! plane ( ! c>d1 = x is 9 ?is position t ! object or sc5e c be p]ceiv$ 9 natural p]spective1 ( natural size1 ( natural =m1 & at natural di/ance4 ,b 9 comb9+ !m ! eyes are look+ at a di/ant object & ! axes are p>allel or ne>ly s4 ,!re=e1 ! eyes are focally adju/$ = a di/ant object b ! light comes f a v ne> object--! pictures on ! c>d4 ,myopic eyes d n exp]i5ce ?is difficulty & x appe>s t normal vision may be tra9$ to ov]come x4 ,normal eyes are aid$ by us+ slightly convex l5ses4 ,su* glasses supplem5t ! l5ses ( ! eye1 mak+ possible a cle> vision ( a ne> object :ile ! eyes are really look+ f> away or1 9 o!r words1 mak+ possible a cle> image ( a ne> object ^u ! ret9a ( ! unadju/$ eye4 ,/]eoscopic pictures are usually s m\nt$ t id5tical po9ts on ! two pictures are f>!r ap>t ?an ! int]ocul> di/ance & !re=e ! two images _c be comb9$ :5 ! optical axes ( ! eyes are p>allel or ne>ly s1 :i* is ! condition :5 look+ at a di/ant object4 ,9 su* a case ! two pictures mu/ be br\ght clos] toge!r4 ,illu/ration3 ,fig4 #b4--,/]eoscopic pictures = comb9+ by conv]g+ or div]g+ ! optical axes4 ,illu/ration3 ,fig4 #c4--,/]eoscopic pictures4 ,9 ,figs4 #b & #c are f\nd dissimil> draw+s ( ! correct dissimil>ity ( /]eoscopic pictures4 ,x is int]e/+ & in/ructive to practice comb9+ ^! ) ! unaid$ eyes4 ,if ,fig4 #b is held at an arm's l5g? & ! eyes are focus$ ^u a po9t sev]al in*es di/ant1 ! axes w be suffici5tly conv]g$ s t ! two images are sup]pos$4 ,x may help to focus ! eyes ^u ! tip ( a f+] until ! /]eoscopic images are comb9$4 ,9 ?is case ( conv]g+ axes ! f9al comb9$ result w be ! appe>ance ( a holl[ tube or ( a %ell ( a truncat$ cone1 app>5tly possess+ ! ?ird dim5sion & be+ p]ceiv$ z app>5tly small] ?an ! actual pictures 9 ! backgr\nd at arm's l5g?4 ,if ! two /]eoscopic pictures are comb9$ by look+ at a po9t f> beyond ! actual position ( ,fig4 #b1 ! comb9$ effect is a solid truncat$ cone b p]ceiv$ z ( ab\t ! same size & at ab\t ! same di/ance f ! eye z ! actual diagrams4 ,9 ! latt] case ! small] end ( ! app>5t solid appe>s to be ne>] ?an ! l>g] end1 b 9 ! =m] case ! rev]se is true1 t is1 ! small] end appe>s to be at a great] di/ance4 ,! same exp]im5ts may be p]=m$ = ,fig4 #c ) simil> results except+ t ?is appe>s to be a %ell "u ! same circum/ances t ,fig4 #b appe>s to be a solid & vice v]sa4 ,a few pati5t trials %\ld be rew>d$ by success1 & if s ! read] c ga9 mu* m und]/&+ f ! actual exp]i5ces ?an f description4 ,! =ego+ discussion ( vision %\ld indicate ! complexity ( ! visual & m5tal activities involv$ 9 ! discrim9ation1 association1 & int]pretation ( ! data obta9$ ?r\gh ! eye4 ,! psy*ology ( visual p]ception is /ill a mu* controv]t$ doma9 b x is believ$ t ! glimpses ( ! process ( vision :i* h be5 af=d$ are suffici5t to enable ! read] to und]/& _m illusions & at lea/ to appreciate m fully ^? ^: explanations rema9 9 d\bt4 ,c]ta9ly ^! glimpses & a k ( ! in=mation :i* visual p]ception actually supplies to u at any mom5t %\ld conv9ce u t ! visual s5se has acquir$ an incomp>able facility = int]pret+ ! objective _w = u4 ,cle>ness ( vision is conf9$ to a small area ab\t ! po9t ( sight1 & x rapidly dim9i%es away f ?is po9t1 images becom+ dim & d\ble4 ,we sweep ?is po9t ( sight backw>d & =w>d & ov] an ext5sive field ( view1 ga!r+ all ! di/9ct impressions into "o m5tal image4 ,9 do+ ?is ! unconsci\s int]pretation ( ! muscul> activity att5d+ accommodation & conv]g5ce ( ! eyes aids 9 giv+ to ?is m5tal picture ! appe>ance ( dep? by e/abli%+ relative di/ances ( v>i\s objects4 ,c]ta9ly ! acquir$ facility is rem>kable4 ,,iv ,,"s ,,types ,,( ,,geometrical ,,illusions ,no simple classification ( illusions is ample or satisfactory1 = !re are _m factors int]wov54 ,= ?is reason no claims are made = ! v>i\s divisions ( ! subject repres5t$ by & 9 ^! *apt]s except+ t ( conv5i5ce4 ,obvi\sly1 "s divisions are necess>y 9 ord] t ! v>iegat$ subject may be pres5table4 ,! classification us$ appe>s to be logical b v evid5tly x _c be p]fectly s :5 ! logic is n :olly available1 ow+ to ! disagreem5t f\nd among ! explanations (f]$ by psy*ologi/s4 ,x may be argu$ t ! geometrical type ( illusion %\ld include _m illusions :i* are discuss$ 9 o!r *apt]s4 ,inde$1 ?is is p]haps true4 ,h[ev]1 x appe>s to suit ! pres5t purpose to introduce ?is phase ( ?is book by a gr\p ( illusions :i* involve plane geometrical figures4 ,if "s ( ! latt] appe> 9 o!r *apt]s1 x is because !y seem to bord] ^u or to include o!r factors beyond ^? app>5tly involv$ 9 ! simple geometrical type4 ,! pres5tation :i* foll[s beg9s 7= ! sake ( cle>ness7 ) a few repres5tative geometrical illusions ( v>i\s types4 ,! ,effect ( ! ,location 9 ! ,visual ,field4--,"o ( ! mo/ common illusions is f\nd 9 ! lett] ,s or figure #h4 ,ord9>ily we are n /rongly consci\s ( a diff]5ce 9 ! size ( ! upp] & l[] p>ts ( ^! *>act]s2 h[ev]1 if we inv]t !m 7#hhhh ,,ssss7 ! diff]5ce is se5 to be l>ge4 ,! "q arises1 ,is ! diff]5ce due fundam5tally to ! locations ( ! two p>ts 9 ! visual field8 ,x sc>cely seems cr$ible t visual p]ception innately appraises ! upp] "p l>g] ?an ! l[]1 or ! l[] small] ?an ! upp] "p :5 ^! small *>act]s are se5 9 _! accu/om$ position4 ,x appe>s to be possible t "h we h examples ( ! effect ( le>n+ or exp]i5ce & t our adaptive visual s5se has become accu/om$ to ov]look ! actual diff]5ce4 ,t is1 = "s reason ?r\gh be+ confront$ ) ?is diff]5ce s _m times1 ! intellect has become adapt$ to x &1 !re=e1 has gr[n to ignore x4 ,reg>dless ( ! explanation1 ! illusion exi/s & ?is is ! po9t ( *ief int]e/4 ,= ! same reason ! curvature ( ! ret9a does n appe> to acc\nt = illusion ?r\gh di/ortion ( ! image1 because ! tra9+ due to exp]i5ce has caus$ great] difficulties ?an ?is to disappe>4 ,we mu/ n ov]look ! trem5d\s corrective influ5ce ( exp]i5ce ^u :i* visual p]ception = ! adult is f\nd$4 ,if we h le>n$ to correct 9 "s cases1 :y n 9 all cases :i* we h enc\nt]$ q g5]ally8 ,illu/ration3 ,fig4 #d4--,! v]tical l9e appe>s long] ?an ! equal horizontal l9e 9 ea* case4 ,?is type ( illusion p]si/s 9 geometrical figures & may be f\nd on e h&4 ,a p]fect squ>e :5 view$ v]tically appe>s too high1 al?\gh ! illusion does n appe> to exi/ 9 ! circle4 ,9 ,fig4 #d ! v]tical l9e appe>s long] ?an ! horizontal l9e ( ! same l5g?4 ,?is may be readily demon/rat$ by ! read] by means ( a v>iety ( figures4 ,a /rik+ case is f\nd 9 ,fig4 #e1 :]e ! height & ! wid? ( ! diagram ( a silk hat are equal4 ,despite ! actual equality ! height appe>s to be mu* great] ?an ! wid?4 ,a pole or a tree is g5]ally apprais$ z ( great] l5g? :5 x is /&+ ?an :5 x lies on ! gr\nd4 ,?is illusion may be demon/rat$ by plac+ a black dot an in* or s above ano!r on a :ite pap]4 ,n[1 at "r angles to ! orig9al dot place ano!r at a horizontal di/ance :i* appe>s equal to ! v]tical di/ance ( ! fir/ dot above ! orig9al4 ,on turn+ ! pap] ?r\gh n9ety degrees or by actual measurem5t1 ! ext5t ( ! illusion w become app>5t4 ,by do+ ?is sev]al times1 us+ v>i\s di/ances1 ?is type ( illusion becomes conv9c+4 ,illu/ration3 ,fig4 #e4--,! v]tical dim5sion is equal to ! horizontal "o1 b ! =m] appe>s great]4 ,! explanation accept$ by "s is t m ef=t is requir$ to raise ! eyes1 or po9t ( sight1 ?r\gh a c]ta9 v]tical di/ance ?an ?r\gh an equal horizontal di/ance4 ,p]haps we unconsci\sly appraise ef=t ( ?is sort 9 t]ms ( di/ance1 b is x n logical to inquire :y we h n ?r\gh exp]i5ce le>n$ to s5se ! diff]5ce betwe5 ! relation ( ef=t to horizontal di/ance & t ( ef=t to v]tical di/ance ?r\gh :i* ! po9t ( sight is mov$8 ,we are do+ ?is cont9u\sly1 s :y d we n le>n to di/+ui%2 fur!rmore1 we h ov]come o!r great ob/acles 9 develop+ our visual s5se4 ,9 ?is complex field ( physiological psy*ology que/ions are n only annoy+1 b (t5 disruptive4 ,z has be5 po9t$ out 9 ,*apt] ,,ii1 images ( objects ly+ ne> ! p]iph]y ( ! visual field are m or less di/ort$1 ow+ to ! /ructure & to c]ta9 defects ( p>ts ( ! eye4 ,= example1 a *eck]bo>d view$ at a prop] di/ance ) respect to its size appe>s q di/ort$ 9 its out] regions4 ,*eap cam]as are likely to cause simil> errors 9 ! images fix$ ^u ! photographic plate4 ,photographs are int]e/+ 9 connection ) visual illusions1 because ( c]ta9 di/ortions & ( ! magnification ( su* aspects z p]spective4 ,incid5tally 9 look+ = illusions1 difficulty is sometimes exp]i5c$ 9 see+ !m :5 ! actual physical tru?s are kn[n2 t is1 9 di/+ui%+ betwe5 :at is actually se5 & :at actually exi/s4 ,! ability to make ?is sep>ation gr[s ) practice b :]e ! difficulty is ob/9ate1 x is well = ! read] to try obs]v]s :o d n suspect ! tru?4 ,illusions ( ,int]rupt$ ,ext5t4--,di/ance & area appe> to v>y 9 ext5t1 dep5d+ ^u :e!r !y are fill$ or empty or are only p>tially fill$4 ,= example1 a s]ies ( dots w g5]ally appe> long] ov]all ?an an equal di/ance betwe5 two po9ts4 ,?is may be easily demon/rat$ by arrang+ ?ree dots 9 a /raight l9e on pap]1 ! two int]v5+ spaces be+ ( equal ext5t1 say ab\t "o or two in*es long4 ,if 9 "o ( ! spaces a s]ies ( a doz5 dots is plac$1 ?is space w appe> long] ?an ! empty space4 ,h[ev]1 if only "o dot is plac$ 9 ! middle ( "o ( ! empty spaces1 ?is space n[ is likely to appe> ( less ext5t ?an ! empty space4 7,see ,fig4 #g47 ,a specific example ( ?is type ( illusion is %[n 9 ,fig4 #f4 ,! fill$ or divid$ space g5]ally appe>s great] ?an ! empty or undivid$ space1 b c]ta9 qualifications ( ?is /atem5t are necess>y4 ,9 a ! divid$ space unque/ionably appe>s great] ?an ! empty space4 ,app>5tly ! fill$ or empty space is m important ?an ! am\nt ( light :i* is receiv$ f ! cle> spaces1 = a black l9e on :ite pap] appe>s long] ?an a :ite space betwe5 two po9ts sep>at$ a di/ance equal to ! l5g? ( ! black l9e4 ,fur!rmore1 app>5tly ! spac+ :i* is ! mo/ obtrusive is mo/ influ5tial 9 caus+ ! divid$ space to appe> great] = a ?an = b4 ,! illusion /ill p]si/s 9 c4 ,illu/ration3 ,fig4 #f4--,! divid$ or fill$ space on ! left appe>s long] ?an ! equal space on ! "r4 ,an idea ( ! magnitude may be ga9$ f c]ta9 exp]im5ts by ,aub]t4 ,he us$ a figure simil> to a ,fig4 #f conta9+ a total ( five %ort l9es4 ,f\r ( !m 7 equally spac$ ov] a di/ance ( #ajj mm4 correspond+ to ! left half ( a1 ,fig4 #f4 ,! rema9+ l9e 0 plac$ at ! extreme "r & def9$ ! limit ( an empty space also #ajj mm4 long4 ,9 all cases1 ! l5g? ( ! empty space appe>$ ab\t t5 p] c5t less ?an t ( ! space occupi$ by ! f\r l9es equally spac$4 ,v>i\s exp]im5t]s obta9 diff]5t results1 & x seems reasonable t ! diff]5ces may be acc\nt$ =1 p>tially at lea/1 by diff]5t degrees ( unconsci\s correction ( ! illusion4 ,?is emphasizes ! desirability ( us+ subjects = su* exp]im5ts :o h no k p]ta9+ to ! illusion4 ,illu/ration3 ,fig4 #g4--,! ?ree l9es are ( equal l5g?4 ,illu/ration3 ,fig4 #h4--,! di/ance betwe5 ! two circles on ! left is equal to ! di/ance betwe5 ! outside edges ( ! two circles on ! "r4 ,z already /at$ !re are app>5t exceptions to any simple rule1 =1 z 9 ! case ( dots cit$ 9 a prec$+ p>agraph1 ! illusion dep5ds ^u ! mann] 9 :i* ! division is made4 ,= example1 9 ,fig4 #g1 a & c are z likely to appe> %ort] ?an b z equal to x4 ,x has be5 conclud$ by c]ta9 inve/igators t :5 subdivision ( a l9e causes x to appe> long]1 ! p>ts into :i* x is divid$ or "s ( !m are likely to appe> %ort] ?an isolat$ l9es ( ! same l5g?4 ,! rev]se ( ?is /atem5t also appe>s to hold4 ,= example 9 ,fig4 #g1 a appe>s %ort] ?an b & ! c5tral "p appe>s l5g!n$1 al?\gh ! total l9e appe>s %ort5$4 ,?is illusion is int5sifi$ by leav+ ! c5tral section blank4 ,a figure ( ?is sort c be readily drawn by ! read] by us+ %ort /raight l9es 9 place ( ! circles 9 ,fig4 #h4 ,9 ?is figure ! space betwe5 ! inside edges ( ! two circles on ! left appe>s l>g] ?an ! ov]all di/ance betwe5 ! outside edges ( ! two circles on ! "r1 despite ! fact t ^! di/ances are equal4 ,x appe>s t m]e int5sity ( ret9al /imulation does n acc\nt = ^! illusions1 b r ! figures :i* we see4 ,illu/ration3 ,fig4 #i4--,?ree squ>es ( equal dim5sions :i* appe> diff]5t 9 area & dim5sion4 ,9 ,fig4 #i ! ?ree squ>es are equal 9 dim5sions b ! diff]5t *>act]s ( ! divisions cause !m to appe> n only unequal1 b no long] squ>es4 ,9 ,fig4 #aj ! di/ance betwe5 ! outside edges ( ! ?ree circles arrang$ horizontally appe>s great] ?an ! empty space betwe5 ! upp] circle & ! left-h& circle ( ! gr\p4 ,illu/ration3 ,fig4 #aj4--,! v]tical di/ance betwe5 ! upp] circle & ! left-h& "o ( ! gr\p is equal to ! ov]all l5g? ( ! gr\p ( ?ree circles4 ,illusions ( ,cont\r4--,! illusions ( ?is type1 or exhibit+ ?is influ5ce1 are q num]\s4 ,9 ,fig4 #aa !re are two semicircles1 "o clos$ by a diamet]1 ! o!r unclos$4 ,! latt] appe>s some:at flatt] & ( slightly great] radius ?an ! clos$ "o4 ,simil>ly 9 ,fig4 #ab ! %ort] portion ( ! int]rupt$ circumf]5ce ( a circle appe>s flatt] & ( great] radius ( curvature ?an ! great] portions4 ,9 ,fig4 #ac ! l5g? ( ! middle space & ( ! op5-sid$ squ>es are equal4 ,9 fact !re are two uncomplet$ squ>es & an empty squ>e betwe51 ! ?ree ( :i* are ( equal dim5sions4 ,h[ev] ! middle space appe>s slightly too high & n>r[2 ! o!r two appe> slightly too l[ & broad4 ,^! figures are relat$ to ! well-kn[n ,muell]-,ly] illusion illu/rat$ 9 ,fig4 #ef4 ,"s ( ! illusions pres5t$ lat] w be se5 to involve ! influ5ce ( cont\r4 ,examples ( ^! are ,figs4 #ee & #fj4 ,9 ! =m]1 ! horizontal base l9e appe>s to sag2 9 ! latt]1 ! areas appe> unequal1 b !y are equal4 ,illu/ration3 ,fig4 #aa4--,two equal semicircles4 ,illu/ration3 ,fig4 #ab4--;,arcs ( ! same circle4 ,illu/ration3 ,fig4 #ac4--,?ree incomplete b equal squ>es4 ,illusions ( ,contra/4--,^? illusions due to brightness contra/ are n includ$ 9 ?is gr\p1 = contra/ "h ref]s to l9es1 angles & areas ( diff]5t sizes4 ,9 g5]al1 p>ts adjac5t to l>ge ext5ts appe> small] & ^? adjac5t to small ext5ts appe> l>g]4 ,a simple case is %[n 9 ,fig4 #ad1 :]e ! middle sections ( ! two l9es are equal1 b t ( ! %ort] l9e appe>s long] ?an t ( ! long] l9e4 ,9 ,fig4 #ae ! two p>ts ( ! connect+ l9e are equal1 b !y d n appe> s4 ,?is illusion is n z positive z ! prec$+ "o &1 9 fact1 ! position ( ! %ort v]tical divid+ l9e may appe> to fluctuate consid]ably4 ,illu/ration3 ,fig4 #ad4--,middle sections ( ! two l9es are equal4 ,illu/ration3 ,fig4 #ae4--;,an effect ( contra/+ areas 7,baldw9's figure74 ,fig4 #af might be consid]$ to be an illusion ( cont\r1 b ! l5g? ( ! top horizontal l9e ( ! l[] figure be+ app>5tly less ?an t ( ! top l9e ( ! upp] figure is due l>gely to contra/+ ! two figures4 ,incid5tally1 x is difficult to believe t ! maximum horizontal wid? ( ! l[] figure is z great z ! maximum height ( ! figure4 ,at ?is po9t x is ( int]e/ to ref] to o!r contra/ illusions su* z ,figs4 #bj1 #eg1 & #ei4 ,illu/ration3 ,fig4 #af4--;,an illusion ( contra/4 ,a /rik+ illusion ( contra/ is %[n 9 ,fig4 #ag1 :]e ! c5tral circles ( ! two figures are equal1 al?\gh ! "o surr\nd$ by ! l>ge circles appe>s mu* small] ?an ! o!r4 ,simil>ly1 9 ,fig4 #ah ! inn] circles ( b & c are equal b t ( b appe>s ! l>g]4 ,! inn] circle ( a appe>s l>g] ?an ! out] circle ( b1 despite _! actual equality4 ,illu/ration3 ,fig4 #ag4--;,equal circles :i* appe> unequal due to contra/ 7,ebb+haus' figure74 ,illu/ration3 ,fig4 #ah4--;,equal circles appe>+ unequal ow+ to contra/+ conc5tric circles4 ,9 ,fig4 #ai ! circle ne>] ! apex ( ! angle appe>s l>g] ?an ! o!r4 ,?is has be5 pres5t$ z "o reason :y ! sun & moon appe> l>g] at ! horizon ?an :5 at high] altitudes4 ,?is explanation mu/ be bas$ ^u ! assumption t we int]pret ! vault ( ! sky to meet at ! horizon 9 a mann] some:at simil> to ! angle b x is difficult to imag9e su* an angle made by ! vault ( ! sky & ! e>?'s horizon4 ,if !re 7 "o 9 reality1 x w\ld n be se5 9 pr(ile4 ,illu/ration3 ,fig4 #ai4--;,circles influ5c$ by position )9 an angle4 ,illu/ration3 ,fig4 #bj4--;,contra/+ angles4 ,if two angles ( equal size are b\nd$ by small & l>ge angles respectively1 ! apex 9 ea* case be+ common to ! inn] & two b\nd+ angles1 ! effect ( contra/ is v app>5t1 z se5 9 ,fig4 #bj4 ,9 ,fig4 #eg are f\nd examples ( effects ( l9es contra/$ z to l5g?4 ,illu/ration3 ,fig4 #ba4--,ow+ to p]spective ! "r angles appe> oblique & vice v]sa4 ,! read] may readily con/ruct an ext5sive v>iety ( illusions ( contra/2 9 fact1 contra/ plays a "p 9 mo/ geometrical-optical illusions4 ,! contra/s may be betwe5 exi/+ l9es1 areas1 etc41 or ! imag9ation may supply "s ( !m4 ,illu/ration3 ,fig4 #bb4--,two equal diagonals :i* appe> unequal4 ,illusions ( ,p]spective4--,z ! complexity ( figures is increas$ ! numb] ( possible illusions is multipli$4 ,9 p]spective we h ! influ5ces ( v>i\s factors su* z l9es1 angles1 & sometimes cont\r & contra/4 ,9 ,fig4 #ba ! sugge/ion due to ! p]spective ( ! cube causes "r angles to appe> oblique & oblique angles to appe> to be "r angles4 ,?is figure is p>ticul>ly illusive4 ,x is int]e/+ to note t ev5 an aft]-image ( a "r-angle cross :5 project$ ^u a wall drawn 9 p]spective 9 a pa9t+ w appe> oblique4 ,illu/ration3 ,fig4 #bc4--;,app>5t v>iations 9 ! di/ance betwe5 two p>allel l9es4 ,a /rik+ illusion involv+ p]spective1 or at lea/ ! influ5ce ( angles1 is %[n 9 ,fig4 #bb4 ,"h ! diagonals ( ! two p>allelograms are ( equal l5g? b ! "o on ! "r appe>s mu* small]4 ,t ,,ax is equal 9 l5g? to ,,ay is readily demon/rat$ by describ+ a circle f ! c5t] ,a & ) a radius equal to ,,ax4 ,x w be f\nd to pass ?r\gh ! po9t ,y4 ,obvi\sly1 geometry ab\nds 9 geometrical-optical illusions4 ,illu/ration3 ,fig4 #bd4--;,a /rik+ illusion ( p]spective4 ,! effect ( contra/ is se5 9 a 9 ,fig4 #bc2 t is1 ! %ort p>allel l9es appe> fur!r ap>t ?an ! pair ( long ones4 ,by add+ ! oblique l9es at ! ends ( ! l[] pair 9 b1 ^! p>allel l9es n[ appe> fur!r ap>t ?an ! horizontal p>allel l9es ( ! small rectangle4 ,! influ5ce ( p]spective is p>ticul>ly app>5t 9 ,fig4 #bd1 :]e natural p]spective l9es are drawn to sugge/ a sc5e4 ,! squ>e columns are ( ! same size b ! fur!r "o1 = example1 be+ app>5tly ! mo/ di/ant & ( ! same physical dim5sions1 actually appe>s mu* l>g]4 ,"h is a case :]e exp]i5ce1 all[+ = a dim9ution ( size ) increas+ di/ance1 actually causes ! column on ! "r to appe> l>g] ?an x really is4 ,! arti/ w f9d ?is illusion ev5 m /rik+ if he draws ?ree human figures ( ! same size b simil>ly dispos$ 9 respect to p]spective l9es4 ,app>5tly conv]g+ l9es influ5ce ^! equal figures 9 proportion z !y sugge/ p]spective4 ,illu/ration3 ,fig4 #be4--;,di/ortion ( a squ>e due to sup]pos$ l9es4 ,al?\gh !y are n necess>ily illusions ( p]spective1 ,figs4 #be & #bf are pres5t$ "h because !y involve simil> influ5ces4 ,9 ,fig4 #be ! holl[ squ>e is sup]pos$ ^u gr\ps ( oblique l9es s arrang$ z to app>5tly di/ort ! squ>e4 ,9 ,fig4 #bf di/ortions ( ! circumf]5ce ( a circle are obta9$ 9 a simil> mann]4 ,illu/ration3 ,fig4 #bf4--;,di/ortion ( a circle due to sup]pos$ l9es4 ,x is int]e/+ to note t we are n p>ticul>ly consci\s ( p]spective1 b x is se5 t x has be5 a factor 9 ! developm5t ( our visual p]ception4 ,9 pro( ( ?is we might recall ! fir/ "t z *ildr5 we 7 ask$ to draw a railroad track trail+ (f 9 ! di/ance4 ,d\btless1 mo/ ( u drew two p>allel l9es in/ead ( conv]g+ ones4 ,a p]son approa*+ u is n s5sibly p]ceiv$ to gr[4 ,he is m likely to be p]ceiv$ all ! "t z ( normal size4 ,! f+] held at "s di/ance may m ?an cov] ! object su* z a di/ant p]son1 b ! f+] is n ord9>ily p]ceiv$ z l>g] ?an ! p]son4 ,( c\rse1 :5 we ?9k ( x we are consci\s ( p]spective & ( ! increase 9 size ( an approa*+ object4 ,:5 a locomotive or automobile approa*es v rapidly1 ?is gr[? is likely to be s /rik+ z to be g5]ally noticeable4 ,! read] may f9d x ( int]e/ at ?is po9t to turn to illu/rations 9 o!r *apt]s4 ,! =ego+ are a few geometrical illusions ( repres5tative types4 ,^! are n all ! types ( illusions by any means & !y are only a few ( an almo/ numb]less ho/4 ,^! h be5 pres5t$ 9 a brief classification 9 ord] t ! read] might n be ov]:elm$ by ! app>5t *aos4 ,v>i\s special & miscellane\s geometrical illusions are pres5t$ 9 lat] *apt]s4 ,v ,,equivocal ,,figures ,_m figures app>5tly *ange 9 appe>ance ow+ to fluctuations 9 att5tion & 9 associations4 ,a human pr(ile 9 intaglio 7,figs4 #gb & #gc7 may appe> z a bas-relief4 ,crease a c>d 9 ! middle to =m an angle & hold x at an arm's l5g?4 ,:5 view$ ) "o eye x c be made to appe> op5 9 "o way or ! o!r2 t is1 ! angle may be made to appe> po9t+ t[>d ! obs]v] or away f him4 ,! m di/ant "p ( an object may be made to appe> ne>] ?an ! rema9+ "p4 ,plane diagrams may seem to be solids4 ,deception ( ?is "* is q easy if ! light-s\rce & o!r extrane\s factors are conceal$ f ! obs]v]4 ,x is v int]e/+ to /udy ^! fluctuat+ figures & to note ! v>i\s extrane\s data :i* lead u to judge correctly4 ,fur!rmore1 x becomes obvi\s t (t5 we see :at we expect to see4 ,= example1 we m commonly enc\nt] relief ?an intaglio2 !re=e1 we are likely to ?9k t we are look+ at ! =m]4 ,prop] consid]ation ( ! position ( ! dom9ant light-s\rce & ( ! %ad[s w usually provide ! data = a correct conclusion4 ,h[ev]1 habit & probability are factors ^: influ5ce is difficult to ov]come4 ,our p]ception is /rongly associat$ ) accu/om$ ways ( see+ objects & :5 ! object is once sugge/$ x grasps our m9d completely 9 its /]eotyp$ =m4 ,/airs1 glasses1 r+s1 cubes1 & intaglios are among ! objects commonly us$ to illu/rate ?is type ( illusion4 ,9 connection ) ?is type1 x is well to realize h[ t5aci\sly we cl+ to our p]ception ( ! real %apes ( objects4 ,= example1 a cube ?r[n into ! air 9 su* a mann] t x pres5ts _m aspects t[>d u is ?r\gh\t its c\rse a cube4 ,illu/ration3 ,fig4 #bg4--;,illu/rat+ fluctuation ( att5tion4 ,! figures :i* exhibit ^! illusions are obvi\sly ^? :i* are capable ( two or m spatial relations4 ,! d\ble int]pretation is m readily accompli%$ by monocul> ?an by b9ocul> vision4 ,fig4 #bg consi/s ( id5tical patt]ns 9 black & :ite4 ,by gaz+ ^u ?is /eadily x w appe> to fluctuate 9 appe>ance f a :ite patt]n ^u a black backgr\nd to a black patt]n ^u a :ite backgr\nd4 ,sometimes fluctuation ( att5tion app>5tly acc\nts = ! *ange &1 9 fact1 ?is c be te/$ by willfully alt]+ ! att5tion f a :ite patt]n to a black "o4 ,incid5tally "o inve/igator f\nd t ! maximum rate ( fluctuation 0 approximately equal to ! pulse rate1 al?\gh no connection betwe5 ! two 0 claim$4 ,x has also be5 f\nd t inv]sion is accompani$ by a *ange 9 refraction ( ! eye4 ,illu/ration3 ,fig4 #bh4--,! gr\p+ ( ! circles fluctuates4 ,ano!r example is %[n 9 ,fig4 #bh4 ,?is may appe> to be :ite circles ^u a black backgr\nd or a black me% ^u a :ite backgr\nd4 ,h[ev]1 ! m /rik+ ph5om5on is ! *ange 9 ! gr\p+ ( ! circles z att5tion fluctuates4 ,we may be consci\s ( holl[ diamonds ( circles1 "o inside ! o!r1 & !n sudd5ly ! patt]n may *ange to gr\ps ( diamonds consi/+ ( f\r circles ea*4 ,p]haps we may be mom5t>ily consci\s ( individual circles2 !n ! patt]n may *ange to a hexagonal "o1 ea* hexagon consi/+ ( sev5 circles--six surr\nd+ a c5tral "o4 ,! patt]n also *anges into p>allel /r+s ( circles1 triangles1 etc4 ,illu/ration3 ,fig4 #bi4--;,cross$ l9es :i* may be int]pret$ 9 two ways4 ,! cross$ l9es 9 ,fig4 #bi c be se5 z "r angles 9 p]spective ) two diff]5t spatial arrangem5ts ( "o or bo? l9es4 ,9 fact !re is q a t5d5cy to see su* cross$ l9es z "r angles 9 p]spective4 ,! two gr\ps on ! "r repres5t a simplifi$ ,zoelln]'s illusion 7,fig4 #cg74 ,! read] may f9d x int]e/+ to sp5d "s "t view+ ^! figures & 9 ex]cis+ 8 ability to fluctuate 8 att5tion4 ,9 fact1 he mu/ call ^u 8 imag9ation 9 ^! cases4 ,sometimes ! *anges are rapid & easy to br+ ab\t4 ,at o!r mom5ts he w enc\nt] an aggravat+ /ubbornness4 ,occasionally !re may appe> a conflict ( two appe>ances simultane\sly 9 ! same figure4 ,! latt] may be obs]v$ occasionally 9 ,fig4 #cj4 ,eye-movem5ts are br\ght =w>d by "s to aid 9 expla9+ ! *anges4 ,illu/ration3 ,fig4 #cj4--,rev]sible cubes4 ,9 ,fig4 #cj a rev]sal ( ! aspect ( ! individual cubes or ( _! p]spective is v app>5t4 ,at r>e mom5ts ! effect ( p]spective may be completely vanqui%$ & ! figure be made to appe> z a plane cross$ by /r+s ( :ite diamonds & zigzag black /rips4 ,! illusion ( ! b5t c>d or p>tially op5 book is se5 9 ,fig4 #ca4 ,! tetrah$ron 9 ,fig4 #cb may appe> ei!r z erect on its base or z lean+ backw>d ) its base se5 f und]nea?4 ,illu/ration3 ,fig4 #ca4--,! rev]sible op5 book 7aft] ,ma*74 ,illu/ration3 ,fig4 #cb4--;,a rev]sible tetrah$ron4 ,! s]ies ( r+s 9 ,fig4 #cc may be imag9$ to =m a tube su* z a %eet-metal pipe ) its axis ly+ 9 ei!r ( two directions4 ,sometimes by clos+ "o eye ! two *anges 9 ?is type ( illusion are m readily br\ght ab\t4 ,x is also int]e/+ to close & op5 ea* eye alt]nately1 at ! same "t try+ to note j :]e ! att5tion is fix$4 ,! famili> /aircase is repres5t$ 9 ,fig4 #cd4 ,x is likely to appe> 9 its usual position & !n sudd5ly to inv]t4 ,x may aid 9 br++ ab\t ! rev]sal to insi/ t "o end ( a /ep is fir/ ne>] ?an ! o!r & !n f>!r away4 ,by focus+ ! att5tion 9 ?is mann] ! fluctuation becomes an easy matt] to obta94 ,illu/ration3 ,fig4 #cc4--,rev]sible p]spective ( a gr\p ( r+s or ( a tube4 ,illu/ration3 ,fig4 #cd4--,s*ro$]'s rev]sible /aircase4 ,9 ,fig4 #ce is a simil> example4 ,fir/ "o "p w appe> solid & ! o!r an empty corn]1 !n sudd5ly bo? are rev]s$4 ,h[ev]1 x is /rik+ to note "o half *anges :ile ! o!r rema9s un*ang$1 ?us produc+ mom5t>ily a r peculi> figure consi/+ ( two solids1 = example1 atta*$ by necess>ily w>p$ surfaces4 ,illu/ration3 ,fig4 #ce4--,?i]y's figure4 ,p]haps ! read] has (t5 witness$ ! /rik+ illusion ( "s portraits :i* 7 made ( subjects look+ directly at ! cam]a or pa9t]4 ,reg>dless ( ! position ( ! obs]v] ! eyes ( ! portrait appe> to be direct$ t[>d him4 ,9 fact1 z ! obs]v] moves1 ! eyes 9 ! picture foll[ him s rel5tlessly z to provoke ev5 a feel+ ( uncann9ess4 ,?is fact is acc\nt$ = by ! abs5ce ( a ?ird dim5sion1 = a sculptur$ model ( a head does n exhibit ?is feature4 ,p]spective plays a "p 9 "s mann]1 b no attempt t[>d explanation w be made4 ,9 ,fig4 #cf are two sket*es ( a face4 ,"o appe>s to be look+ at ! obs]v]1 b ! o!r does n4 ,if ! read] w cov] ! l[] p>ts ( ! two figures1 leav+ only ! two pairs ( eyes %[+1 bo? pairs w ev5tually appe> to be look+ at ! obs]v]4 ,p]haps ! read] w be consci\s ( m5tal ef=t & ! lapse ( a few mom5ts be=e ! eyes on ! left are made to appe> to be look+ directly at him4 ,al?\gh x is n claim$ t ?is illusion is caus$ by ! same conditions z ^? imm$iately prec$+1 x involves att5tion4 ,at lea/1 x is fluctuat+ 9 appe>ance & !re=e is equivocal4 ,x is int]e/+ to note ! influ5ce ( ! o!r features 7bel[ ! eyes74 ,! p]spective ( ^! is a p[]ful influ5ce 9 direct+ ! eyes ( ! sket*4 ,9 ! =ego+ only def9ite illusions h be5 pres5t$ :i* are univ]sally witness$ by normal p]sons4 ,!re are no halluc9atory phases 9 ! conditions or causes4 ,x is difficult to divide ^! ) def9it5ess f c]ta9 illusions ( dep? z discuss$ 9 ,*apt] ,,vii4 ,! latt] und\bt$ly are sometimes entw9$ to "s ext5t ) halluc9atory phases2 9 fact1 x is d\btful if !y are n always halluc9ations to "s degree4 ,halluc9ations are n ( int]e/ f ! viewpo9t ( ?is book1 b illusions ( dep? are treat$ because !y are ( int]e/4 ,!y are ei!r halluc9ations or are on ! bord]-l9e betwe5 halluc9ations & ^? illusions :i* are almo/ univ]sally exp]i5c$ by normal p]sons "u simil> conditions4 ,! latt] /atem5t does n hold = illusions ( dep? 9 :i* objects may be se5 alt]nately ne> & f>1 l>ge & small1 etc41 al?\gh !y are n necess>ily pure halluc9ations z di/+ui%$ f ! types ( illusions reg>d+ :i* !re is g5]al p]ceptual agreem5t4 ,illu/ration3 ,fig4 #cf4--;,illu/rat+ c]ta9 influ5ces ^u ! app>5t direction ( vision4 ,by cov]+ all b ! eyes ! latt] appe> to be drawn alike 9 bo? sket*es4 ,9 explanation ( ! illusory ph5om5a p]ta9+ to su* geometrical figures z are discuss$ 9 ! =ego+ p>agraphs1 *iefly two diff]5t k9ds ( hypo!ses h be5 (f]$4 ,!y are respectively psy*ological & physiological1 al?\gh !re is m or less ( a mixture ( ! two 9 mo/ attempts t[>d explanation4 ,! psy*ological hypo!ses introduce su* factors z att5tion1 imag9ation1 judgm5t1 & w4 ,h]+ & also ,helmholtz claim t ! k9d ( inv]sion :i* occurs is l>gely a matt] ( *ance or ( volition4 ,! latt] holds t ! p]ception ( p]spective figures is influ5c$ by imag9ation or ! images ( memory4 ,t is1 if "o =m ( ! figure is vividly imag9$ ! p]ception ( x is imm95t4 ,helmholtz has /at$ t1 ,glanc+ at a figure we obs]ve spontane\sly "o or ! o!r =m ( p]spective & usually ! "o t is associat$ 9 our memory ) ! greate/ numb] ( images4 ,! physiological hypo!ses dep5d l>gely ^u su* factors z accommodation & eye-movem5t4 ,neck] held to ! =m] z ! *ief cause4 ,he has /at$ t ! "p ( ! figure ^: image lies ne> ! fovea is e/imat$ z ne>] ?an ^? portions 9 ! p]iph]al regions ( ! visual field4 ,?is hypo!sis is op5 to s]i\s objections4 ,wundt cont5ds t ! inv]sion is caus$ by *anges 9 ! po9ts & l9es ( fixation4 ,he says1 ,! image ( ! ret9a ought to h a det]m9$ position if a p]spective illusion is to appe>2 b ! =m ( ?is illusion is entirely dep5d5t on motion & direction4 ,"s hypo!ses int]weave ! kn[n facts ( ! n]v\s sy/em ) psy*ological facts b "s ( ^! are examples ( a common anomaly 9 !orization1 = facts plus facts d n necess>ily result 9 a correct !ory4 ,t is1 two sets ( facts int]wov5 d n necess>ily yield an explanation :i* is correct4 ,,vi ,,! ,,influ5ce ,,( ,,angles ,z previ\sly /at$1 no satisfactory classification ( visual illusions exi/s1 b 9 ord] to cov] ! subject1 divisions are necess>y4 ,= ?is reason ! read] is introduc$ 9 ?is *apt] to ! effects att5d+ ! pres5ce ( angles4 ,by no means does x foll[ t ?is gr\p repres5ts ano!r type1 = x really includes _m illusions ( sev]al types4 ,! reason = ?is gr\p+ is t angles play an important "p1 directly or indirectly1 9 ! production ( illusions4 ,= a long "t _m geometrical illusions 7 acc\nt$ = by ov]e/imation or und]e/imation ( angles1 b ?is view has (t5 be5 f\nd to be inadequate4 ,h[ev]1 x _c be d5i$ t _m illusions are due at lea/ to ! pres5ce ( angles4 ,app>5tly ,zoelln] 0 ! fir/ to describe an illusion :i* is illu/rat$ 9 simple =m 9 ,fig4 #bi & m elaborately 9 ,figs4 #cg to #dj4 ,! two figures at ! "r ( ,fig4 #bi 7 drawn = ano!r purpose & are n design$ favorably = ! effect1 al?\gh x may be detect$ :5 ! figure is held at a di/ance4 ,zoelln] accid5tally notic$ ! illusion on a patt]n design$ = a pr9t = dress-goods4 ,! illusion is b slightly noticeable 9 ,fig4 #bi1 b by multiply+ ! numb] ( l9es 7& angles7 ! long p>allel l9es appe> to div]ge 9 ! direction t ! cross+ l9es conv]ge4 ,zoelln] /udi$ ! case ?or\ghly & e/abli%$ v>i\s facts4 ,he f\nd t ! illusion is greate/ :5 ! long p>allel l9es are incl9$ ab\t #de degrees to ! horizontal4 ,?is may be accompli%$ = ,fig4 #cg1 by turn+ ! page 7held 9 a v]tical plane7 ?r\gh an angle ( #de degrees f normal4 ,! illusion vani%es :5 held too f> f ! eye to di/+ui% ! %ort cross+ l9es1 & its /r5g? v>ies ) ! incl9ation ( ! oblique l9es to ! ma9 p>allels4 ,! mo/ effective angle betwe5 ! %ort cross+ l9es & ! ma9 p>allels appe>s to be approximately #cj degrees4 ,9 ,fig4 #cg !re are two illusions ( direction4 ,! p>allel v]tical /rips appe> unp>allel & ! "r & left portions ( ! oblique cross-l9es appe> to be %ift$ v]tically4 ,x is int]e/+ to note t /eady fixation dim9i%es & ev5 de/roys ! illusion4 ,illu/ration3 ,fig4 #cg4--,zoelln]'s illusion ( direction4 ,! maximum effectiv5ess ( ! illusion1 :5 ! figure is held s t ! ma9 p>allel l9es are at an incl9ation ( ab\t #de degrees to ! horizontal 0 acc\nt$ = by ,zoelln] z ! result ( less visual exp]i5ce 9 oblique directions4 ,he insi/$ t x takes less "t & is easi] to inf] div]g5ce or conv]g5ce ?an p>allelism4 ,?is explanation appe>s to be disprov$ by a figure 9 :i* slightly div]g5t l9es are us$ in/ead ( p>allel ones4 ,ow+ to ! effect ( ! oblique cross+ l9es1 ! div]g+ l9es may be made to appe> p>allel4 ,fur!rmore x is difficult to atta* mu* importance to ,zoelln]'s explanation because ! illusion is visible "u ! extremely brief illum9ation provid$ by "o electric sp>k4 ,( c\rse1 ! duration ( ! physiological reaction is d\btless great] ?an t ( ! sp>k1 b at be/ ! "t is v %ort4 ,h]+ expla9$ ! ,zoelln] illusion z due to ! curvature ( ! ret9a1 & ! result+ diff]5ce 9 ! ret9al images1 & held t acute angles appe> relatively too l>ge & obtuse ones too small4 ,! latt] has be5 f\nd to h limitations 9 ! explanation ( c]ta9 illusions4 ,?is ,zoelln] illusion is v /rik+ & may be con/ruct$ 9 a v>iety ( =ms4 ,9 ,fig4 #cg ! effect is q app>5t & x is int]e/+ to view ! figure at v>i\s angles4 ,= example1 hold ! figure s t ! broad p>allel l9es are v]tical4 ,! illusion is v pron\nc$ 9 ?is position2 h[ev]1 on tilt+ ! page backw>d ! illusion f9ally disappe>s4 ,9 ,fig4 #ch ! %ort oblique l9es d n cross ! long p>allel l9es & to make ! illusion m /rik+1 ! obliquity ( ! %ort l9es is rev]s$ at ! middle ( ! long p>allel l9es4 ,v>iations ( ?is figure are pres5t$ 9 ,figs4 #ci & #dj4 ,9 ?is case by /eady fixation ! p]spective effect is increas$ b !re is a t5d5cy = ! p>allel l9es to appe> m ne>ly truly p>allel ?an :5 ! po9t ( sight is p]mitt$ to roam ov] ! figures4 ,illu/ration3 ,fig4 #ch4--,p>allel l9es :i* d n appe> s4 ,illu/ration3 ,fig4 #ci4--,wundt's illusion ( direction4 ,illu/ration3 ,fig4 #dj4--;,h]+'s illusion ( direction4 ,_m inve/igations ( ! ,zoelln] illusion are record$ 9 ! lit]ature4 ,f ^! x is obvi\s t ! result is due to ! additive effects ( _m simple illusions ( angle4 ,9 ord] to give an idea ( ! mann] 9 :i* su* an illusion may be built up ! reason+ ( ,ja/r[#a w be pres5t$ 9 cond5s$ =m4 ,:5 two /raight l9es su* z ,a & ,b 9 ,fig4 #da are sep>at$ by a space x is usually possible to connect ! two m5tally & to det]m9e :e!r or n1 if connect$1 !y w\ld lie on a /raight l9e4 ,h[ev]1 if ano!r l9e is connect$ to "o1 ?us =m+ an angle z ,c does ) ,a1 ! l9es :i* appe>$ to be cont9u\s 7z ,a & ,b orig9ally7 no long] appe> s4 ,! conv]se is also true1 = l9es :i* are n 9 ! same /raight l9e may be made to appe> to be by ! addition ( ano!r l9e =m+ a prop] angle4 ,all ^! v>iations _c be %[n 9 a s+le figure1 b ! read] w f9d x int]e/+ to draw !m4 ,fur!rmore1 ! lett]s us$ on ! diagram 9 ord] to make ! description cle>] may be confus+ & ^! c be elim9at$ by r$raw+ ! figure4 ,9 ,fig4 #da ! obtuse angle ,,ac t5ds to tilt ,a d[nw>d1 s app>5tly if ,a 7 prolong$ x w\ld fall bel[ ,b4 ,simil>ly1 ,c appe>s to fall to ! "r ( ,d4 ,illu/ration3 ,fig4 #da4--,simple effect ( angles4 ,?is illusion app>5tly is due to ! pres5ce ( ! angle & ! effect is produc$ by ! pres5ce ( "r & acute angles to a less degree4 ,! illusion decreases or increases 9 g5]al z ! angle decreases or increases respectively4 ,al?\gh x is n safe to pres5t simple /atem5ts 9 a field s complex z t ( visual illusion :]e explanations are /ill controv]sial1 x is p]haps possible to g5]alize z ,ja/r[ did 9 ! =ego+ case z foll[s3 ,if ! direction ( an angle is t ( ! l9e bisect+ x & po9t+ t[>d ! apex1 ! direction ( ! sides ( an angle w app>5tly be deviat$ t[>d ! direction ( ! angle4 ,! deviation app>5tly is great] ) obtuse ?an ) acute angles1 & :5 obtuse & acute angles are s plac$ 9 a figure z to give rise to opposite deviations1 ! great] angle w be ! dom9ant influ5ce4 ,al?\gh ! illusion 9 su* simple cases z ,fig4 #da is slight1 x is q noticeable4 ,! effect ( ! addition ( _m ( ^! slight individual influ5ces is obvi\s 9 accompany+ figures ( great] complexity4 ,^! individual effects c be s multipli$ & comb9$ t _m illusory figures may be devis$4 ,9 ,fig4 #db ! oblique l9es are add$ to bo? horizontal l9es 9 su* a mann] t ,a is tilt$ d[nw>d at ! angle & ,b is tilt$ upw>d at ! angle 7! lett]s correspond+ to simil> l9es 9 ,fig4 #da74 ,9 ?is mann] !y appe> to be deviat$ consid]ably out ( _! true /raight l9e4 ,if ! read] w draw a /raight l9e ne>ly p>allel to ,d 9 ,fig4 #da & to ! "r1 he w f9d x helpful4 ,?is l9e %\ld be drawn to appe> to be a cont9uation ( ,c :5 ! page is held s ,d is approximately horizontal4 ,?is is a simple & effective means ( te/+ ! magnitude ( ! illusion1 = x is measur$ by ! degree ( app>5t deviation betwe5 ,d & ! l9e drawn adjac5t to x1 :i* ! eye w tol]ate4 ,ano!r me?od ( obta9+ su* a measurem5t is to beg9 ) only ! angle & to draw ! app>5t cont9uation ( "o ( its l9es ) a space int]v5+4 ,?is deviation f ! true cont9uation may !n be readily det]m9$4 ,illu/ration3 ,fig4 #db4 ,! effect ( two angles 9 tilt+ ! horizontal l9es4 ,illu/ration3 ,fig4 #dc4 ,! effect ( cross$ l9es ^u _! respective app>5t directions4 ,a m complex case is f\nd 9 ,fig4 #dc :]e ! effect ( an obtuse angle ,,acd is to make ! cont9uation ( ,,ab app>5tly fall bel[ ,,fg & ! effect ( ! acute angle is ! rev]se4 ,h[ev]1 ! net result is t due to ! prepond]ance ( ! effect ( ! l>g] angle ov] t ( ! small]4 ,! l9e ,,ec adds no?+1 = x m]ely introduces two angles :i* re9=ce ^? above ,,ab4 ,! l9e ,,bc may be omitt$ or cov]$ )\t appreciably affect+ ! illusion4 ,illu/ration3 ,fig4 #dd4--;,ano!r /ep t[>d ! ,zoelln] illusion4 ,9 ,fig4 #dd two obtuse angles are arrang$ s t _! effects are additive1 ) ! result t ! horizontal l9es app>5tly deviate maximally = su* a simple case4 ,?us x is se5 t ! t5d5cy ( ! sides ( an angle to be app>5tly deviat$ t[>d ! direction ( ! angle may result 9 an app>5t div]g5ce f p>allelism z well z 9 mak+ cont9u\s l9es appe> discont9u\s4 ,! illusion 9 ,fig4 #dd may be /r5g!n$ by add+ m l9es p>allel to ! oblique l9es4 ,?is is demon/rat$ 9 ,fig4 #ch & 9 o!r illu/rations4 ,9 ?is mann] /rik+ illusions are built up4 ,illu/ration3 ,fig4 #de4--,! two diagonals w\ld meet on ! left v]tical l9e4 ,illu/ration3 ,fig4 #df4--,pogg5dorff's illusion4 ,:i* oblique l9e on ! "r is ! prolongation ( ! oblique l9e on ! left8 ,if oblique l9es are ext5d$ across v]tical ones1 z 9 ,figs4 #de & #df1 ! illusion is se5 to be v /rik+4 ,9 ,fig4 #de ! oblique l9e on ! "r if ext5d$ w\ld meet ! upp] end ( ! oblique l9e on ! left2 h[ev]1 ! app>5t po9t ( int]section is some:at l[] ?an x is 9 reality4 ,9 ,fig4 #df ! oblique l9e on ! left is 9 ! same /raight l9e ) ! l[] oblique l9e on ! "r4 ,! l9e drawn p>allel to ! latt] furni%es an idea ( ! ext5t ( ! illusion4 ,?is is ! well-kn[n ,pogg5dorff illusion4 ,! upp] oblique l9e on ! "r actually appe>s to be approximately ! cont9uation ( ! upp] oblique l9e on ! "r4 ,! explanation ( ?is illusion on ! simple basis ( und]e/imation or ov]e/imation ( angles is op5 to criticism4 ,if ,fig4 #df is held s t ! int]cept$ l9e is horizontal or v]tical1 ! illusion disappe>s or at lea/ is greatly r$uc$4 ,x is difficult to reconcile ?is disappe>ance ( ! illusion = c]ta9 positions ( ! figure ) ! !ory t ! illusion is due to an incorrect appraisal ( ! angles4 ,illu/ration3 ,fig4 #dg4--;,a /raight l9e appe>s to sag4 ,accord+ to ,judd1#b ^? portions ( ! p>allels ly+ on ! obtuse-angle side ( ! int]cept$ l9e w be ov]e/imat$ :5 horizontal or v]tical di/ances along ! p>allel l9es are ! subjects ( att5tion1 z !y are 9 ! usual positions ( ! ,pogg5dorff figure4 ,he holds fur!r t ! ov]e/imation ( ?is di/ance along ! p>allels 7! two v]tical l9es7 & ! und]e/imation ( ! oblique di/ance across ! int]val are suffici5t to provide a full explanation ( ! illusion4 ,! disappe>ance & appe>ance ( ! illusion1 z ! position ( ! figure is v>i$ appe>s to demon/rate ! fact t l9es produce illusions only :5 !y h a direct influ5ce on ! direction 9 :i* ! att5tion is turn$4 ,t is1 :5 ?is ,pogg5dorff figure is 9 su* a position t ! int]cept$ l9e is horizontal1 ! incorrect e/imation ( di/ance along ! p>allels has no direct be>+ on ! di/ance to :i* ! att5tion is direct$4 ,9 ?is case ,judd holds t ! entire influ5ce ( ! p>allels is absorb$ 9 aid+ ! int]cept$ l9e 9 c>ry+ ! eye across ! int]val4 ,= a detail$ acc\nt ! read] is ref]r$ to ! orig9al pap]4 ,"s o!r illusions are n[ pres5t$ to demon/rate fur!r ! effect ( ! pres5ce ( angles4 ,d\btless1 9 "s ( ^!1 o!r causes contribute m or less to ! total result4 ,9 ,fig4 #dg a s]ies ( conc5tric arcs ( circles end 9 a /raight l9e4 ,! result is t ! /raight l9e appe>s to sag p]ceptibly4 ,incid5tally1 x may be int]e/+ = ! read] to asc]ta9 :e!r or n !re is any d\bt 9 8 m9d z to ! arcs appe>+ to belong to circles4 ,to ! au?or ! arcs appe> di/ort$ f ^? ( true circles4 ,illu/ration3 ,fig4 #dh4--;,di/ortions ( cont\r due to contact ) o!r cont\rs4 ,9 ,fig4 #dh ! b\nd+ figure is a true circle b x appe>s to be di/ort$ or d5t$ inw>d :]e ! angles ( ! hexagon meet x4 ,simil>ly1 ! sides ( ! hexagon appe> to sag inw>d :]e ! corn]s ( ! rectangle meet !m4 ,! influ5ces :i* h be5 emphasiz$ app>5tly are responsible = ! illusions 9 ,figs4 #di1 #ej & #ea4 ,x is int]e/+ to note ! disappe>ance ( ! illusion1 z ! plane ( ,fig4 #di is v>i$ f v]tical t[>d ! horizontal4 ,t is1 x is v app>5t :5 view$ p]p5dicul>ly to ! plane ( ! page1 ! latt] be+ held v]tically1 b z ! page is tilt$ backw>d ! illusion decreases & f9ally disappe>s4 ,illu/ration3 ,fig4 #di4--;,an illusion ( direction4 ,illu/ration3 ,fig4 #ej4--,twi/$-cord illusion4 ,^! are /raight cords4 ,illu/ration3 ,fig4 #ea4--,twi/$-cord illusion4 ,^! are conc5tric circles4 ,! illusions 9 ,figs4 #ej & #ea are commonly t]m$ twi/$ cord effects4 ,a cord may be made by twi/+ two /r&s :i* are :ite & black 7or any d>k color7 respectively4 ,?is may be sup]pos$ ^u v>i\s backgr\nds ) /rik+ results4 ,9 ,fig4 #ej ! /raight cords appe> b5t 9 ! middle1 ow+ to a rev]sal ( ! twi/4 ,su* a figure may be easily made by us+ cord & a *eck]$ clo?4 ,9 ,fig4 #ea x is difficult to conv9ce ! intellect t ! cords are n arrang$ 9 ! =m ( conc5tric circles1 b ?is becomes evid5t :5 "o ( !m is trac$ out4 ,! influ5ce ( ! illusion is s p[]ful t x is ev5 difficult to foll[ "o ( ! circles ) ! po9t ( a p5cil ar\nd its entire circumf]5ce4 ,! cord appe>s to =m a spiral or a helix se5 9 p]spective4 ,illu/ration3 ,fig4 #eb4--;,a spiral :5 rotat$ appe>s to exp& or contract1 dep5d+ ^u direction ( rotation4 ,a /rik+ illusion is obta9$ by revolv+ ! spiral %[n 9 ,fig4 #eb ab\t its c5t]4 ,?is may be consid]$ z an effect ( angles because ! curvature & consequ5tly ! angle ( ! spiral is cont9ually *ang+4 ,!re is a peculi> movem5t or progression t[>d ! c5t] :5 revolv$ 9 "o direction4 ,:5 ! direction ( rotation is rev]s$ ! movem5t is t[>d ! ext]ior ( ! figure2 t is1 !re is a seem+ expansion4 ,angles appe> to modify our judgm5ts ( ! l5g? ( l9es z well z ( _! direction4 ,( c\rse1 x mu/ be admitt$ t "s ( ^! illusions might be classifi$ "u ^? ( contra/ & o!rs4 ,9 fact1 x has be5 /at$ t classification is difficult b x appe>s logical to discuss ! effect ( angles 9 ?is *apt] ap>t f ! divisions pres5t$ 9 ! prec$+ *apt]s4 ,?is decision 0 rea*$ because ! effect ( angles c\ld be se5 9 _m ( ! illusions :i* w\ld m logically be gr\p$ "u ! classification pres5t$ 9 ! prec$+ *apt]s4 ,illu/ration3 ,fig4 #ec4--;,angles affect ! app>5t l5g? ( l9es4 ,9 ,fig4 #ec ! ?ree horizontal l9es are ( equal l5g? b !y appe> unequal4 ,?is mu/ be due prim>ily to ! size ( ! angles made by ! l9es at ! ends4 ,)9 c]ta9 limits1 ! great] ! angle ! great] is ! app>5t elongation ( ! c5tral horizontal portion4 ,?is g5]alization appe>s to apply ev5 :5 ! angle is less ?an a "r angle1 al?\gh !re appe>s to be less /r5g? to ! illusions ) ^! small] angles ?an ) ! l>g] angles4 ,o!r factors :i* contribute to ! ext5t ( ! illusion are ! positions ( ! figures1 ! di/ance betwe5 !m1 & ! juxtaposition ( c]ta9 l9es4 ,! illusion /ill exi/s if ! horizontal l9es are remov$ & also if ! figures are cut out ( pap] aft] jo9+ ! l[] ends ( ! %ort l9es 9 ea* case4 ,illu/ration3 ,fig4 #ed4--,! horizontal l9e appe>s to tilt d[nw>d t[>d ! ends4 ,illu/ration3 ,fig4 #ee4--,! horizontal l9e appe>s to sag 9 ! middle4 ,9 ,fig4 #ed ! horizontal /raight l9e appe>s to consi/ ( two l9es tilt+ slightly upw>d t[>d ! c5t]4 ,?is w be se5 to be 9 agreem5t ) ! g5]al proposition t ! sides ( an angle are deviat$ 9 ! direction ( ! angle4 ,9 ?is case x %\ld be not$ t "o ( ! obtuse angles to be consid]$ is ,,abc & t ! effect ( ?is is to tilt ! l9e ,,bd d[nw>d f ! c5t]4 ,9 ,fig4 #ee ! horizontal l9e appe>s to tilt upw>d t[>d its extremities or to sag 9 ! middle4 ,! explanation 9 ord] to h>monize ) ! =ego+ mu/ be bas$ ^u ! assumption t our judgm5ts may be influ5c$ by ?+s n pres5t b imag9$4 ,9 ?is case only "o side ( ea* obtuse angle is pres5t1 ! o!r side be+ =m$ by cont9u+ ! horizontal l9e bo? ways by means ( ! imag9ation4 ,t we d ?is unconsci\sly is atte/$ to by _m exp]i5ces4 ,= example1 we (t5 f9d ourselves imag9+ a horizontal1 a v]tical1 or a c5t] ^u :i* to base a p5d+ judgm5t4 ,a discussion ( ! influ5ce ( angles mu/ include a ref]5ce to ! well-kn[n ,muell]-,ly] illusion pres5t$ 9 ,fig4 #ef4 ,x is obvi\s 9 a t ! horizontal "p on ! left appe>s consid]ably long] ?an t "p 9 ! "r half ( ! diagram4 ,! influ5ce ( angles 9 ?is illusion c be easily te/$ by v>y+ ! direction ( ! l9es at ! ends ( ! two portions4 ,illu/ration3 ,fig4 #ef4--,! ,muell]-,ly] illusion4 ,9 all ^! figures ! influ5ce ( angles is obvi\s4 ,?is does n mean t !y are always solely or ev5 prim>ily responsible = ! illusion4 ,9 fact1 ! illusion ( ,pogg5dorff 7,fig4 #df7 may be due to ! incorrect e/imation ( c]ta9 l9e> di/ances1 b ! angles make ?is errone\s judgm5t possible1 or at lea/ contribute t[>d x4 ,_m discussions ( ! !ories or explanations ( ^! figures are available 9 sci5tific lit]ature ( :i* "o by ,judd#b may be tak5 z repres5tative4 ,he holds t ! false e/imation ( angles 9 ! ,pogg5dorff figure is m]ely a second>y effect1 n always pres5t1 & 9 no case ! s\rce ( ! illusion2 fur!rmore1 t ! illusion may be expla9$ z due to ! incorrect l9e> di/ances1 & may be r$uc$ to ! type ( illusion f\nd 9 ! ,muell]-,ly] figure4 ,c]ta9ly !re are grave dang]s 9 expla9+ an illusion on ! basis ( an app>5tly simple op]ation4 ,9 ,fig4 #ef1 b is made up ( ! two p>ts ( ! ,muell]-,ly] illusion4 ,a small dot may be plac$ equally di/ant f ! inside extremities ( ! horizontal l9es4 ,x is int]e/+ to note t ov]e/imation ( di/ance )9 ! figure is accompani$ ) und]e/imation outside ! figure &1 conv]sely1 ov]e/imation )9 ! figure is accompani$ by und]e/imation 9 ! neighbor+ space4 ,if ! small dot is object$ to z provid+ an additional ,muell]-,ly] figure ( ! empty space1 ?is dot may be omitt$4 ,z a sub/itute an obs]v] may try to locate a po9t midway betwe5 ! inside extremities ( ! horizontal l9es4 ,! error 9 locat+ ?is po9t w %[ t ! illusion is pres5t 9 ?is empty space4 ,9 ?is connection x is int]e/+ to note "s o!r illusions4 ,9 ,fig4 #eg ! influ5ce ( sev]al factors are evid5t4 ,two obvi\sly important ones are 7#a7 ! angles made by ! %ort l9es at ! extremities ( ! ext]ior l9es p>allel to ! sides ( ! l>ge triangle1 & 7#b7 ! influ5ce ( contra/ ( ! pairs ( adjac5t p>allel l9es4 ,! effect %[n 9 ,fig4 #ec is se5 to be augm5t$ by ! addition ( contra/ ( adjac5t l9es ( unequal l5g?4 ,an int]e/+ v>iation ( ! effect ( ! pres5ce ( angles is se5 9 ,fig4 #eh4 ,! two l9es =m+ angles ) ! horizontal are ( equal l5g? b due to _! relative positions1 !y d n appe> s4 ,x w\ld be q mislead+ to say t ?is illusion is m]ely due to angles4 ,obvi\sly1 x is due to ! pres5ce ( ! two oblique l9es4 ,x is ( int]e/ to turn to ,figs4 #be1 #bf & v>i\s illusions ( p]spective4 ,illu/ration3 ,fig4 #eg4--;,comb9$ influ5ce ( angles & contra/+ l5g?s4 ,illu/ration3 ,fig4 #eh4--,two equal oblique l9es appe> unequal because ( _! diff]5t positions4 ,at ?is po9t a digression appe>s to be necess>y &1 !re=e1 ,fig4 #ei is introduc$4 ,"h ! areas ( ! two figures are equal4 ,! judgm5t ( area is likely to be influ5c$ by juxtapos$ l9es & !re=e1 z 9 ?is case1 ! l[] appe>s l>g] ?an ! upp] "o4 ,simil>ly two trapezoids ( equal dim5sions & areas may be con/ruct$4 ,if ea* is con/ruct$ s t x re/s ^u its long] p>allel & "o figure is above ! o!r & only slightly sep>at$1 ! m9d is tempt$ to be influ5c$ by comp>+ ! juxtapos$ base ( ! upp] ) ! top ( ! l[] trapezoid4 ,! =m] dim5sion be+ great] ?an ! latt]1 ! l[] figure appe>s small] ?an ! upp] "o4 ,angles mu/ necess>ily play a "p 9 ^! illusions1 al?\gh x is admitt$ t o!r factors may be prom95t or ev5 dom9ant4 ,illu/ration3 ,fig4 #ei4--;,an illusion ( area4 ,?is appe>s to be a conv5i5t place to ins]t an illusion ( area bas$1 d\btless1 ^u =m1 b angles mu/ play a "p 9 ! illusions2 at lea/ !y are responsible = ! =m4 ,9 ,fig4 #fj ! five figures are con/ruct$ s z to be approximately equal 9 area4 ,h[ev]1 !y appe> unequal 9 ?is respect4 ,9 comp>+ areas1 we _c escape ! influ5ce ( ! l5g? & directions ( l9es :i* b\nd ^! areas1 & also1 ! effect ( contra/s 9 l5g?s & directions4 ,angles play a "p 9 all ^!1 al?\gh v indirectly 9 "s cases4 ,illu/ration3 ,fig4 #fj4--;,five equal areas %[+ ! influ5ce ( angles & contra/+ l5g?s4 ,to "s ext5t ! =ego+ is a digression f ! ma9 int5t ( ?is *apt]1 b x appe>s wor? :ile to introduce ^! indirect effects ( ! pres5ce ( angles 7real or imag9>y7 9 ord] to emphasize ! complexity ( influ5ces & _! subtl5ess4 ,direction is 9 ! la/ analysis an effect ( angle2 t is1 ! direction ( a l9e is measur$ by ! angle x makes ) "s ref]5ce l9e1 ! latt] be+ real or imag9>y4 ,9 ,fig4 #fa1 ! effect ( div]t+ or direct+ att5tion by "s subtle =ce1 su* z sugge/ion1 is demon/rat$4 ,?is =ce appe>s to contract or exp& an area4 ,! circle on ! left appe>s small] ?an ! o!r4 ,( c\rse !re is ! effect ( empty space comp>$ ) p>tially fill$ space1 b ?is _c be avoid$ 9 ?is case4 ,h[ev]1 x c be %[n t ! sugge/ions produc$ by ! arr[s t5d to produce app>5t r$uction or expansion ( areas4 ,note ! use ( arr[s 9 adv]tisem5ts4 ,illu/ration3 ,fig4 #fa4--,%[+ ! effect ( direct+ ! att5tion4 ,al?\gh !ory is subord9at$ to facts 9 ?is book1 a glimpse "h & !re %\ld be int]e/+ & helpful4 ,aft] hav+ be5 introduc$ to v>i\s types & influ5ces1 p]haps ! read] may bett] grasp ! tr5d ( !ories4 ,! p]spective !ory assumes1 & correctly s1 t simple diagrams (t5 sugge/ objects 9 ?ree dim5sions1 & t ! introduction ( an imag9>y ?ird dim5sion effects *anges 9 ! appe>ance ( l9es & angles4 ,t is1 l5g?s & directions ( l9es are app>5tly alt]$ by ! influ5ce ( l9es & angles1 :i* d n actually exi/4 ,t ?is is true may be prov$ 9 v>i\s cases4 ,9 fact ! read] has d\btless be5 conv9c$ ( ?is 9 connection ) "s ( ! illusions already discuss$4 ,v]tical l9es (t5 repres5t l9es ext5d+ away f ! obs]v]1 :o sees !m =e%ort5$ & !re=e !y may seem long] ?an horizontal l9es ( equal l5g?1 :i* are n subject to =e%ort5+4 ,?is c\ld expla9 su* illusions z se5 9 ,figs4 #d & #e4 ,h[ev] ?is !ory is n z easily appli$ to _m illusions4 ,accord+ to ,?i]y's p]spective !ory a l9e t appe>s ne>] is se5 z small] & a l9e t seems to be fur!r away is p]ceiv$ z long]4 ,if ! left portion ( b1 ,fig4 #ef1 be reproduc$ ) long] oblique l9es at ! ends b ) ! same l5g? ( horizontal l9es1 x w appe> clos] & ! horizontal l9es w be judg$ z %ort]4 ,! read] w f9d x int]e/+ to draw a numb] ( ^! portions ( ! ,muell]-,ly] figure ) ! horizontal l9e 9 ea* case ( ! same l5g? b ) long] & long] obliques at ! ends4 ,! dynamic !ory ( ,lipps gives an important role to ! inn] activity ( ! obs]v]1 :i* is n necess>ily sep>at$ f ! objects view$1 b may be felt z be+ 9 ! objects4 ,t is1 9 view+ a figure ! obs]v] unconsci\sly sep>ates x f surr\nd+ space & !re=e creates some?+ def9ite 9 ! latt]1 z a limit+ activity4 ,^! two ?+s1 "o real 7! object7 & "o imag9>y1 are balanc$ aga9/ ea* o!r4 ,a v]tical l9e may sugge/ a necess>y resi/ance aga9/ gravitational =ce1 ) ! result t ! l9e appe>s long] ?an a horizontal "o re/+ 9 peace4 ,! difficulty ) ?is !ory is t x all[s too mu* opportunity = purely philosophical explanations1 :i* are likely to run to ! fanciful4 ,x has ! d\btful advantage ( be+ able to expla9 illusions equally well if !y are actually rev]s$ f :at !y are4 ,= example1 gravity c\ld ei!r contract or elongate ! v]tical l9e1 dep5d+ ^u ! *oice ( viewpo9t4 ,! confusion !ory dep5ds ^u att5tion & beg9s ) ! difficulty ( isolat+ f illusory figures ! portions to be judg$4 ,amid ! complexity ( ! figure ! att5tion _c easily be fix$ on ! portions to be judg$4 ,?is results 9 confusion4 ,= example1 if areas ( diff]5t %apes su* z ^? 9 ,fig4 #fj are to be comp>$1 x is difficult to become oblivi\s ( =m or ( compactness4 ,9 try+ to see ! two *ief p>allel l9es 9 ,fig4 #ch1 9 _! true p>allelism ! att5tion is be+ subject$ to div]sion1 by ! %ort oblique p>allels ) a compromis+ result4 ,surely ?is !ory expla9s "s illusions successfully1 b x is n s successful ) "s ( ! illusions ( contra/4 ,! fact t practice 9 mak+ judgm5ts 9 su* cases z ,figs4 #de & #ef r$uces ! illusion ev5 to ultimate disappe>ance1 argues 9 favor ( ! confusion !ory4 ,p]haps ! obs]v] devotes himself m or less consci\sly to isolat+ ! p>ticul> feature to be judg$ & f9ally atta9s ! ability to d s4 ,accord+ to ,au]ba*'s indirect-vision !ory ! eyes 9 judg+ ! two halves ( ! horizontal l9e 9 a1 ,fig4 #ef1 involunt>ily draw imag9>y l9es p>allel to ?is l9e b above or bel[ x4 ,obvi\sly ! two p>ts ( su* l9es are unequal 9 ! same mann] z ! horizontal l9e 9 ! ,muell]-,ly] figure appe>s divid$ into two unequal p>ts4 ,some:at analog\s to ?is 9 "s cases is ,brunot's mean-di/ance !ory4 ,accord+ to ?is we e/abli% c5t]s ( gravity 9 figures & ^! influ5ce our judgm5ts4 ,^! are glimpses ( c]ta9 tr5ds ( !ories4 ,none is a complete success or failure4 ,ea* expla9s "s illusions satisfactorily1 b n necess>ily exclusively4 ,= ! pres5t1 we w be cont5t ) ^! glimpses ( ! purely !oretical aspects ( visual illusions4 ,,vii ,,illusions ,,( ,,dep? ,,& ,,( ,,di/ance ,besides ! s-call$ geometrical illusions discuss$ 9 ! prec$+ *apt]s1 !re is an int]e/+ gr\p 9 :i* ! p]ception ( ! ?ird dim5sion is 9 error4 ,:5 any ( ! ord9>y crit]ia ( relief or ( di/ance are app>5tly modifi$1 illusions ( ?is k9d are possible4 ,!re are _m illusions ( ?is sort1 su* z ! loom+ ( objects 9 a fog2 ! app>5t enl>gem5t ( ! sun & moon ne> ! horizon2 ! flatt5+ ( ! vault ( ! sky2 ! intaglio se5 z relief2 ! alt]ation ( relief ) light+2 & v>i\s *anges 9 ! l&scape :5 reg>d$ ) ! head inv]t$4 ,al?\gh "s ( ! crit]ia = ! p]ception ( dep? or ( di/ance h already be5 po9t$ out1 especially 9 ,*apt] ,,iii1 ^! w be m5tion$ aga94 ,di/ance or dep? is indicat$ by ! di/ribution ( light & %ade1 & an unusual object l an intaglio is likely to be mi/ak5 = relief :i* is m common4 ,an analysis ( ! light+ w usually reveal ! real =m ( ! object4 7,see ,figs4 #gj1 #ga1 #gb1 #gc1 #gf & #gg47 ,9 ?is connection x is int]e/+ to comp>e photographic negatives ) _! correspond+ positive pr9ts4 ,di/ance is (t5 e/imat$ by ! def9ition & color ( objects se5 ?r\gh great dep?s ( air 7a]ial p]spective74 ,^! di/ant objects are blurr$ by ! irregul> refraction ( ! light-rays ?r\gh non-homog5e\s atmosph]e4 ,!y are obscur$ to "s degree by ! veil ( brightness due to ! illum9at$ du/1 smoke1 etc41 9 ! atmosph]e4 ,!y are also t9t$ 7app>5tly7 by ! sup]position ( a t9t$ atmosph]e4 ,?us we h dim di/ance1 blue peaks1 azure dep?s ( sky1 etc41 repres5t$ 9 photographs1 pa9t+s1 & writ+s4 ,incid5tally1 ! sky above is blue = ! same g5]al reasons t ! atmosph]e1 int]v5+ betwe5 ! obs]v] & a di/ant horizon1 is blui%4 ,! ludicr\s errors made 9 e/imat+ di/ances 9 su* regions z ! ,rockies is usually acc\nt$ = by ! r>e cle>ness & homog5eity ( ! atmosph]e4 ,h[ev]1 is ! latt] a full explanation8 ,to "s ext5t we judge unkn[n size by e/imat$ di/ance1 & unkn[n di/ance by e/imat$ size4 ,:5 a p]son is view+ a great m\nta9 peak = ! fir/ "t1 is he n likely to assume x to be comp>able 9 size to ! hills ) :i* he has be5 famili>8 ,ev5 by all[+ consid]able1 is he n likely to greatly und]e/imate ! size ( ! m\nta9 &1 z a direct consequ5ce1 to und]e/imate ! di/ance proportionately8 ,?is incorrect judgm5t w\ld naturally be facilitat$ by ! abs5ce ( dimness & blu5ess due to ! atmosph]ic haze4 ,angul> p]spective1 :i* app>5tly v>ies ! =ms ( angles & produces ! div]g5ce ( l9es1 contributes mu* in=mation 9 reg>d to relative & absolute di/ances f ! eye ( ! v>i\s objects or ! p>ts ( an object4 ,= example1 a rectangle may appe> z a rhomboid4 ,x is obvi\s t c]ta9 data p]ta9+ to ! objects view$ mu/ be assum$1 & if ! assumptions are incorrect1 illusions w result4 ,^! judgm5ts also involve1 z mo/ judgm5ts d1 o!r data ext]nal to ! objects view$4 ,p]haps ^! incorrect judgm5ts are delusions r ?an illusions1 because visual p]ception has be5 delud$ by mis9=mation suppli$ by ! intellect4 ,size or l9e> p]spective is a factor 9 ! p]ception ( dep? or ( di/ance4 ,z has be5 /at$1 if we "k ! size exp]i5ce det]m9es ! di/ance2 & conv]sely1 if we "k ! di/ance we may e/imate ! size4 ,obvi\sly e/imates are involv$ & ^! :5 incorrect lead to false p]ception or int]pretation4 ,z an object approa*es1 ! axes ( ! eyes conv]ge m & m & ! eye-l5s mu/ be ?ick5$ m & m to keep ! object 9 focus4 ,z /at$ 9 ,*apt] ,,iii1 we h le>n$ to int]pret ^! accompany+ s5sations ( muscul> adju/m5t4 ,?is may be demon/rat$ by hold+ an object at an arm's l5g? & !n br++ x rapidly t[>d ! eyes1 keep+ x 9 focus all ! "t4 ,! s5sations ( conv]g5ce & accommodation are q int5se4 ,! two eyes look at a sc5e f two diff]5t po9ts ( view respectively & _! images d n p]fectly agree1 z has be5 %[n 9 ,figs4 #b & #c4 ,?is b9ocul> disp>ity is responsible to "s degree = ! p]ception ( dep?1 z ! /]eoscope has demon/rat$4 ,if two sph]es ( ! same size are susp5d$ on invisible /r+s1 "o at six feet1 ! o!r at sev5 feet away1 "o eye sees ! two balls 9 ! same plane1 b "o appe>s l>g] ?an ! o!r4 ,) b9ocul> vision ! balls appe> at diff]5t di/ances1 b judgm5t appraises !m z ( approximately equal size4 ,at t di/ance ! focal adju/m5t is n mu* diff]5t = bo? balls1 s t ! muscul> movem5t1 due to focus+ ! eye1 plays a small "p 9 ! e/imation ( ! relative di/ance4 ,b9ocul> disp>ity & conv]g5ce are ! prim>y factors4 ,"s h held t ! p]ception ( dep?1 t is1 ( a relative di/ance1 arises f ! process ( unconsci\sly runn+ ! po9t ( sight back & =?4 ,h[ev]1 ?is view1 unmodifi$1 appe>s unt5able :5 x is consid]$ t a sc5e illum9at$ by a lightn+ fla% 7( ! ord] ( magnitude ( a ?\s&? ( a second7 is se5 ev5 9 ?is brief mom5t to h dep?4 ,objects are se5 9 relief1 9 actual relation z to di/ance & 9 normal p]spective1 ev5 "u ! extremely brief illum9ation ( an electric sp>k 7( ! ord] ( magnitude ( "o tw5ty-?\s&? ( a second74 ,?is c also be demon/rat$ by view+ /]eoscopic pictures ) a /]eoscope1 ! illum9ation be+ furni%$ by an electric sp>k4 ,"u ^! circum/ances relief & p]spective are q satisfactory4 ,surely 9 ^! brief int]vals ! po9t ( sight _c d mu* survey+ ( a sc5e4 ,p>allax aids 9 ! p]ception ( dep? or di/ance4 ,if ! head be mov$ lat]ally ! view or sc5e *anges slightly4 ,objects or portions ( objects previ\sly hidd5 by o!rs may n[ become visible4 ,objects at v>i\s di/ances appe> to move ne>] or fur!r ap>t4 ,we h come to int]pret ^! app>5t movem5ts ( objects 9 a sc5e 9 t]ms ( relative di/ances2 t is1 ! relative am\nt ( p>allactic displacem5t is a measure ( ! relative di/ances ( ! objects4 ,! relative di/ances or dep? locations ( diff]5t p>ts ( an object c be p]ceiv$ z fluctuat+ or ev5 rev]s+4 ,?is is due to fluctuations 9 att5tion1 & illusions ( rev]sible p]spective are ( ?is class4 ,x is q impossible = "o to fix 8 att5tion 9 p]fect cont9uity ^u any object4 ,!re are _m involunt>y eye-movem5ts :i* _c be ov]come & "u normal conditions c]ta9 details are likely to occupy ! focus ( att5tion alt]nately or successively4 ,?is applies equally well to ! auditory s5se & p]haps to ! o!r s5ses4 ,emotional color+ has mu* to d ) ! fixation ( att5tion2 t :i* we admire1 desire1 love1 hate1 etc41 is likely to dwell m 9 ! focus ( att5tion ?an t :i* /irs our emotions less4 ,a slight sugge/ion ( =w>d & backw>d movem5ts c be produc$ by successively int]cept+ ! vision ( "o eye by an opaque c>d or o!r conv5i5t object4 ,x has be5 sugge/$ t ! illusion is due to ! consequ5t v>iations 9 ! t5sion ( conv]g5ce4 ,?ird dim5sional movem5ts may be produc$ = b9ocul> or monocul> vision dur+ eye-closure4 ,!y are also produc$ by op5+ ! eyes z widely z possible1 by pressure on ! eye-balls1 & by /ress+ ! eyelids4 ,h[ev]1 ^! are n important & are m]ely m5tion$ 9 pass+4 ,an increase 9 ! brightness ( an object is accompani$ by an app>5t movem5t t[>d ! obs]v]1 & conv]sely a decrease 9 brightness produces an app>5t movem5t 9 ! opposite direction4 ,^! effects may be witness$ ^u view+ ! gl[+ end ( a cig> :i* is be+ smok$ by "s "o a few y>ds away 9 ! d>kness4 ,rapidly mov+ ?9 cl\ds may produce su* an effect by v>y+ ! brightness ( ! moon4 ,"s peculi> impressions ( ?is nature may be felt :ile wat*+ ! fla%+ light ( "s light-h\ses or ( o!r signal+ /ations4 ,x has be5 sugge/$ t we naturally appraise bright] objects z ne>] ?an objects less bright4 ,h[ev]1 is x n int]e/+ to attribute ! app>5t movem5t to irradiation8 7,see ,*apt] ,,viii47 ,a bright object appe>s l>g] ?an a d>k object ( ! same size & at ! same di/ance4 ,:5 ! same object v>ies 9 brightness x rema9s 9 consci\sness ! same object & !re=e ( con/ant size2 h[ev]1 ! app>5t increase 9 size z x becomes bright] mu/ be acc\nt$ = 9 "s mann] & !re is only "o way op54 ,x mu/ be attribut$ a less] di/ance ?an =m]ly & !re=e ! sudd5 increase 9 brightness m$iates a consci\sness ( a movem5t =w>d1 t is1 t[>d ! obs]v]4 ,if two simil> objects1 su* z ! po9ts ( a compass1 are view$ b9ocul>ly & _! lat]al di/ance ap>t is alt]$1 ! obs]v] is consci\s ( a ?ird dim5sional movem5t4 ,inasmu* z ! accommodation is unalt]$ b conv]g5ce mu/ be v>i$ z ! lat]al di/ance betwe5 ! two1 ! explanation ( ! illusion mu/ consid] ! latt]4 ,! pair ( compass-po9ts are v conv5i5t = mak+ a demon/ration ( ?is pron\nc$ illusion4 ,! relation ( size & di/ance easily acc\nts = ! illusion4 ,obvi\sly ?is type ( illusion _c be illu/rat$ effectively by means ( diagrams1 s ! read] mu/ be cont5t to wat* = !m himself4 ,"s p]sons are able volunt>ily to produce illusory movem5ts 9 ! ?ird dim5sion1 b su* p]sons are r>e4 ,_m p]sons h exp]i5c$ involunt>y illusions ( dep?4 ,c>r f\nd1 9 a s]ies ( classes compris+ #cej /ud5ts1 #eh p]sons :o _h exp]i5c$ involunt>y dep? illusions at "s "t 9 _! lives4 ,five ( ^! also possess$ complete volunt>y control ov] ! ph5om5on4 ,! circum/ances att5d+ visual illusions ( dep? are n ! same = v>i\s cases1 & ! illusions v>y widely 9 _! features4 ,l o!r phases ( ! subject1 ?is has be5 treat$ 9 _m pap]s1 b ( ^! only "o w be specifically m5tion$1 = x w suffice4 ,c>r#c has /udi$ ?is type ( illusion comp>atively rec5tly & app>5tly q g5]ally1 & 8 "w w be drawn ^u = examples ( ?is type4 ,app>5tly !y may be divid$ into f\r classes3 7#a7 ,^? ( pure di/ance2 t is1 an object may appe> to be locat$ at v>y+ di/ances f ! obs]v]1 b no movem5t is p]ceiv$4 ,= example1 a p]son might be se5 fir/ at ! true di/ance2 he might be se5 next v close 9 front ( ! eyes2 !n he might sudd5ly appe> to be q remote2 7#b7 illusions ( pure motion2 t is1 objects are p]ceiv$ z mov+ 9 a c]ta9 direction )\t any app>5t *ange 9 location4 ,9 o!r words1 !y appe> to move1 b !y d n appe> to trav]se space2 7#c7 illusions ( movem5t :i* include a *ange 9 location4 ,?is appe>s to be ! mo/ common illusion ( dep?2 7#d7 ^? includ+ a comb9ation ( ! fir/ & ?ird classes4 ,= example1 ! object might fir/ appe> to move away f its true location & is p]ceiv$ at "s remote place4 ,%ortly x may appe> 9 its true orig9al position1 b ?is *ange 9 location does n involve any s5se ( motion4 ,^! peculi> illusions ( dep? are n z g5]ally exp]i5c$ z ^? describ$ 9 prec$+ *apt]s4 ,a geometrical illusion1 especially if x is pron\nc$1 is likely to be p]ceiv$ q univ]sally1 b ^! illusions ( dep? are ei!r m difficult to notice or m dep5d5t ^u psy*ological peculi>ities f> f univ]sal among p4 ,x is int]e/+ to note ! p]c5tages comput$ f ,c>r's /ati/ics obta9$ ^u int]rogat+ #cej /ud5ts4 ,( ^!1 #ag p] c5t _h exp]i5c$ dep?-illusions & betwe5 "o & two p] c5t _h volunt>y control ( ! ph5om5on4 ,( ! #dh :o _h exp]i5c$ illusions ( ?is type & 7 able to submit detail$ descriptions1 #be p] c5t belong$ to class 7#a7 ( ^? describ$ 9 ! prec$+ p>agraph2 #d p] c5t to class 7#b72 #eb p] c5t to class 7#c72 & #ag p] c5t to class 7#d74 ,usually ! illusion involves all objects 9 ! visual field b ) "s subjects ! field is contract$ or ! objects 9 ! p]iph]y ( ! field are unaffect$4 ,= mo/ p]sons ^! illusions involve normal p]ceptual objects1 al?\gh x appe>s t !y are phases ( halluc9atory orig94 ,inasmu* z ^! illusions _c be illu/rat$ diagrammatically we c d no bett] ?an to cond5se "s ( ! descriptions obta9$ & report$ by ,c>r4#c ,a case 9 :i* ! p]iph]al objects rema9 visible & /ation>y at _! true positions :ile ! c5tral portion ( ! field p>ticipates 9 ! illusion is z foll[s3 ,! obs]v] on a cle> day 0 gaz+ d[n a /reet :i* end$ a block away1 a r[ ( h\ses =m+ ! backgr\nd at ! end ( ! /reet4 ,! obs]v] 0 talk+ to & look+ directly at a companion only a %ort di/ance away4 ,soon ?is p]son 7app>5tly7 began to move d[n ! /reet1 until %e rea*$ ! backgr\nd ( h\ses at ! end1 & !n sl[ly came back to h] orig9al position4 ,! movem5t 9 bo? directions 0 di/9ctly p]ceiv$4 ,dur+ ! illusory movem5t !re 0 no vagu5ess ( outl9e or cont\r1 no blurr+ or confusion ( features2 ! p]son obs]v$1 seem$ di/9ct & sub/antial 9 "* dur+ ! illusion4 ,! p]ceiv$ object mov$ 9 relation to surr\nd+ objects2 !re 0 no movem5t ( ! visual field z a :ole4 ,! p]son decreas$ 9 size dur+ ! backw>d movem5t & increas$ 9 size dur+ ! =w>d return movem5t4 ,) _m p]sons :o exp]i5ce illusions ( dep?1 ! objects appe> to move to1 or appe> at1 "s def9ite position & rema9 !re until ! illusion is volunt>ily ov]come1 or until x disappe>s )\t volunt>y action4 ,a cond5sation ( a typical description ( ?is g5]al type pres5t$ by ,c>r is z foll[s3 ,all visual objects sudd5ly rec$e to ! app>5t di/ance ( ! horizon & rema9 9 t position sev]al m9utes1 return+ at ! end ( ?is p]iod to _! orig9al positions4 ,?is return movem5t is v sl[ at ! beg9n+1 b ! latt] phase ( ! movem5t is q rapid4 ,if ! subject closes h] eyes :ile ! objects appe> at _! di/ant position %e _c ev5 imag9e ^? objects locat$ any:]e except at _! app>5t di/ant position4 ,9 all cases 7enc\nt]$ by ,c>r7 ! motion 9 bo? directions is an actual exp]i5ce reality & ! subject 0 helpless z to initiat+1 /opp+1 or modify+ ! c\rse ( ! illusion 9 any way4 ,objects & ev5 visual images 7:i* are subject to ! same illusions7 decrease 9 size 9 proportion to ! am\nt ( backw>d movem5t & gr[ l>g] aga9 on _! return movem5t4 ,! objects are always cle>ly def9$ z if 9 good focus4 ,9 ?is p>ticul> case ! illusion occurr$ ab\t twice a ye>1 "u a v>iety ( conditions ( illum9ation1 at v>i\s times ( ! day1 b app>5tly "u conditions ( a r pron\nc$ fatigue4 ,9 reg>d to ! v>iation 9 ! size ( objects1 _m :o h exp]i5c$ ^! illusions ( dep? te/ify t ! size seems to *ange 9 proportion to ! app>5t di/ance1 accord+ to ! law ( p]spective4 ,"s p]sons appe> 9 d\bt z to ?is *ange & a few h exp]i5c$ ! peculi> anomaly ( decreas+ size z ! objects app>5tly approa*$4 ,_m p]sons :o h exp]i5c$ ^! peculi> illusions report no *ange 9 ! di/9ctness ( objects2 almo/ z _m are unc]ta9 reg>d+ ?is po9t2 & z _m report a *ange 9 di/9ctness4 ,app>5tly !re are phases ( halluc9atory orig9 s t !re is a wide v>iety ( exp]i5ces among ^? subject to ?is type ( illusion4 ,accord+ to ,c>r's inve/igation int]nal conditions alone are responsible = ! illusion ) m p]sons ?an ^? due to ext]nal conditions alone4 ,) "s p]sons a comb9ation ( int]nal & ext]nal conditions seem to be a necessity4 ,fixation ( vision appe>s to be an ess5tial objective condition = _m obs]v]s4 ,t is1 ! illusion appe>$ :ile fixat+ a speak] or s+] 9 a *ur* or a !at]4 ,) o!rs ! illusion occurs :ile read+4 ,"s report$ t fixation ^u *eck]$ or o!r regul>ly patt]n$ objects 0 an ess5tial condition4 ,among ! subjective conditions report$ z ess5tial are /eady fixation1 conc5tration ( att5tion1 complete m5tal absorption1 dreamy m5tal ab/raction1 & fatigue4 ,ocul> defects d n appe> to be ess5tial1 = ! illusions h be5 exp]i5c$ by _m ^: eyes 7 kn[n to be free f any abnormalities4 ,p]iod ( life does n appe> to h any prim>y influ5ce1 = ^? :o are subject to ^! peculi> illusions (t5 h exp]i5c$ !m ?r\gh\t _m ye>s4 ,9 "s cases x is evid5t t ! illusions occur dur+ a con/ra9$ eye position1 :ile ly+ d[n1 imm$iately ^u aris+ f b$ 9 ! morn+1 & ^u op5+ ! eyes aft] hav+ _h !m clos$ = "s "t4 ,h[ev]1 ! necessity = ^! conditions are exceptional4 ,! control ( ^! illusions ( dep?1 t is1 ! ability to create or to de/roy !m1 appe>s to be totally lack+ = mo/ ( ^? :o h exp]i5c$ !m4 ,"s c influ5ce !m1 a few c de/roy !m1 a few c indirectly initiate !m1 b ^? :o c bo? create & de/roy !m appe> to be r>e4 ,x may seem to ! read] t ! latt] "p ( ?is *apt] dep>ts f ! ma9 tr5d ( ?is book1 = mo/ ( ^! illusions ( dep? are to a degree ( halluc9atory orig94 ,fur!rmore x has be5 ! int5tion to discuss only ^? types ( illusions :i* are exp]i5c$ q uni=mly & univ]sally4 ,! digression ( ?is *apt] is excus$ on ! basis ( af=d+ a glimpse along ! bord]l& ( ^? gr\ps ( illusions :i* are ne>ly univ]sally exp]i5c$4 ,_m o!r phases ( dep? illusions h be5 record$ 9 sci5tific lit]ature4 ,! excell5t records pres5t$ by ,c>r c\ld be drawn ^u = fur!r glimpses1 b x appe>s t no m space %\ld be giv5 to ?is exceptional type4 ,! read] %\ld be suffici5tly =ew>n$ ( ?is type & %\ld be able to take x into acc\nt if peculi>ities 9 o!r types appe> to be expla9able 9 ?is mann]4 ,h[ev]1 9 clos+ x is well to emphasize ! fact t ! halluc9atory aspect ( dep? illusions is practically abs5t 9 types ( illusions to :i* att5tion is conf9$ 9 o!r *apt]s4 ,,viii ,,irradiation ,,& ,,brightness-,,contra/ ,_m int]e/+ & /rik+ illusions owe _! exi/5ce to contra/s 9 brightness4 ,! visual ph5om5on ( irradiation does n /rictly belong to ?is gr\p1 b x is s closely relat$ to x & s dep5d5t ^u brightness-contra/ t x is includ$4 ,a d>k l9e or spot w appe> d>k] 9 g5]al z ! brightness ( its environm5t is increas$2 or conv]sely1 a :ite spot surr\nd$ by a d>k environm5t w appe> bright] z ! latt] is d>k5$4 ,9 o!r words1 black & :ite1 :5 juxtapos$1 mutually re9=ce ea* o!r4 ,black pr9t on a :ite page appe>s mu* d>k] ?an x really is4 ,?is may be prov$ by pun*+ a hole 9 a black velvet clo? & lay+ ?is hole ov] a black portion ( a l>ge lett]4 ,! ink :i* appe>$ s black 9 ! pr9t1 :5 ! latt] 0 surr\nd$ by ! :ite pap]1 n[ appe>s only a d>k gray4 ,incid5tally a hole 9 a box l9$ ) black velvet is mu* d>k] ?an a piece ( ! black velvet surr\nd+ ! hole4 ,! effects ( brightness-contra/ are p>ticul>ly /rik+ :5 demon/rat$ by means ( light+1 a simple app>atus be+ illu/rat$ diagrammatically 9 ,fig4 #fb4 ,= example1 if a hole ,h is cut 9 an opaque :ite blott+ pap] & a l>ge piece ( ! :ite blott+ pap] is plac$ at ,c1 ! eye :5 plac$ be=e ! op5+ at ! "r w see ! op5+ at ,h fill$ ) ! backgr\nd ,c4 ,! hole ,h may be cut 9 ?9 metal1 pa9t$ a dull :ite1 & may be ( ! %ape ( a />4 ,?is %ape provides an intimacy betwe5 ! hole & its environm5t :i* t5ds to augm5t ! effects ( contra/s4 ,r & ,f are respectively ! re> & front lamps4 ,t is1 ! lamps ,r illum9ate ,c1 :i* fills ! hole & app>5tly is ! hole2 & ! lamps ,f illum9ate ! diffus+ :ite environm5t ,e4 ,! two sets ( lamps may be controll$ by sep>ate rheo/ats1 b if ! latt] are unavailable ! lamps 7sev]al 9 ea* set7 may be arrang$ s t by turn+ ea* "o (f or on1 a range ( contra/s 9 brightness betwe5 ,e & ,h 79 reality ,c7 may be obta9$4 7,by us+ color$ lamps & color$ pap]s z discuss$ 9 ,*apt] ,,ix ! m>vel\s effects ( color-contra/ may be sup]pos$ ^u ^? ( brightness-contra/47 ,illu/ration3 ,fig4 #fb4--,simple app>atus = demon/rat+ ! rem>kable effects ( contra/s 9 brightness & color4 ,if1 = example1 ,c is v feebly illum9at$ & ,e is v bright1 ,c w be pron\nc$ black2 b :5 ! lamps ,f are ext+ui%$ & no light is p]mitt$ to rea* ,e1 ! contra/ is rev]s$1 & ,c may actually appe> :ite4 ,( c\rse1 x is obvi\s t :ite & black are relative t]ms z enc\nt]$ 9 su* a case4 ,9 fact 9 brightness-contra/s relative & n absolute values ( brightness are usually ! m important4 ,9 ord] to m9imize ! /ray light :i* em]ges f ,h1 x is well to pa9t ! inside ( bo? comp>tm5ts black ) ! exception ( suffici5tly l>ge areas ( ,c & ,e4 ,! use ( black velvet in/ead ( black pa9t is sometimes advisable4 ,x is also well to scre5 ! lamps z sugge/$ 9 ! diagram4 ,?is simple app>atus w demon/rate "s v /rik+ effects ( contra/s 9 brightness & w s]ve1 also1 to demon/rate ev5 m int]e/+ effects ( contra/s 9 color4 ,two opposite contra/s obta9able by means ( a simple app>atus illu/rat$ 9 ,fig4 #fb may be %[n simultane\sly by means ( :ite1 black1 & gray pap]s arrang$ z 9 ,fig4 #fc4 ,9 ?is figure ! gray is repres5t$ by ! p>tially black ,vs1 ea* ( :i* conta9s equal am\nts ( black & ( :ite4 ,:5 held at "s di/ance ?is s]ves z a gray & ! same effect is app>5t z is describ$ = ! case ( actually gray ,vs4 ,an excell5t demon/ration may be made by ! read] by us+ two ,vs1 cut f ! same %eet ( gray pap]1 & pa/$ respectively ^u :ite & black backgr\nds1 z 9 ,fig4 #fc4 ,x w be app>5t t ! "o amid ! black environm5t appe>s mu* bright] ?an ! "o 7same gray7 amid ! :ite environm5t4 ,?is c be demon/rat$ easily to an audi5ce by means ( a figure two feet long4 ,x is int]e/+ to c>ry ! exp]im5t fur!r & place a ,v ( mu* d>k] gray on ! black backgr\nd ?an ! ,v on ! :ite backgr\nd4 ,! p]si/5cy ( ! illusion is f\nd to be rem>kable1 = x w exi/ ev5 :5 ! "o ,v is actually a mu* d>k] gray ?an ! o!r4 ,to become conv9c$ t ! two grays are ( ! same brightness 9 ,fig4 #fc1 x is only necess>y to pun* two holes 9 a :ite or gray c>d at su* a di/ance ap>t t !y w lie respectively ov] portions ( ! two ,vs :5 ! c>d is laid ^u ,fig4 #fc4 ,! grays 9 ! holes %\ld n[ appe> alike because _! environm5ts are simil>4 ,illu/ration3 ,fig4 #fc4--;,illu/rat+ brightness-contra/4 ,! importance ( contra/s 9 brightness & 9 color _c be ov]emphasiz$1 & x appe>s c]ta9 t no "o c fully realize _! effectiv5ess )\t witness+ x 9 a mann] simil> to t sugge/$ 9 ,fig4 #fb4 ,illu/ration3 ,fig4 #fd4--;,an effect ( brightness-contra/4 ,note ! d>k5+ ( ! int]sections ( ! :ite /rips4 ,_m illusions ( brightness-contra/ are visible on e h&4 ,= example1 ! po9t at :i* ! mullions ( a w9d[ cross w be se5 to appe> bright] ?an ! rema9+ portions ( !m :5 view$ aga9/ a bright sky4 ,conv]sely1 9 ,fig4 #fd1 d>k spots appe> :]e ! :ite b>s cross4 ,?is is purely an illusion & ! same type may be witness$ by ! obs]vant _m times a day4 ,9 ,fig4 #fd x is ( int]e/ to note t ! illusion is weak = ! cross+ ^u :i* ! po9t ( sight re/s1 b by av]t$ vision ! illusion is prom95t = ! o!r cross+s4 ,?is is "o ( ! effects :i* dep5ds ^u ! location 9 ! visual field4 ,no brightness-contra/s are se5 correctly & (t5 ! illusions are v /rik+4 ,if a s]ies ( gray pap]s is arrang$ f black to :ite1 ) ! successive pieces ov]lapp$ or o!rwise juxtapos$1 a s]ies ( /eps ( uni=m brightness is n se54 ,an in/rum5t w\ld det]m9e ! brightness ( ea* z uni=m1 b to ! eye ! s]ies w\ld appe> some:at flut$4 ,t is1 :]e a light gray jo9$ a d>k] gray ! edge ( ! =m] w\ld appe> light] ?an its actual brightness1 & ! edge ( ! d>k] gray w\ld appe> d>k] ?an x %\ld4 ,?is may also be demon/rat$ by lay+ a doz5 pieces ( :ite tissue pap] 9 a pile 9 su* a mann] t a s]ies ( #a1 #b1 #c1 #d1 etc41 ?ickness w\ld be produc$4 ,on view+ ?is by transmitt$ light a s]ies ( grays is se51 & ! effect ( contra/ is q app>5t4 ,su* a patt]n c be made photographically by rotat+ be=e a photographic plate a disk ) op5+s arrang$ prop]ly 9 /eps4 ,_m demon/rations ( ! *ief illusion ( brightness-contra/ are visible at night "u gl>+ light+ conditions4 ,x is difficult or impossible to see objects beyond automobile headlights1 & adjac5t to !m1 9 ! visual field4 ,objects simil>ly locat$ 9 respect to any surface suffici5tly bright are m or less obscur$4 ,*>act]s writt5 ^u a blackbo>d1 plac$ betwe5 two w9d[s1 may be invisible if ! surfaces se5 ?r\gh ! w9d[ are q bright1 unless a suffici5t quantity ( light rea*es ! blackbo>d f o!r s\rces4 ,/age-sett+s h be5 *ang$ 9 p]fect obscurity be=e an audi5ce by turn+ on a r[ ( bright lights at ! edge ( ! /age-op5+4 ,! t]m bl9d+ light owes its orig9 to ?is effect ( brightness-contra/4 ,! l9e ( juncture betwe5 a bright & a d>k surface may n be se5 z a %>p l9e1 b z a n>r[ b& ( gray4 ,:5 ?is is true x is possible t an undue am\nt ( area is cr$it$ to ! :ite4 ,9 prec$+ p>agraphs we h se5 ! peculi> effect at ! bord]-l9es ( a s]ies ( grays4 ,?is may h some?+ to d ) ! e/imate2 h[ev]1 irradiation may be due to excitation ( ret9al rods & cones adjac5t to1 b n actually )9 ! bright image4 ,a rem>kable effect :i* may be p>tially attributable to irradiation c be produc$ by cross+ a grat+ ( p>allel black l9es ) an oblique black l9e4 ,at ! actual cross+s ! black appe>s to run up ! n>r[ angle some:at l ink w\ld "u ! influ5ce ( surface t5sion4 ,?is is p>ticul>ly /rik+ :5 two grat+s or ev5 two ord9>y fly-scre5s are sup]pos$4 ,! effect is visible :5 pass+ two picket-f5ces1 "o beyond ! o!r4 ,if a d>k object is held s t a /raight edge appe>s to cross a c&le-flame or o!r light-s\rce1 at ?is portion ! /raight edge w appe> to h a not* 9 x4 ,irradiation 9 g5]al has be5 def9$ z ! lat]al diffusion ( n]v\s /imuli beyond ! actual /imulus4 ,x is n conf9$ to ! visual s5se b irradiation = ?is s5se is a t]m appli$ to ! app>5t enl>gem5t ( bright surfaces at ! exp5se ( adjac5t d>k] surfaces4 ,! cresc5t ( ! new moon appe>s l>g] 9 radius ?an ! fa9t outl9e ( ! d>k] portion :i* is feebly illum9at$ *iefly by light reflect$ f ! e>?'s surface4 ,a filam5t ( a lamp appe>s to gr[ 9 size z ! curr5t ?r\gh x is sl[ly increas$ f a z]o value2 t is1 z x increases 9 brightness4 ,9 ,fig4 #fe ! small inn] squ>es are ( ! same size b ! :ite squ>e appe>s l>g] ?an ! black "o4 ,x seems t ?is app>5t increase is made at ! exp5se ( ! adjac5t d>k area4 ,?is ph5om5on or illusion is /ronge/ :5 ! brightness is mo/ int5se1 & is said to be greate/ :5 ! accommodation is imp]fect4 ,a v int5se light-s\rce may appe> _m times l>g] ?an its actual physical size4 ,illu/ration3 ,fig4 #fe4--,! ph5om5on ( irradiation4 ,d\btless a numb] ( factors may play a "p 9 ?is ph5om5on4 ,x appe>s possible t !re is a rapid spread+ ( ! excitation ov] ! ret9a ext5d+ q beyond ! bord] ( ! m int5sely /imulat$ region1 b ?is mu/ be practically in/antane\s 9 ord] to satisfy results ( exp]im5ts4 ,eye-movem5ts may play "s "p =1 despite ! mo/ s]i\s ef=ts to fixate ! po9t ( sight1 a fr+e w appe> on ! bord]s ( images :i* is c]ta9ly due to involunt>y eye-movem5ts4 ,irradiation has also be5 ascrib$ to sph]ical ab]ration 9 ! eye-l5s & to diffraction ( light at ! pupil4 ,pr9t$ type appe>s consid]ably r$uc$ 9 size :5 ! pupil is dilat$ ) atrop9 & is re/or$ to normal appe>ance :5 a small artificial pupil is plac$ be=e ! dilat$ pupil4 ,x has be5 sugge/$ t *romatic ab]ration 9 ! eye-l5s is a contributory cause1 b ?is _c be v important1 = ! illusion is visible ) mono*romatic light :i* elim9ates *romatic ab]ration4 ,! exp]im5tal evid5ce appe>s to indicate t ! ph5om5on is ( a physical nature4 ,!re are v>iations 9 ! effects attributable to radiation1 & x is difficult to r$uce !m to simple t]ms4 ,p]haps x may aid ! read] to h be=e him ! classification pres5t$ by ,boswell4#d ,he describes ! v>ieties ( irradiation z foll[s3 #a4 ,v rapid spread+ ( ! excitation ov] ! ret9a ext5d+ f> beyond ! bord] ( ! /imulat$ region & occurr+ imm$iately ^u impact ( ! /imulat+ light4 #b4 ,irradiation )9 ! /imulat$ portion ( ! ret9a aft] ! =m ( a figure becomes di/9ctly p]ceptible4 #c4 ,emanations ( decreas+ int5sity ext5d !mselves outw>d & backw>d f a mov+ image until lo/ 9 ! d>kness ( ! backgr\nd4 #d4 ,a well kn[n =m ( irradiation :i* occurs :5 a surface ( great] int5sity enl>ges itself at ! exp5se ( "o ( less int5sity4 #e4 ,a =m hav+ _m ( ! *>act]i/ics ( ! fir/ type1 b occurr+ only aft] long p]iods ( /imulation1 ( ! magnitude ( #cj to #fj seconds or m4 ,,ix ,,color ,9 ord] to simplify ! pres5tation ( ! g5]al subject1 discussions ( color h be5 omitt$ 9 s f> z possible f ! prec$+ *apt]s4 ,!re are almo/ numb]less ph5om5a involv+ color1 _m ( :i* are illusions1 or seem+ly s4 ,x w be obvi\s t _m are errors ( s5se2 "s are errors ( judgm5t2 o!rs are errors due to defects ( ! optical sy/em ( ! eye2 & _m may be ascrib$ to c]ta9 *>act]i/ics ( ! visual process4 ,x is n ! int5tion to cov] ! entire field 9 detail2 inde$1 ?is c\ld n be done )9 ! conf9es ( a l>ge volume4 ,h[ev]1 sub/antial glimpses ( ! m important phases ( color z relat$ to illusions are pres5t$ 9 ?is *apt]4 ,9 ! e>ly *apt]s p]ta9+ to ! eye & to vision "s ( ! foll[+ po9ts 7 necess>ily t\*$ ^u1 b ! repetition 9 ! p>agraphs :i* foll[ is avoid$ z mu* z possible4 ,simultane\s ,contra/4--,t ! life ( color is due to contra/ is demon/rable 9 _m ways4 ,if a room is illum9at$ by deep r$ light1 at fir/ ?is color is v vivid 9 consci\sness2 h[ev]1 gradually x becomes less saturat$4 ,aft] a half h\r ! color is app>5tly a mu* fad$ r$ b ^u em]g+ f ! room into "o normally light$1 ! latt] appe>s v m>k$ly gre5i% 9 t9t4 ,! reason t ! pure r$ light does n appe> z /rongly color$ z x really is1 is due to ! lack ( contra/4 ,9 a simil> mann] at night we see :ite objects z :ite ev5 "u ! yell[i% artificial light4 ,! latt] appe>s v yell[ 9 color :5 x is fir/ turn$ on z daylight wanes b z d>kness falls & "t elapses x gradually assumes a colorless appe>ance4 ,an app>atus con/ruct$ aft] ! plan ( ,fig4 #fb is v effective = demon/rat+ ! rem>kable effects ( color-contra/ b "s additions w add consid]ably to its conv5i5ce4 ,if ! lamps ,f are divid$ into ?ree circuits1 ea* emitt+1 respectively1 r$1 gre51 & blue prim>y colors1 x is possible by means ( controll+ rheo/ats to illum9ate ,e1 ! environm5t1 ) light ( any hue 7includ+ purple71 ( any saturation1 & ( a wide range ( int5sities or result+ brightnesses4 ,?us we h a v simple app>atus = quickly provid+ almo/ numb]less environm5ts = ,h4 ,! same s*eme c be appli$ to lamps ,r1 ) ! result t a va/ array ( colors may be se5 ?r\gh ! hole ,h4 ,if ! hole is ! %ape ( ! /> 9 ,fig4 #ff x w be f\nd v effective4 ,! obs]v] w actually see a /> ( any desir$ color amid an environm5t ( any desir$ color4 ,c>e %\ld be tak5 to h ! /> cut 9 v ?9 mat]ial 9 ord] to elim9ate conspicu\s b\nd>y l9es4 ,x is q satisfactory to use a s]ies ( color$ pap]s on a slide at ,c & ord9>y cle> lamps at ,r4 ,by means ( ?is app>atus bo? contra/s--hue & brightness--may be demon/rat$4 ,( c\rse1 = black & :ite only brightness-contra/ is pres5t2 b 9 g5]al :]e !re is color-contra/ !re is also brightness-contra/4 ,! latt] may be r$uc$ or ev5 elim9at$ if ! brightness ( ! /> & ( its surr\nd+s are made equal1 b x is difficult to make a satisfactory balance 9 ?is respect4 ,assum+1 h[ev]1 t brightness-contra/ is elim9at$1 we h left only hue & saturation contra/1 or :at w be t]m$ 7r loosely1 x is admitt$7 color-contra/4 ,illu/ration3 ,fig4 #ff4--;,an excell5t patt]n = demon/rat+ color-contra/4 ,if ! surr\nd+s are d>k &1 = example1 an orange /> is se5 alone1 x does n appe> v colorful4 ,h[ev]1 if ! surr\nd+s are n[ made bright ) :ite light1 ! /> appe>s q saturat$4 ,) blue or gre5 light ! orange /> appe>s ev5 m int5sely orange1 b :5 ! color-contra/ is r$uc$1 z 9 ! case ( yell[ or r$ surr\nd+s1 ! vividness ( ! orange /> aga9 decreases4 ,?is may be summ>iz$ by /at+ t two widely diff]5t colors view$ 9 ?is mann] w mutually affect ea* o!r s t !y appe> /ill m diff]5t 9 hue4 ,if _! hues are close toge!r spectrally ?is effect is n z app>5t4 ,= example1 if orange & gre5 are contra/$1 ! orange w appe> r$di% 9 hue & ! gre5 w appe> blui%4 ,let u n[ assume ! /> to be :ite1 & t ! surr\nd+s are ( any color ( approximately ! same brightness4 ,! /> :i* is really :ite w n[ appe> decid$ly t9t$ & ( a hue approximately complem5t>y to t ( ! surr\nd+s4 ,:5 ! latt] are ( a gre5 color ! :ite /> w assume a purpli% t+e2 :5 r$ ! :ite /> w appe> ( a blue-gre5 t9t2 :5 yell[ ! :ite /> w appe> blui%4 ,?is is an illusion 9 any s5se ( ! t]m4 ,! /r5g? ( ?is illusion caus$ by simultane\s contra/ is v rem>kable4 ,= example1 if a grayi% purple /> is view$ amid int5se gre5 surr\nd+s x w appe> ri*ly purple1 b :5 ! surr\nd+s are *ang$ to a ri* purple ! grayi% purple /> w ev5 appe> gre5i%4 ,! app>5t *ange ( a color to its complem5t>y by m]ely alt]+ its environm5t is really a rem>kable illusion4 ,! importance ( simultane\s contra/ is easily demon/rat$ ^u a pa9t+ by isolat+ any color$ object f its surr\nd+s by means ( a hole 9 a gray c>d4 ,= example1 an orange fl[]-pot amid ! gre5 foliage ( its surr\nd+s w appe> decid$ly diff]5t 9 color & brightness ?an :5 view$ ?r\gh a hole 9 a :ite1 black1 or gray c>dbo>d4 ,by means ( color$ pap]s ! same color may be plac$ 9 _m diff]5t environm5ts & ! v>i\s contra/s may be view$ simultane\sly4 ,! ext5t ( ! illusion is v evid5t :5 reveal$ 9 ?is simple mann]4 ,h[ev]1 too mu* emphasis _c be giv5 to ,figs4 #fb & #ff z a p[]ful means = realiz+ ! greate/ effects4 ,aft]-images4--,aft] look+ at bright objects we see aft]-images ( ! same size & =m :i* v>y m or less 9 color4 ,^! aft]-images are due to p]si/5ce or fatigue ( ! visual process1 dep5d+ ^u conditions4 ,aft] look+ at ! sun = a mom5t a v bright aft]-image is se54 ,und\bt$ly ?is at fir/ is due to a p]si/5ce ( ! visual process1 b z x decays x cont9u\sly *anges color & f9ally its pres5ce is due to fatigue4 ,aft]-images may be se5 aft] look+ int5tly at any object & !n direct+ ! eyes t[>d a blank surface su* z a wall4 ,a picture-frame w be se5 z a rectangul> aft]-image2 a *eck]$ patt]n w be se5 z a *eck]$ aft]-image4 ,:5 ^! aft]-images are project$ ^u o!r objects x is obvi\s t ! appe>ance ( ! latt] is app>5tly alt]$ especially :5 ! obs]v] is n consci\s ( ! aft]-image4 ,! effects are se5 9 pa9t+s & _m peculi> ph5om5a 9 ! v>i\s arts are directly traceable to aft]-images4 ,x appe>s unnecess>y to detail ! _m effects = ! explanations or at lea/ ! g5]al pr9ciples ( aft]-images are s simple t ! read] %\ld easily r5d] an analysis ( any giv5 case4 ,let u assume t vision is fix$ ^u a gre5 squ>e ^u a gray or :ite backgr\nd4 ,despite ! utmo/ ef=t on ! "p ( ! obs]v] to gaze fix$ly ^u ?is gre5 squ>e1 ! latt] w beg9 to appe> fr+$ ) a p9ki% bord]4 ,?is is due to ! aft]-image ( ! gre5 squ>e & x is displac$ slightly due to involunt>y eye-movem5ts4 ,aft] gaz+ z /eadily z possible = a half m9ute1 or ev5 less1 if ! po9t ( sight is turn$ to ! :ite pap] a p9k squ>e is se5 ^u x4 ,fur!rmore1 ?is p9k squ>e moves ov] ! field ) ! po9t ( sight4 ,?is is ! type mo/ g5]ally notic$4 ,aft]-images h be5 classifi$ z positive & negative4 ,! =m] are ^? 9 :i* ! di/ribution ( light & %ade is ! same z 9 ! orig9al object4 ,^? 9 :i* ?is di/ribution is rev]s$1 z 9 ! photographic negative1 are t]m$ negative4 ,aft]-images und]go a v>iety ( *anges 9 color b 9 g5]al !re are two important /ates4 ,9 "o ! color is ! same z 9 ! orig9al object & 9 ! o!r x is approximately complem5t>y to ! orig9al color4 ,9 g5]al ! negative aft]-image is approximately complem5t>y 9 color to ! color ( ! orig9al object4 ,aft]-images are be/ obs]v$ :5 ! eyes are well re/$1 z 9 ! morn+ ^u awak5+4 ,) a little practice 9 giv+ att5tion to !m1 !y c be se5 float+ 9 ! air1 9 ! indef9ite field ( ! clos$ eyes1 ^u a wall1 or else:]e1 & ! *anges 9 ! brightness & color c be readily foll[$4 ,negative aft]-images are sometimes v p]si/5t & !re=e are m commonly notic$ ?an positive ones4 ,! positive aft]-image is due to ret9al in]tia1 t is1 to ! p]si/5cy ( ! visual process aft] ! actual /imulus has be5 remov$4 ,x is ( relatively brief duration4 ,if an aft]-image ( a w9d[ is project$ on a :ite area x is likely to appe> z a negative :5 project$ ^u a :ite backgr\nd1 & z a positive ^u a d>k backgr\nd1 su* z is readily provid$ by clos+ ! eyes4 ,x may be ( int]e/ = ! read] to obta9 an aft]-image ( a bright surface ( a light-s\rce & /udy its color *anges ) ! eye clos$4 ,^u repeat+ ! exp]im5t ! progression ( colors w be f\nd to be always ! same = ! same conditions4 ,! duration ( ! aft]-image w be f\nd to v>y ) ! brightness & p]iod ( fixation ( ! object4 ,x is int]e/+ to note t an aft]-image is se5 ) difficulty :5 ! eyes are 9 motion1 b x becomes q conspicu\s :5 ! eyes are br\ght to re/4 ,an aft]-image due to ! /imulation ( only "o eye sometimes seems to be se5 by ! o!r eye4 ,naturally ?is has giv5 rise to ! sugge/ion t ! seat ( aft]-images is c5tral r ?an p]iph]al2 t is1 9 ! bra9 r ?an at ! ret9a4 ,h[ev]1 ?is is n g5]ally ! case & ! exp]im5tal evid5ce weighs heavily aga9/ ?is conclusion4 ,if ,fig4 #eb is revolv$ ab\t its c5t] & fixat$ = "s "t /rik+ effects are obta9$ ^u look+ away sudd5ly ^u any object4 ,! latt] w appe> to %r9k if ! spiral has seem$ to run outw>d1 or to exp& if ! spiral has seem$ to run inw>d4 ,^! are cle>ly aft]-images ( motion4 ,z /at$ else:]e1 we may h illusions ( aft]-images z well z ( ! orig9al images4 ,= example1 if a cle>ly def9$ plane geometrical figure su* z a cross or squ>e is bright en\gh to produce a /rong aft]-image1 ! latt] :5 project$ ^u a p]spective draw+ w appe> di/ort$2 t is1 x is likely to appe> 9 p]spective4 ,a simple way ( demon/rat+ aft]-images & _! duration is to move ! object produc+ !m4 ,= example1 ext+ui% a mat* & move ! gl[+ end4 ,if obs]v$ c>efully )\t mov+ ! eye a blui% aft]-image w be se5 to foll[ ! gl[+ end ( ! mat*4 ,9 ?is case ! eyes %\ld be direct$ /raight ahead :ile ! /imulus is mov+ & ! obs]vation mu/ be made by av]t$ or indirect vision4 ,gr[? & ,decay ( ,s5sation4--,al?\gh _m aft]-images may n be consid]$ to be illusions 9 ! s5se 9 :i* ! t]m is us$ "h1 !re are _m illusions 9 :i* !y at lea/ play a "p4 ,fur!rmore1 x is ! int5tion ?r\gh\t ^! *apt]s to adh]e to a discussion ( /atic illusions1 x is difficult to avoid t\*+ occasionally ^u motion4 ,! eyes are 9 motion mo/ ( ! "t1 h5ce1 c]ta9 effects ( an illusory nature may be sup]pos$ ^u /ation>y objects4 ,! p]si/5ce ( vision has be5 demon/rat$ by e small boy z he wav$ a gl[+ /ick seiz$ f a bonfire4 ,fireworks owe mu* ( _! beauty to ?is ph5om5on4 ,a rapidly revolv+ spok$ :eel may appe> to be a m or less transp>5t disk1 b occasionally :5 a rapid eye-movem5t moves ! po9t ( sight ) suffici5t spe$ 9 ! direction ( motion1 ! spokes reappe> mom5t>ily4 ,motion-pictures owe _! success to ?is visual prop]ty--! p]si/5ce ( vision4 ,if a lant]n-slide picture be focus$ ^u black velvet or ^u a d>k doorway1 ! project$ image w n be se54 ,h[ev]1 if a :ite rod be mov$ rapidly en\gh 9 ! plane ( ! image1 ! latt] may be se5 9 its entirety4 ,! mixture ( colors1 by rotat+ !m on disks1 owes its possibility to ! p]si/5ce ( ! color-s5sations beyond ! p]iod ( actual /imulation4 ,! fact t x takes "t = s5sations ( light to gr[ & decay is n z important "h z ! fact t ! rates ( gr[?1 & also ( decay1 v>y = diff]5t colors4 ,9 g5]al1 ! gr[? & ! decay are n ( simil> or uni=m rates4 ,fur!rmore1 ! s5sation (t5 initially ov]%oots its f9al /eady value1 ! am\nt ( ov]%oot+ dep5d+ ^u ! int5sity & color ( ! /imulus4 ,^! effects may be witness$ 9 _! ext5sive v>iety by rotat+ disks s con/ruct$ t black & v>i\s colors /imulate ! ret9a 9 def9ite ord]s4 ,an int]e/+ case ( ?is k9d may be demon/rat$ by rotat+ ! disk %[n 9 ,fig4 #fg4 ,not)/&+ ! fact t ^! are only black & :ite /imuli1 a s]ies ( color$ r+s is se5 v>y+ f a r$di% *ocolate to a blue-gre54 ,exp]im5t w det]m9e ! be/ spe$1 :i* is r sl[ "u a mod]ate int5sity ( illum9ation4 ,! r$di% r+s w be out]mo/ & ! blue-gre5 r+s inn]mo/ :5 ! disk is rotat$ 9 "o direction4 ,^u rev]s+ ! direction ( rotation ! positions ( ^! color$ r+s w be rev]s$4 ,by us+ v>i\s colors1 su* z r$ & gre5 = ! :ite & black respectively1 o!r colors w be produc$1 "s ( :i* are v /rik+4 ,! complete explanation ( ! ph5om5on is n cle>1 ow+ to ! d\bt :i* exi/s conc]n+ _m ( ! ph5om5a ( color-vision1 b x appe>s c]ta9 t ! diff]5ce 9 ! rates ( gr[? & decay ( ! v>i\s color-s5sations 7! :ite /imulus includes all ! spectral hues ( ! illum9ant7 is at lea/ p>tially1 if n :olly1 responsible4 ,illu/ration3 ,fig4 #fg4--;,by rotat+ ?is ,mason 7black & :ite7 disk color-s5sations are produc$4 ,an int]e/+ effect1 p]haps due :olly or 9 "p to ! diff]5ces 9 ! rates ( gr[? & decay ( color-s5sations1 may be obs]v$ :5 a color$ patt]n is mov$ "u a l[ int5sity ( illum9ation1 ! eyes rema9+ focus$ ^u a po9t 9 space at ab\t ! same di/ance z ! object4 ,a squ>e ( r$ pap] pa/$ 9 ! c5t] ( a l>g] piece ( blue-gre5 pap] is a satisfactory object4 ,on mov+ ?is object g5tly1 keep+ ! po9t ( sight fix$ 9 its plane ( movem5t1 ! c5tral r$ squ>e w appe> to %ake l jelly & a decid$ trail ( color w appe> to cl+ to ! lagg+ edge ( ! c5tral squ>e4 ,p]haps *romatic ab]ration plays "s "p 9 mak+ ?is effect s conspicu\s4 ,a simil> case w be not$ 9 a photographic d>k-room illum9at$ by r$ light ^u obs]v+ ! self-lum9\s dial ( a wat* or clock4 ,:5 ! latt] is mov$ 9 ! plane ( ! dial1 ! gre5i% lum9\s figures appe> sep>at$ f ! r$ dial & seem to lag beh9d dur+ ! movem5t4 ,= su* demon/rations x is well to exp]im5t some:at by v>y+ ! int5sity ( ! illum9ation & ! spe$ ( movem5t4 ,relatively l[ values ( ea* appe> to be be/4 ,al?\gh ! v>i\s color-s5sations gr[ & decay at diff]5t rates1 ! latt] dep5d ^u conditions4 ,x appe>s t blue-s5sation rises v rapidly & greatly ov]%oots its f9al /eady value = a giv5 /imulus4 ,r$ ranks next & gre5 ?ird 9 ?is respect4 ,! ov]%oot+ appe>s to be great] = ! great] int5sity ( ! /imulus4 ,! "t requir$ = ! s5sation to rea* a /eady value dep5ds bo? ^u ! spectral "* & ! brightness ( ! color b is usually less ?an a second4 ,*romatic ,ab]ration4--,x is well kn[n t ! eye focuses diff]5t spectral colors at diff]5t po9ts4 ,?is is true ( any simple l5s & ! defect is ov]come 9 ! manufacture ( optical in/rum5ts by comb9+ two l5ses consi/+ respectively ( glasses diff]+ consid]ably 9 refractive index4 ,if a :ite object is view$ by ! eye1 x %\ld appe> ) a purpli% fr+e2 h[ev]1 ! effect is obs]v$ m readily by view+ a light-s\rce ?r\gh a purple filt] :i* transmits only violet & r$ light4 ,! light-s\rce w h a r$ or a violet fr+e1 dep5d+ ^u ! accommodation or focus ( ! eye4 ,?is effect is p]haps be/ witness$ on view+ a l9e spectrum su* z t ( ! m]cury arc1 focus$ ^u a gr\nd glass4 ,! violet & blue l9es are n se5 9 good focus :5 ! eyes are focus$ ^u ! gre5 & yell[ l9es4 ,fur!rmore1 ! =m] c be se5 9 excell5t focus at a di/ance too %ort = accommodat+ ! eyes to ! gre5 & ! yell[ l9es4 ,?is exp]im5t %[s t ! focal l5g? ( ! optical sy/em ( ! eye is consid]ably %ort] = ! spectral hues ( %ort] wave-l5g? 7violet1 blue7 ?an = ^? ( long] wave-l5g? 7su* z yell[74 ,n>r[ slits cov]$ ) diffus+ glass & illum9at$ respectively by fairly pure blue1 gre51 yell[1 & r$ lights may be sub/itut$4 ,! effect may be demon/rat$ by try+ to focus f9e detail su* z pr9t :5 two adjac5t areas are illum9at$ by blue & r$ lights respectively4 ,x is also obs]v$ :5 f9e detail su* z black l9es are held close to ! eye = color$ fr+es are se54 ,?is optical defect is responsible = c]ta9 visual illusions4 ,an excell5t demon/ration ( *romatic ab]ration 9 ! eye is f\nd by view+ f9e detail ?r\gh a purple filt]4 ,n[ if a r$ filt] be sup]pos$ on ! purple "o only ! r$ light is transmitt$4 ,not)/&+ ! decrease 9 illum9ation or r ( light rea*+ ! eye1 measurem5t %[s t f9] detail c be discrim9at$ ?an 9 ! fir/ case4 ,a simil> result is f\nd on sup]pos+ a blue filt] ^u ! purple "o4 ,retir+ & ,advanc+ ,colors4--,= ye>s ! arti/ & ! decorator h felt t c]ta9 colors seem to advance ne>] ?an o!rs or t ! latt] seem to retire m ?an ! =m]4 ,! au?or#e obta9$ actual measurem5ts ( ?is ph5om5on1 b ! evid5ce also indicat$ t ! effects 7 n ! same = all p]sons4 ,! ph5om5on is v noticeable 9 ! case ( ! image ( a color$ lant]n-slide project$ ^u a scre5 & is readily obs]v$ :5 ! image consi/s ( lett]s ( v>i\s colors4 ,9 ! case ( r$ & gre5 lett]s1 = example1 ! =m] appe> 7to mo/ p]sons7 to be consid]ably ne>] ! obs]v] ?an ! gre5 lett]s4 ,x has appe>$ to ! writ] t ! illusion is app>5t ev5 = :ite lett]s ^u a d>k backgr\nd4 ,9 g5]al1 ! colors ^: dom9ant hues are ( ! %ort] wave-l5g?s 7violet1 blue1 blue-gre51 gre57 are retir+ & ^? ^: dom9ant hues are ( ! long] wave-l5g?s 7yell[1 orange1 r$7 are advanc+4 ,illu/ration3 ,fig4 #fh4--;,= demon/rat+ retir+ & advanc+ colors4 ,9 ord] to obta9 exp]im5tal measurem5ts two light-tight boxes1 ea* conta9+ a light-s\rce1 7 arrang$ to run indep5d5tly ^u tracks4 ,ov] ! front end ( ea* a diaphragm 0 plac$ s t ! obs]v] saw two *>act]s z 9 ,fig4 #fh4 ,a saturat$ r$ filt] 0 plac$ ov] "o & a saturat$ blue filt] ov] ! o!r4 ,9 a d>k room ! obs]v] saw a blue ,e & a r$ ,h /&+ out 9 ! d>kness4 ,"o ( ^! boxes 0 fa/5$ s z to be immovable & ! obs]v] mov$ ! o!r to & fro by means ( a cord ov] pulleys until ! two *>act]s appe>$ equi-di/ant f him4 ,?is 0 done = a s]ies ( di/ances ( ! /ation>y box f ! obs]v]'s eye4 ,ne>ly all ! obs]v]s 7)\t be+ acqua9t$ ) ! positions7 7 oblig$ to set ! r$ ,h fur!r beh9d ! blue ,e 9 ord] t bo? appe>$ at ! same di/ance4 ,?is add$ di/ance = ! r$ ,h 0 z mu* z #b4d feet :5 ! blue ,e 0 at a di/ance ( #bd feet4 ,9 o!r words ! diff]5ce 9 ! positions ( ! two 0 z mu* z #aj p] c5t ( ! total di/ance 9 ?is case4 ,_m o!r int]e/+ data 7 obta9$ b mo/ ( ^! are n p>ticul>ly ( int]e/ "h4 ,"s ( ! exp]im5ts t5d$ to %[ ! effect ( c]ta9 optical defects 9 ! eye & ! v>iations & ev5 rev]sal ( ! effect = "s p]sons 7 acc\nt$ = by diff]5ces 9 ! curvatures1 etc41 ( c]ta9 eye-m$ia = ! obs]v]s4 ,^! details are n ( int]e/ "h b x may be ( int]e/ to "k t ! ph5om5on may be acc\nt$ = by ! *romatic ab]ration 9 ! eye4 ,?is may n be ! true explanation1 or x may be only p>tially correct4 ,p]haps "s ( ! illusion is purely psy*ological 9 orig94 ,c]ta9ly ! illusion is v app>5t to mo/ c>eful obs]v]s4 ,color-s5sibility ( ! ,ret9a4--,?is aspect 0 t\*$ ^u 9 ,*apt] ,,iii1 b ! diff]5ces 9 ! s5sibility ( v>i\s areas ( ! ret9a to v>i\s colors are ( suffici5t importance to be discuss$ fur!r4 ,! ability to di/+ui% light & color gradually fades or decreases at ! p]iph]y ( ! visual field1 b ! actual areas ( ! fields ( p]ception v>y consid]ably1 dep5d+ ^u ! hue or spectral "* ( ! light rea*+ ! ret9a4 ,! extreme p]iph]al region ( ! visual field is color-bl9d2 t is1 color ceases to be p]ceiv$ be=e brightness-p]ception vani%es 9 ! outskirts ( ! visual field4 ,^! fields = v>i\s colors dep5d 9 size & cont\r n only ^u ! hue or spectral "* ( ! light-/imuli b also ^u ! int5sity & p]haps ^u ! size ( ! /imuli4 ,!re is "s disagreem5t z to ! relative sizes ( ^! fields b x appe>s t !y increase 9 size 9 ! foll[+ ord]3 gre51 r$1 blue1 :ite 7colorless74 ,! p]=mances ( aft]-images1 & ! rates ( gr[? & decay ( s5sation v>y = diff]5t colors & = diff]5t areas ( ! ret9a1 b x w\ld be t$i\s to p]use ! _m details ( ^! aspects ( vision4 ,!y are m5tion$ 9 ord] t ! read] may take !m into acc\nt 9 any specific case4 ,z already /at$1 ! c5tral "p ( ! visual field--! fovea ^u :i* we dep5d = acute vision--conta9s a yell[i% pigm5tation1 :i* is responsible = ! t]m yell[ spot4 ,?is op]ates z a yell[ filt] = ?is c5tral area & modifies ! appe>ance ( visual fields q ! same z if a simil> yell[ filt] 0 plac$ 9 ! c5tral position ( ! field ( vision4 ,! effect ( ! selectivity ( ! yell[ spot is noticeable 9 view+ c]ta9 colors4 ,purk9je ,effect4--,! relative s5sibility ( ! ret9a v>ies = diff]5t colors ) a *ange 9 brightness2 or x may be bett] to /ate t ! relative s5sations = v>i\s colors alt]s z ! brightness values are r$uc$ to a l[ int5sity4 ,= example1 if a r$di% purple 7consi/+ ( r$ & blue or violet rays7 be illum9at$ 9 su* a mann] t ! int5sity ( illum9ation1 & consequ5tly its brightness1 may be r$uc$ f normal to a l[ value 7approximat+ moonlight conditions71 x w be se5 to v>y f r$di% purple to violet4 ,9 do+ ?is its appe>ance *anges ?r\gh ! range ( purples f r$di% to violet4 ,?is c be accompli%$ by ori5tation ( ! purple surface ?r\gh\t v>i\s angles ) respect to ! direction ( light or by r$uc+ ! illum9ation by means ( scre5s4 ,9 g5]al ! ,purk9je effect may be describ$ z an increas+ s5sibility ( ! ret9a = light ( %ort] wave-l5g?s 7violet1 blue1 gre57 z ! brightness decreases1 or a correspond+ decreas+ s5sibility = light ( long] wave-l5g?s 7yell[1 orange1 r$74 ,! effect may be se5 on any color$ surfaces at twilight illum9ation4 ,a blue & a r$ fl[]1 :i* appe> ( ! same brightness be=e sunset w beg9 to appe> unequal 9 ?is respect z twilight deep5s4 ,! r$ w become d>k] m rapidly ?an ! blue if !re are no appreciable *anges 9 ! color ( ! daylight4 ,f9ally all color disappe>s4 ,x is bett] to p]=m ?is exp]im5t "u artificial light1 9 ord] t ! spectral "* ( ! illum9ant may be c]ta9 to rema9 con/ant4 ,9 ?is case rheo/ats mu/ n be us$ = dimm+ ! light because ( ! att5dant *anges 9 color or quality ( ! light4 ,! ,purk9je effect may be notic$ by ! c>eful obs]v] & x is responsible = c]ta9 illusions4 ,app>5tly x _c op]ate ov] "o portion ( ! ret9a1 :ile ! rema9d] is /imulat$ by normal int5sities ( light4 ,ret9al ,rivalry4--,_m curi\s effects may be obta9$ by /imulat+ ! two ret9as ) lights1 respectively diff]5t 9 color4 ,= example1 x is int]e/+ to place a blue glass be=e "o eye & a yell[ or r$ "o be=e ! o!r4 ,! two indep5d5t monocul> fields /rive = supremacy & ?is rivalry is q impressive4 ,= a mom5t ! :ole field may appe> ( "o color & !n sudd5ly x w appe> ( ! o!r color4 ,app>5tly ! fluctuation ( att5tion is a factor4 ,usually x does n seem to be possible to rea* a quiesc5t /ate or a p]fect mixture ( ! two colors 9 ?is mann]4 ,! dep5d5ce ( "o monocul> field ^u ! o!r1 & also _! indep5d5ce1 are emphasiz$ by ?is exp]im5t4 ,x is ( int]e/ to consid] ! illusions ( rev]sible p]spective & o!rs 9 ,*apt] ,v 9 ?is connection4 ,illu/ration3 ,fig4 #fi4--;,by comb9+ ^! /]eoscopically ! effect ( metallic lu/re 7simil> to graphite 9 ?is case7 is obta9$4 ,"o ( ! int]e/+ results ( ret9al rivalry is f\nd 9 comb9+ two /]eoscopic pictures 9 black & :ite ) ! black & :ite rev]s$ 9 "o ( !m4 ,! app>5tly solid object w appe> to possess lu/re4 ,! exp]im5t may be tri$ ) ,fig4 #fi by comb9+ ! two /]eoscopic pictures by conv]g+ or div]g+ ! axes ( ! eyes z describ$ 9 connection ) ,figs4 #b & #c4 ,x w be not$ t 9 ord] = two /]eoscopic pictures1 :5 comb9$1 to produce a p]fect effect ( ?ree dim5sions _! dissimil>ity mu/ be no m ?an t exi/+ betwe5 ! two views f ! two eyes respectively4 ,! dissimil>ity 9 ,fig4 #fi is correct z to p]spective1 b ! rev]sal ( :ite & black 9 "o ( !m produces an effect beyond t ( true ?ird dim5sion4 ,:5 ! colors are s arrang$ 9 su* pictures z to be q diff]5t 9 ! two ! effects are /rik+4 ,!re is1 9 su* cases1 an effect beyond t ( p]fect b9ocul> comb9ation4 ,by means ( ! /]eoscope x is possible to atta9 b9ocul> mixture ( colors b ?is is usually difficult to accompli%4 ,! difficulty decreases z ! brightness & saturation ( ! colors decrease & is less = colors :i* d n diff] mu* 9 hue & 9 brightness4 ,^! effects may be /udi$ at any mom5t1 = x is only necess>y to ?r[ ! eyes out ( focus = any object & to note ! results4 ,_m simple exp]im5ts may be arrang$ = a /]eoscope1 us+ black & :ite1 & v>i\s comb9ations ( colors4 ,= example1 ,fig4 #fe may be comb9$ by means ( d\ble images 7produc$ by conv]g+ or div]g+ ! optical axes7 s t ! two inn] squ>es are co9cid5t4 ,actual obs]vation is mu* m satisfactory ?an a detail$ description4 ,miscellane\s4--,!re are _m int]e/+ effects due to diffraction ( light by edges ( objects1 by me%es su* z a wire scre5 or a h&k]*ief1 by ! eye-m$ia1 etc4 ,on look+ at a v bright small light-s\rce x may be se5 to be surr\nd$ by _m colors4 ,/ream]s ( light appe> to radiate f brilliant s\rces & all bright areas color$ or colorless1 :5 view$ amid d>k surr\nd+s1 appe> to be surr\nd$ by diffuse bru%es ( light4 ,^! bru%es are likely to be ( a blui% t9t4 ,_m ( ^! ph5om5a are readily expla9$1 b ?is _c be done safely )\t kn[+ or recogniz+ all conditions4 ,_m are n easily expla9$1 especially :5 report$ by o!rs1 :o may n recognize c]ta9 important conditions4 ,= example1 au!ntic obs]v]s h report$ t black lett]s on :ite pap] appe>$ vivid r$ on a :ite backgr\nd1 "u c]ta9 conditions4 ,( ! latt]1 ! app>5tly important "o 0 sun's rays fall+ aslant ! =ehead4 ,:5 ! eyes 7 %ad$ ) ! h& ! lett]s imm$iately appe>$ black z !y %\ld4 ,! influ5ce ( ! color ( an object ^u its app>5t weight is relatively slight1 b !re is evid5ce ( a t5d5cy to judge a r$ or black object to be slightly heavi] ?an a yell[ or blue object ( ! same weight4 ,x appe>s t hue is a m9or factor 9 influ5c+ ! judgm5t & t !re is no correlation betwe5 ! affective quality ( a color & its influ5ce ^u app>5t weight4 ,al?\gh ! scanty evid5ce available attributes b a slight influ5ce to color 9 ?is respect1 x is ( int]e/ 9 pass+ z a rem9d] ( ! _m subtle factors :i* are at "w modify+ our judgm5ts4 ,x ,,light+ ,x %\ld be obvi\s by ?is "t t ! light+ ( objects or ( a sc5e c alone produce an illusion1 & t x c 9 /ill m cases contribute t[>d an illusion4 ,fur!rmore1 !re are _m cases ( illusions 9 light+ due to brightness & color4 ,_m effects ( light+ h be5 describ$ else:]e ) detail$ analyses ( ! und]ly+ pr9ciples1 b a cond5s$ survey apply+ p>ticul>ly to illusions w be pres5t$ "h4 ,! comp>ison ( intaglio ) l[ relief has be5 m5tion$ sev]al times 9 prec$+ *apt]s4 ,examples ( ^! z relat$ to light+ are f\nd 9 ,figs4 #gj to #gc4 ,fig4 #gj repres5ts a bas-relief light$ f above & ,fig4 #ga w\ld ord9>ily be tak5 to repres5t a bas-relief light$ f bel[4 ,h[ev]1 ! latt] 0 made f a photograph ( ! mold 7intaglio7 f :i* ! bas-relief 0 made & ,fig4 #ga really repres5ts an intaglio light$ f above4 ,simil>ly ,fig4 #gb repres5ts ! bas-relief light$ f ! left & ,fig4 #gc ord9>ily w\ld be tak5 to be a bas-relief light$ f ! "r4 ,h[ev]1 ,fig4 #gc 0 made f a photograph ( an intaglio light$ f ! left4 ,^! amply demon/rate ! effect ( light+ z an influ5ce ^u ! appe>ance ( objects & !y indicate ! importance ( correct assumptions 9 arriv+ at a correct judgm5t4 ,9 ^! cases ! concealm5t ( ! light-s\rce & ! commonness ( bas-relief z comp>$ ) intaglio are ! causes = ! illusion or ! error 9 judgm5t4 ,c]ta9ly 9 ^! cases ! visual s5se deliv]s its data correctly4 ,illu/ration3 ,fig4 #gj4--;,a bas-relief light$ f above4 ,illu/ration3 ,fig4 #ga4--;,an intaglio light$ f above4 ,illu/ration3 ,fig4 #gb4--;,a bas-relief light$ f ! left4 ,illu/ration3 ,fig4 #gc4--;,an intaglio light$ f ! left4 ,illu/ration3 ,fig4 #gd4--;a4 ,a disk 7above7 & a sph]e 7bel[7 light$ f ov]head4 b4 ,a disk & a sph]e light$ by p]fectly diffus$ light4 ,9 ,fig4 #gd ! upp] object is a disk & ! l[] is a sph]e4 ,9 a ,fig4 #gd ! light+ is due to a s\rce ( light ( r small physical dim5sions directly above ! objects4 ,! same objects illum9at$ by means ( highly diffus$ light 7t is1 light f _m directions & ( uni=m int5sity7 appe> z 9 b4 ,bo? objects n[ appe> z disks4 ,x is obvi\s t "u appropriate light+ a disk might be tak5 = a sph]e & vice v]sa1 dep5d+ ^u :i* dom9ates ! judgm5t or ^u ! =mulation ( ! att5dant assumptions4 ,incid5tally an appe>ance q simil> to t ( a1 ,fig4 #gd is obta9$ :5 ! light-s\rce is ne> ! obs]v]2 t is1 :5 x lies ne> ! l9e ( sight4 ,illu/ration3 ,fig4 #ge4--;,a concave hemisph]ical cup on ! left & a convex hemisph]e on ! "r light$ by a light-s\rce ( l>ge angle su* z a w9d[4 ,some:at simil> to ! confusion ( intaglio ) bas-relief is ! confusion ( ! two hemisph]ical objects illu/rat$ 9 ,fig4 #ge4 ,! "o on ! left is concave t[>d ! obs]v]4 ,9 o!r words1 bo? c\ld be hemisph]ical %ells--"o a mold = ! o!r4 ,"u ! light+ :i* exi/$ :5 ! orig9al photographs 7 made !y c\ld bo? be tak5 = hemisph]es4 ,! light+ 0 due to a l>ge light-s\rce at ! left1 b if ! object on ! left is assum$ 7incorrectly7 to be a hemisph]e convex t[>d ! obs]v] or a sph]e1 x mu/ be consid]$ to be light$ f ! "r1 :i* is also an incorrect assumption4 ,obvi\sly1 if ! direction ( ! dom9ant light is cle> to ! obs]v]1 he is n likely to make ! error 9 judgm5t4 ,incid5tally ! object on ! "r might be assum$ to be a sph]e because a sph]e is m commonly enc\nt]$ ?an a hemisph]e4 ,illu/ration3 ,fig4 #gf4--,! same z ,fig4 #ge1 b light$ by a v small light-s\rce4 ,! same objects are repres5t$ 9 ,fig4 #gf light$ f ! left by means ( a light-s\rce ( relatively small dim5sions2 t is1 a s\rce subt5d+ a relatively small solid-angle at ! objects4 ,9 ?is case ! %>p %ad[ due to ! edge ( ! hemisph]ical cup 7on ! left7 is likely to cause ! obs]v] to inquire fur!r be=e submitt+ 8 judgm5t4 ,! m gradual modulation ( light & %ade z 9 ! case ( a sph]e or a hemisph]e convex t[>d ! obs]v] is n pres5t 9 ! case ( ! cup4 ,?is %\ld be suffici5t in=mation = ! c>eful obs]v] to guide him1 or at lea/ to prev5t him f arriv+ at ! def9ite conclusion t ! left-h& object is a hemisph]e ) its convex side t[>d him4 ,fur!rmore x %\ld be not$ t we (t5 jump at ! conclusion t an object is a sph]e ev5 ?\gh we see ) "o eye practically only a hemisph]e & ) two eyes h>dly en\gh m to ju/ify su* a conclusion4 ,h[ev]1 sph]es are m commonly enc\nt]$ ?an hemisph]es1 s we take a *ance )\t really admitt+ or ev5 recogniz+ t we d4 ,! =ego+ figures illu/rate sev]al phases :i* influ5ce our judgm5ts & ! wond] is t we d n make m errors ?an we d4 ,( c\rse1 exp]i5ce plays a l>ge "p & =tunately exp]i5ce c be dep5d$ ^u 9 mo/ cases2 h[ev]1 9 ! o!r cases x leads u a/ray to a great] ext5t ?an if we _h less ( x4 ,! photograph]1 p]haps1 recognizes m ?an anyone else ! pitfalls ( light+ b x is un=tunate t he is n bett] acqua9t$ ) ! fundam5tals und]ly+ ! control ( light4 ,improp] light+ does produce app>5t incongru\s effects b adequately controll$ x is a p[]ful m$ium ^: pot5tiality has n be5 fully realiz$4 ,! photograph] aims to illum9ate & to pose ! subject ) respect to ! s\rce or s\rces ( light s t undesirable features are suppress$ & desirable results are obta9$4 ,f9ally 8 "w mu/ be accept$ by o!rs & ! latt]1 be+ human1 possess 7unadmitt$ly ( c\rse7 a desire to be good look+4 ,light+ may be a p[]ful flatt]] :5 well controll$ & may be a base reveal] or ev5 a creator ( ugl9ess4 ,incid5tally1 ! photograph] is always "u ! h&icap ( supply+ a lik5ess to an individual :o p]haps nev] sees ?is same lik5ess 9 a mirror4 ,9 o!r words1 ! image :i* a p]son sees ( himself 9 a mirror is n ! same 9 g5]al t ! photograph] supplies him 9 ! photographic portrait4 ,! portrait c be a true lik5ess b ! mirror$ image 9 g5]al _c be4 ,9 ! mirror !re is a rev]sal ( ! p>ts f "r to left4 ,= example1 a sc> on ! "r *eek ( ! actual face appe>s on ! left *eek 9 ! mirror4 ,faces are n usually symmetrical & ?is rev]sal causes an individual to be famili> ) 8 own facial *>act]i/ics 9 ?is rev]s$ =m4 ,?is influ5ce is v m>k$ 9 "s cases4 ,= example1 suppose ! left side ( a companion's face to be some:at p>alyz$ on "o side due to illness4 ,we h become m or less oblivi\s to ! alt]$ expression ( ! left side by see+ x s (t54 ,h[ev]1 if we cat* a glimpse ( ?is companion's face 9 ! mirror & ! alt]$ expression ( ! left side n[ appe>s ^u ! "r side ( ! face1 ! contra/ makes ! fact v conspicu\s4 ,p]haps ?is acc\nts = ! diff]5ce :i* exi/s betwe5 ! op9ions ( ! photograph] 7or fri5ds7 & ( ! subject ( ! portrait4 ,all ! illusions ( brightness-contra/ may be produc$ by light+4 ,surfaces & details may appe> l>g] or small]1 h>% or almo/ oblit]at$1 heavy or light2 9 fact1 light+ plays an important "p 9 influ5c+ ! mood or expression ( a room4 ,a ceil+ may be lift$ by light or x may hang l[ & ?reat5+ :5 d>k1 due to relatively little light rea*+ x4 ,columns may appe> d>k on a light backgr\nd or vice v]sa1 & ^! illu/rate ! effects ( irradiation4 ,a giv5 room may be giv5 a v>iety ( moods or expressions by v>y+ ! light+ & inasmu* z ! room & its physical *>act]i/ics h n be5 alt]$1 ! v>i\s moods may be consid]$ to be illusions4 ,x %\ld be obvi\s t light+ is a pot5t factor4 ,9 connection ) light+ x %\ld be not$ t contra/s play a prom95t role z !y always d4 ,^! h be5 discuss$ 9 o!r *apt]s1 b x appe>s advantage\s to recall "s ( ! *ief features4 ,! effect ( contra/ is always 9 ! direction ( /ill great] contra/4 ,t is1 black t5ds to make its surr\nd+s :ite2 r$ t5ds to make its surr\nd+s blue-gre5 7complem5t>y71 etc4 ,! contra/-effect is greate/ :5 ! two surfaces are juxtapos$ & ! elim9ation ( b\nd>y l9es ( o!r colors 7includ+ black or :ite7 increases its magnitude4 ,! contra/-effect ( colors is mo/ conspicu\s :5 !re is no brightness-contra/1 t is1 :5 ! two surfaces are ( equal brightness & !re=e diff] *iefly 9 hue4 ,?is effect is also greate/ = saturat$ colors4 ,x has be5 /at$ t cold colors produce /rong] contra/-effects ?an w>m colors1 b exp]im5tal evid5ce is n suffici5tly pl5tiful & dep5dable to v]ify ?is /atem5t4 ,z ! int5sity ( illum9ation increases1 colors appe> to become less saturat$4 ,= example1 a pure r$ object "u ! noonday sun is likely to be pa9t$ an orange r$ by ! arti/ because x does n appe> z saturat$ z x w\ld "u a mu* l[] int5sity ( illum9ation4 ,9 g5]al1 black & :ite are ! f9al appe>ances ( colors = respectively v l[ & v high brightness4 ,z ! int5sity ( illum9ation decreases1 hue f9ally disappe>s & ) cont9u$ decrease ! color approa*es black4 ,conv]sely1 z ! int5sity ( illum9ation increases1 a color becomes app>5tly less & less saturat$ & t5ds t[>d :ite4 ,= example1 on view+ ! sun ?r\gh a color$ glass ! sun appe>s ( a mu* less saturat$ color ?an ! haze ne> ! sun or a :ite object illum9at$ by sunlight4 ,visual adaptation also plays a prom95t "p1 & x may be /at$ t all s5sations ( light t5d t[>d a middle gray & all s5sations ( color t5d t[>d neutrality or a complete disappe>ance ( hue4 ,! t5d5cy ( s5sations ( light t[>d a middle gray is n z easily recogniz$ z *anges 9 color b v>i\s facts support ?is conclusion4 ,9 light+ x is important to recognize ! t5d5cy ( color t[>d neutrality4 ,= example1 a w>m yell[ light soon disappe>s z a hue & only its subtle influ5ce is left2 h[ev]1 a yell[ vase /ill appe>s yell[ because x is contra/$ ) objects ( o!r colors4 ,9 ! case ( color$ light ! light falls ^u ev]y?+ visible1 & if !re is no o!r light-s\rce ( ano!r color ) :i* to contra/ x1 its color appe>s gradually to fade4 ,?is is an excell5t example ( ! trem5d\s p[] & importance ( contra/4 ,x is ! life ( color & x mu/ be fully appreciat$ if ! pot5tiality ( light+ is to be drawn ^u z x %\ld be4 ,physical measurem5ts are z ess5tial 9 light+ z 9 o!r phases ( human endeavor = =m+ a solid f\ndation1 b 9 all ^! activities :]e visual p]ception plays an important "p judgm5t is f9ally ! means = appraisal4 ,:]ev] ! psy*ological aspect is prom95t physical measurem5ts are likely to be mislead+ if !y d n agree ) m5tal appraisals4 ,( c\rse ! physical measurem5ts %\ld be made & accumulat$ b !y %\ld be consid]$ n alone b 9 connection ) psy*ological effects4 ,! photomet] may %[ a v adequate int5sity ( illum9ation2 nev]!less see+ may be unsatisfactory or ev5 impossible4 ,an illum9ation ( a few foot-c&les "u prop] conditions at a giv5 surface is q adequate = read+2 h[ev]1 ?is surface may appe> q d>k if ! surr\nd+s are bright en\gh4 ,9 su* a case ! photomet] yield$ results q likely to be mis9t]pret$ z satisfactory4 ,x %\ld be obvi\s t _m illusions discuss$ 9 prec$+ *apt]s are ( int]e/ 9 ?is connection4 ,an int]e/+ example ( ! illusion ( color may be easily demon/rat$ by means ( a yell[ filt]4 ,= ?is purpose a can>y glass is q satisfactory4 ,:5 su* a filt] is plac$ be=e ! eyes a daytime sc5e outdoors1 = example1 is likely to appe> to be illum9at$ to a great] int5sity ?an :5 ! eyes are n look+ ?r\gh ! filt]4 ,?is is true = a glass us$ by ! au?or not)/&+ ! fact t ! filt] transmits only ab\t "o-half z mu* light z a p]fectly cle> colorless glass4 ,9 o!r words1 ! brightnesses ( objects 9 ! sc5e are r$uc$ on ! av]age ab\t fifty p] c5t1 /ill ! subject is impress$ ) an app>5t increase 9 ! int5sity ( illum9ation 7& 9 brightness7 :5 ! filt] is plac$ be=e ! eyes4 ,( c\rse1 ! actual r$uction 9 brightness dep5ds ^u ! color ( ! object4 ,9 su* a case z ! =ego+1 true explanations are likely to involve _m factors4 ,= ?is reason explanations are usually t$i\s if !y are to be suffici5tly qualifi$ to be reasonably ne> complet5ess4 ,9 ?is case x appe>s t ! yell[ filt] may cause "o to appraise ! int5sity ( illum9ation z hav+ increas$1 by associat+ su* an influ5ce z ! sun com+ out f beh9d a cl\d4 ,if we look into ! dep?s :]e light & color accumulat$ _! psy*ological p[]s1 we are confront$ on e h& by associations _m ( :i* are m or less obscure1 & !re=e are subtly influ5tial4 ,! psy*ological p[]s ( colors c\ld h be5 discuss$ m g5]ally 9 ! prec$+ *apt]1 b inasmu* z !y c be demon/rat$ m effectively by light+ 7& aft] all ! effect is "o ( light 9 any case7 !y w be discuss$ briefly "h4 ,!y h be5 pres5t$ m at l5g? else:]e4 ,x is well kn[n t ! arti/1 decorator1 & o!rs speak ( w>m & cold colors1 & ^! effects h a firm psy*ological f\ndation4 ,= example1 if a c]ta9 room be illum9at$ by means ( blue light1 x does seem cold]4 ,a !at] illum9at$ by means ( blui% light seems consid]ably cool] to ! audi5ce ?an is indicat$ by ! !rmomet]4 ,if ?is light+ is resort$ to 9 ! summ] "t ! !at] w be m invit+ &1 aft] all1 9 su* a case x makes little diff]5ce :at ! !rmomet] indicates4 ,! cold light has produc$ an illusion ( coolness4 ,simil>ly w>m light1 su* z yell[ or orange1 is responsible = ! opposite feel+ & x is easily demon/rat$ t an illusion ( high] temp]ature may be produc$ by its use4 ,z already /at$1 color-s*emes 9 ! decorations & furni%+s produce simil> effects b 9 g5]al !y are m p[]ful :5 ! prim>y light is color$4 ,9 ! latt] case no object is ov]look$ = ev5 ! h&s & faces ( ! be+s 9 ! room are color$ by ! light4 ,9 ! case ( color-s*emes n all objects are t+$ ) ! desir$ w>m or cold color4 ,9 ! =ego+1 associations play a prom95t role4 ,! sky has be5 blue ?r\gh\t ! numb]less c5turies dur+ :i* ! human organism evolv$4 ,! blue-sky dur+ all ^! c5turies has t+$ ! %ad[s outdoors a blui% color4 ,t %ade is relatively cool we "k by exp]i5ce & p]haps we associate coolness or cold ) ! a]ial realm4 ,^! are glimpses ( influ5ces :i* h cooep]at$ t[>d creat+ ! psy*ological effect ( coldness 9 ! case ( blui% light4 ,by contra/ ) skylight1 sunlight is yell[i%1 & a place 9 ! sun is relatively w>m4 ,s\? rooms are usually w>m] ?an nor? rooms 9 ?is hemisph]e :5 artificial heat is abs5t & ! psy*ological effect ( w>m? has naturally gr[n out ( ^! & simil> influ5ces4 ,we c\ld g fur!r into ! psy*ology ( light & color & conjecture reg>d+ effects directly attributable to color1 su* z excitem5t1 depression1 & tranquillity4 ,9 s do+ we w\ld be l$ f> a/ray f illusions 9 ! s5se ( ! t]m z us$ "h4 ,al?\gh ?is t]m z us$ "h is /ill some:at re/rict$1 x is broad] 9 scope ?an 9 its usual applications4 ,h[ev]1 x is n broad en\gh to lead f> into ! _m devi\s highways & byways ( light & color4 ,if we did make ^! excursions we w\ld f9d associations almo/ univ]sally answ]+ ! que/ions4 ,! "q w\ld arise z to innate p[]s ( colors & we w\ld f9d ourselves wond]+ if all ^! p[]s 7 acquir$ 7?r\gh associations7 & :e!r or n "s 7 innate4 ,& aft] _m int]e/+ views ( ! intricate subject we w\ld likely conclude t ! "q ( ! innat5ess ( "s ( ! p[]s ( color mu/ be left unansw]$4 ,z an example let u take ! case ( ! re/fulness or depression due to blue4 ,we note t ! blue sky is q s]5e or tranquil & we f9d t ! delicate s5sibilities ( poets v]ify ?is impression4 ,?is association c\ld acc\nt = ! impression or feel+ ( tranquillity associat$ ) blue4 ,on proce$+ fur!r1 we w\ld f9d nature's solitudes (t5 t+$ ) ! blue skylight1 = ^! solitudes are usually 9 ! %ade4 ,?us _! re/fulness or ev5 depressiv5ess may be acc\nt$ =--p>tially at lea/4 ,^! brief glimpses are pres5t$ 9 ord] t !y may sugge/ to ! read] ano!r tr5d ( ?\ght :5 c]ta9 illusions ( light & color are held up = analysis4 ,besides ^! our individual exp]i5ces :i* h mold$ our likes & dislikes mu/ be tak5 into acc\nt4 ,?is phase ( light & color has be5 treat$ else:]e4#f ,a v unusual k9d ( optical illusion is illu/rat$ by ! ph5om5on ( ! app>5t end+ ( a se>*light beam :i* has attract$ mu* att5tion 9 connection ) ! p[]ful se>*lights us$ = locat+ a]oplanes 7,fig4 #gg74 ,= ye>s ! app>5t end+ has m or less c>elessly be5 attribut$ to ! dim9ution ( ! d5sity ( atmosph]ic fog or haze1 b rec5tly ,k>r]#ac has sugge/$ :at appe>s to be ! correct explanation4 ,:5 ! beam ( light f a p[]ful se>*light is direct$ into space1 its pa? is visible ow+ to ! scatt]+ ( "s ( ! light by du/ & moi/ure p>ticles & ! molecules ( ! air itself4 ,:ile obvi\sly ! beam itself mu/ g on indef9itely1 its lum9\s pa? appe>s to end abruptly at no v great di/ance f ! s\rce4 ,?is is true :e!r ! beam is photograph$ or view$ ) ! nak$ eye4 ,illu/ration3 ,fig4 #gg4--;,app>5t end+ ( a se>*light beam4 ,! fact t ! appe>ance ( ! beam is no diff]5t :5 x is direct$ horizontally ?an :5 direct$ v]tically prov$ t ! common assumption p]ta9+ to ! end+ ( ! haze or fog is unt5able4 ,fur!rmore1 photometric measurem5ts on ! diff]5t portions ( ! beam z se5 f a position ne> ! se>*light %[ t ! beam is actually bright] at its out] t]m9ation ?an ne> its orig94 ,aga91 ! app>5t l5g? ( ! beam v>ies ) ! position ( ! obs]v]1 & be>s a direct ratio to 8 di/ance f ! se>*light4 ,! fact is1 t ! lum9\s pa? ( ! beam has no def9ite end+1 & ext5ds to a v great di/ance--practically to inf9ity4 ,x appe>s to be %>ply cut (f = ! same reason t ! b\nd>y betwe5 e>? & sky 9 a flat l&scape is a %>p l9e4 ,j z ! horizon rec$es :5 ! l&scape is view$ f an elevation1 s ! beam appe>s long] :5 "o's di/ance f x is increas$4 ,! out] portion appe>s bright]1 because "h ! l9e ( sight pi]ces x to great dep?4 ,t ! end+ ( ! beam appe>s close at h& is no d\bt p>tly due to ! brightness di/ribution1 b is also a matt] ( p]spective aris+ f ! mann] 9 :i* ! beam is adju/$4 ,se>*light op]ators 9 ! army 7 in/ruct$ to adju/ ! light to ?r[ a p>allel beam4 ,accord+ly1 ! adju/m5ts 7 s made t ! beam appe>$ ! same wid? at its out] extremity z at its base4 ,! result seems to be a %ort p>allel %aft ( light1 b is really a div]g5t cone ( inf9ite ext5t1 its angle ( div]g5ce be+ su* z exactly to (fset ! effects ( p]spective4 ,if ! beam 7 a truly p>allel "o x w\ld seem to come to a po9t1 j z ! edges ( a long /raight /ret* ( c\ntry road seem to meet at ! horizon4 ,if ! sides ( ! road 7 n p>allel1 b div]g$ f ! obs]v]'s eye at exactly ! rate at :i* !y ord9>ily w\ld appe> to conv]ge1 !n ! road w\ld seem to be z wide :]e x pass$ out at ! horizon z at ! obs]v]'s feet4 ,if !re 7 no o!r means 9 ! l&scape ( judg+ ! di/ance ( ! horizon ?an by ! p]spective af=d$ by ! road1 x w\ld likely be inf]r$ t ! road only ext5d$ a %ort di/ance on ! level1 & !n w5t d[n a hill1 t is1 pass$ abruptly f ! obs]v]'s view4 ,^! conditions obta9 ideally 9 ! case ( ! se>*light beam4 ,!re is no o!r means ( judg+ ! position 9 space ( ! end ( an unob/ruct$ se>*light beam ?an by ! p]spective ( ! beam itself1 & ! op]ator 9 adju/+ x to appe> p>allel elim9ates ! p]spective4 ,! angle at :i* ! beam mu/ div]ge to appe> p>allel to an obs]v] dep5ds ^u ! di/ance ( ! obs]v] f ! se>*light4 ,a beam :i* seems p>allel to a p]son close to x w n appe> s at a di/ance4 ,?is fact probably acc\nts = ! difficulties enc\nt]$ dur+ se>*light drill 9 ! army 9 gett+ a beam :i* satisfi$ bo? ! private op]at+ ! lamp & ! (fic] d[n ! field z to its p>allelity4 ,to summ>ize1 ! app>5t abrupt end+ ( a se>*light beam is purely an optical illusion4 ,x really has no end+2 x ext5ds to inf9ity4 ,,xi ,,nature ,visual illusions ab\nd ev]y:]e1 & !re are a numb] ( special int]e/ 9 nature4 ,inasmu* z ^! are repres5tative ( a wide range ( conditions & are usually )9 ! possible exp]i5ce ( ne>ly ev]yone daily1 !y appe> wor?y ( special consid]ation4 ,"s ( ^! h be5 casually m5tion$ 9 o!r *apt]s b fur!r data may be ( int]e/4 ,no agreem5t has be5 rea*$ 9 "s cases 9 ! _m sugge/$ explanations & little or no attempt ( ?is "* w be made 9 ! foll[+ p>agraphs4 ,_m illusions :i* may be se5 9 nature w be pass$ by because _! exi/5ce %\ld be obvi\s aft] read+ ! prec$+ *apt]s4 ,= example1 a tree appe>s long] :5 /&+ ?an aft] x has be5 fell$ = ! same reason t we ov]e/imate v]tical l9es 9 comp>ison ) horizontal ones4 ,! app>5t movem5t ( ! sun1 moon1 & />s1 :5 cl\ds are float+ pa/1 is a p[]ful1 ?\gh commonplace1 illusion b we are m specifically int]e/$ 9 /atic illusions4 ,h[ev]1 x is ( int]e/ to recall ! effect ( involunt>y eye-movem5ts or ( fluctuation 9 fixation because ?is factor 9 vision is important 9 _m illusions4 ,x is demon/rat$ by ly+ face upw>d on a />lit night & fix+ ! gaze ^u a />4 ,! latt] appe>s to move m or less j]kily ov] its d>k backgr\nd4 ,! magnitude & involunt>y nature ( ^! eye-movem5ts is demon/rat$ 9 ?is mann] v effectively4 ,! effect sometimes kn[n z a]ial p]spective has be5 m5tion$ h]et(ore4 ,! atmosph]e is n p]fectly transp>5t or colorless & is n homog5e\s f an optical /&po9t4 ,x scatt]s rays ( ! %ort] wave-l5g?s m ?an ^? ( ! long] wave-l5g?s4 ,h5ce x appe>s ( a blui% t9t & any?+ se5 ?r\gh great di/ances ( x t5ds t[>d a r$di% color4 ,! blue sky & ! r$ness ( ! sett+ sun are results ( ?is cause4 ,di/ant signal-lights are r$d5$1 due to ! decrease 9 ! rays ( %ort] wave-l5g? by scatt]+4 ,app>5tly we h come to e/imate di/ance to "s ext5t ?r\gh ! am\nt ( blurr+ & t9t+ sup]pos$ ^u ! di/ant sc5e4 ,9 ! high ,rockies :]e ! atmosph]e is unusually cle>1 /ret*es ( fifty miles ( atmosph]e ly+ betwe5 ! obs]v] & ! di/ant peaks w %[ v little haze4 ,a p]son inexp]i5c$ 9 ! region is likely to con/rue ?is abs5ce ( haze z a %ort] di/ance ?an ! reality & _m amus+ incid5ts & ludicr\s mi/akes are *>g$ aga9/ ! t5d]foot 9 ! ,rockies4 ,aft] misjudg+ di/ance s (t5 to 8 own discomfiture a t\ri/ is said to h be5 f\nd disrob+ prep>atory to swimm+ across an irrigation dit*4 ,he _h lo/ confid5ce 9 8 judgm5t ( di/ance & 0 go+ to assume ! risk ( jump+ across :at appe>$ to be a dit* b :at might be a broad riv]4 ,( c\rse1 ?is /ory might n be true b x s]ves z well z any to emphasize ! illusion :i* arises :5 ! famili> haze is n pres5t 9 /range t]ritory4 ,x is a common exp]i5ce t ?+s loom 9 a fog1 t is1 t !y appe> l>g] ?an !y really are4 ,an explanation :i* has be5 (f]$ is t ( an excess ( a]ial p]spective :i* causes u to ov]e/imate di/ance & !re=e to ov]e/imate size4 ,if ?is explanation is correct1 x is q 9 ! same mann] t 9 cle> atmosph]e 9 ! m\nta9s we und]e/imate di/ance &1 consequ5tly1 size4 ,h[ev]1 ano!r factor may ent] 9 ! latt] case1 = ! illusion is conf9$ *iefly to newcom]s2 t is1 9 "t "o le>ns to judge correctly4 ,on ent]+ a region ( real m\nta9s ! fir/ "t1 ! newcom]'s previ\s exp]i5ce ) ^! =mations is conf9$ to hills relatively mu* small]4 ,ev5 all[+ consid]ably = a great] size :5 view+ ! maje/ic peaks = ! fir/ "t1 he _c be expect$ to ?9k 9 t]ms ( peaks _m times l>g] ?an 8 famili> hills4 ,?us und]e/imat+ ! size ( ! great peaks1 he und]e/imates ! di/ance4 ,! r>ity ( ! atmosph]ic haze aids him 9 mak+ ?is mi/ake4 ,?is is n (f]$ z a sub/itute = a]ial p]spective z ! prim>y cause ( ! illusion b x appe>s to ! au?or t x is a cause :i* mu/ be tak5 into acc\nt4 ,! app>5t =m ( ! sky has attract$ ! att5tion ( _m sci5tific inve/igators = c5turies4 ,!re are _m conflict+ op9ions z to ! causes ( ?is appe>ance ( =m1 b !re is g5]al agreem5t t ! sky appe>s usually z a flatt5$ vault4 ,! sky is bright1 due to scatt]+ ( light by actual p>ticles ( solid matt] & moi/ure & possibly by molecules ( gas4 ,lack ( optical homog5eity due to v>y+ refractive index is likely to be p>tially responsible4 ,usually a prom95t lay] ( haze ab\t a mile 9 ?ickness 7al?\gh ?is v>ies consid]ably7 lies next to ! e>?'s surface4 ,! top ( ?is haze is fairly well def9$ z a]ial travel]s "k1 b ! sky above is /ill f> f black1 indicat+ scatt]$ light & illum9at$ p>ticles /ill high]4 ,z "o cont9ues to asc5d1 !reby leav+ m & m ( ! lum9\s haze beh9d1 ! sky becomes d>k] & d>k]4 ,(t5 at altitudes ( f\r or five miles ! sky is v d>k & ! sun is pi]c+ly bright4 ,usually !re is little or no bright haze adjac5t to ! sun at ^! high altitudes z is commonly se5 f ! e>?'s surface4 ,at ^! high altitudes ! au?or is n consci\s ( a flatt5$ vault z at ! e>?'s surface b ! illusion ( a hemisph]ical dome /ill p]si/s4 ,!re is "s agreem5t t ! dome ( ! sky appe>s less depress$ at ! z5i? by night ?an by day4 ,?is is 9 accord ) ! au?or's obs]vation at v high altitudes on occasions :5 ! sky 0 mu* d>k] ?an :5 view$ f ! e>?'s surface4 ,demb] & ,uibe assum$ ! app>5t %ape z a "p ( a sph]e 7ju/ify+ ?is assumption to _! satisfaction7 & obta9$ e/imates ( ! app>5t depression at ! z5i?4 ,!y e/imat$ ! middle po9t ( ! arc f ! z5i? to ! horizon & !n measur$ ! angul> altitude ( t po9t4 ,!y f\nd t ! degree ( cle>ness ( ! sky has consid]able influ5ce ^u ! app>5t height & !y /ate t ! sky appe>s high] 9 ! sub-tropics ?an 9 ,g]many4 ,on v cle> moonless nights !y f\nd t ! %ape ( ! sky-dome diff]s little f t ( a hemisph]e4 ,!y conclud$ t ! ph5om5on is app>5tly due to optical conditions ( ! atmosph]e :i* h n be5 det]m9$4 ,x is ( int]e/ to note ! appe>ance ( ! sky :5 cumulus cl\ds are pres5t4 ,! bases ( ^! v>y 9 height1 b are f\nd at altitudes f ?ree to five ?\s& feet4 ,!y appe> to =m a flat ro( ( cl\ds b5d+ d[nw>d at ! horizon1 ?us giv+ ! appe>ance ( a vault$ b flatt5$ dome4 ,?is app>5t %ape does n diff] mu* 9 cle> wea!r1 p]haps due l>gely to ! accu/om$ness ( ! eye & to ! degradation ( color f blue to gray t[>d ! horizon4 ,fur!rmore ! l[] sky is usually mu* bright] ?an ! z5i? & ! latt] be+ d>k] appe>s to hang l[]4 ,x is ( int]e/ to note h[ p]si/5t is ! illusion ( a flatt5$ dome1 = :5 "o rises rapidly 9 ! air &1 )9 a few m9utes1 is on ! level ) ! cl\ds or ! d5se l[-ly+ haze1 he is mildly surpris$ to f9d ^! are levels & n vault$ ro(s4 ,despite ! fact t by _m previ\s exp]i5ces he has le>n$ :at to expect1 ! feel+ ( mild surprise is born ea* "t on asc5d+ rapidly4 ,! appe>ance ( ! flatt5$ vault ( ! sky is held by "s to acc\nt = ! app>5t enl>gem5t ( ! sun1 moon1 & ! con/ellations at ! horizon4 ,t is1 !y appe> m di/ant at ! horizon & we in/9ctively appraise !m z be+ l>g] ?an :5 !y are at high] altitudes4 ,x is c]ta9 t ^! heav5ly bodies d appe> mu* l>g] :5 !y are ris+ or sett+ ?an :5 !y are ne>] ! z5i?4 ,9 fact1 ?is is "o ( ! mo/ rem>kable & surpris+ illusions :i* exi/4 ,fur!rmore ?is app>5t enl>gem5t has be5 not$ univ]sally1 /ill _m p]sons h attribut$ x to an actual optical magnification4 ,al?\gh we are m famili> ) ?is enl>gem5t 9 connection ) ! sun & moon1 x /ill p]si/s ) ! con/ellations4 ,= example1 ,orion is app>5tly v l>ge2 9 fact1 ?is is ! orig9 ( ! "n4 ,t ?is enl>gem5t is an illusion c be %[n 9 sev]al ways b t x is solely due to ! influ5ce ( ! app>5t flatt5$ =m ( ! sky may be d\bt$4 ,c]ta9ly ! moon appe>s greatly enl>g$ :ile ne> ! horizon1 ev5 :5 !re is d\bt z to an appreciable appe>ance ( flatt5+ ( ! sky-dome4 ,_m peculi> conditions & prejudices mu/ be tak5 into acc\nt4 ,= example1 if v>i\s p]sons are ask$ to give an idea ( h[ l>ge is ! disk ( ! sun or moon1 _! answ]s w\ld v>y usually ) ! head ( a b>rel z ! maximum4 ,h[ev]1 ! size ( a tree at a di/ant sky-l9e might unhesitat+ly be giv5 z ?irty feet4 ,at ! horizon we in/9ctively comp>e ! size ( ! sun1 moon1 & con/ellations ) hills1 trees1 h\ses1 & o!r objects1 b :5 ! =m] are high t[>d ! z5i? 9 ! empty sky we may judge !m 9 _! isolat$ position to be ne>]1 h5ce small]4 ,normally ! ret9al image gr[s l>g] z ! object approa*es1 b ?is same s5sation also arises :5 an object gr[s 9 size )\t alt]+ its di/ance4 ,if ! moon be view$ ?r\gh field-glasses ! image is l>g] ?an 9 ! case ( ! unaid$ eyes1 b x is q common = obs]v]s to /ate t x appe>s small]4 ,! enl>gem5t may be int]pret$ z approa* & inasmu* z we1 ?r\gh habit1 all[ = enl>gem5t z an object approa*es1 we also mu/ r$uce x 9 our imag9ation to its natural size4 ,p]haps 9 ?is case we ov]do ?is r$uction4 ,james /ates t ! increas$ app>5t size ( ! moon ne> ! horizon is a result ( association & probability4 ,x is se5 ?r\gh vapor\s air & looks dimm] & duski] ?an :5 x rides on high2 & x is se5 ov] fields1 trees1 h$ges1 /reams1 & ! l1 :i* break up ! int]v5+ space & makes u ! bett] realize ! latt]'s ext5t4 ,bo? ^! causes may make ! moon seem m di/ant :5 x is at l[ altitudes & z its visual angle gr[s less1 we may ?9k t x mu/ be a l>g] body & we s p]ceive x4 ,c]ta9ly x looks p>ticul>ly l>ge :5 a well-kn[n object is silh\ett$ aga9/ its disk4 ,be=e proce$+ fur!r ) explanations1 x may be ( int]e/ to turn to ,fig4 #gh :i* is an accurate trac+ ( ! pa? ( ! moon's image across a photographic plate4 ,! cam]a 0 plac$ 9 a fix$ position & ! image ( ! moon's disk on ris+ 0 accurately focus$ on a pan*romatic plate4 ,a d5se r$ filt] 0 ma9ta9$ ov] ! l5s ?r\gh\t 9 ord] to elim9ate ! effect ( selective absorption ( ! atmosph]e4 ,b ! slighte/ enl>gem5t 0 detect$ 9 ! wid? ( ! pa? ne> ! horizon z comp>$ ) t at ! highe/ altitude4 ,?is copy 0 made because x 0 ?\ght bett] = reproduction ?an ! photograph :i* w\ld require a half-tone4 ,?is is positive evid5ce t ! ph5om5on is an illusion4 ,illu/ration3 ,fig4 #gh4--;,an accurate trac+ f a photograph 7cont9ual exposure7 ( ! moon ris+4 ,simil>ly ,fig4 #gi is a copy ( a negative ( sev]al exposures ( ! sun4 ,ow+ to ! great] brightness1 cont9u\s exposure 0 n consid]$ feasible4 ,a pan*romatic plate & r$ filt] 0 us$ z 9 ! case ( ! moon4 ,! v>i\s exposures 7 made )\t o!rwise adju/+ ! cam]a4 ,aga9 no enl>gem5t at ! horizon 0 f\nd4 ,illu/ration3 ,fig4 #gi4--;,accurate trac+s f a photograph 7%ort exposures at int]vals7 ( ! sun sett+4 ,al?\gh ! =ego+ is conclusive evid5ce ( ! illusory "* ( ! enl>gem5t !re are o!r ways ( mak+ measurem5ts4 ,on view+ ! sun at ! horizon a bright aft]-image is obta9$4 ,?is may n[ be project$ ^u ! sky z a backgr\nd at any desir$ altitude4 ,x w appe> mu* small] at ! z5i? ?an ! sun appe>s at ! horizon4 ,c]ta9ly ?is is a simple & conclusive demon/ration ( ! illusion4 ,9 ?is case ! aft]-image ( ! sun or ! sun itself w usually appe> at lea/ twice z l>ge z ! aft]-image at ! z5i?4 ,if ! v>iation 9 ! position ( ! eyes is held to acc\nt = ! illusion1 ?is explanation may be support$ by us+ a horizontal telescope ) adju/able cross-hairs1 & a mirror4 ,by v>y+ ! position ( ! latt] ! disk ( ! sun may be measur$ at any altitude )\t v>y+ ! position ( ! eye4 ,:5 ev]y?+ is elim9at$ f ! field b ! moon's disk1 x is f\nd to be con/ant 9 size4 ,h[ev]1 ?is is n conclusive evid5ce t ! v>iation 9 ! position ( ! l9e ( sight acc\nts = ! illusion4 ,z a demon/ration ( ! abs5ce ( enl>gem5t ( ! size ( ! moon ne> ! horizon "s h br\ght =w>d measurem5ts ( ! lun> circles & simil> ph5om5a4 ,^! are said to be unaffect$ by ! altitude ( ! moon except = refraction4 ,b ev5 ?is does n *ange ! horizontal diamet] & actually dim9i%es ! v]tical "o4 ,! moon is fur!r away :5 ne> ! horizon ?an :5 at ! z5i?1 ! maximum increase 9 di/ance be+ "o-half ! diamet] ( ! e>?4 ,?is w\ld make ! moon appe> ab\t "o-sixtie?1 or "o-half m9ute ( arc small] at ! horizon ?an at ! z5i?4 ,?is is n only 9 ! wrong direction to aid 9 acc\nt+ = ! app>5t enl>gem5t1 b x is s slight z to be imp]ceptible to ! unaid$ eye4 ,ne>ly two c5turies ago ,rob]t ,smi? & 8 colleagues conclud$ t ! sky appe>s ab\t ?ree times z f> away at ! horizon z at ! z5i?4 ,!y f\nd t ! relative app>5t diamet]s ( ! sun & ( ! moon v>i$ ) altitude z foll[s3 ,altitude ,relative app>5t diamet] #j deg4 7horizon7 #ajj #ae #fh #cj #ej #de #dj #fj #cd #ge #ca #ij 7z5i?7 #cj ,illu/ration3 ,fig4 #hj4--;,explanation (f]$ by ,smi? ( ! app>5t enl>gem5t ( heav5ly bodies ne> ! horizon4 ,!y also f\nd a simil> relation betwe5 ! altitude & ! app>5t size ( con/ellations4 ,fig4 #hj is a reproduction ( a diagram :i* ,smi? submitt$ z illu/rat+ ! cause ( ! illusion ( app>5t enl>gem5t ( heav5ly bodies ne> ! horizon4 ,if ! sky seems to be a flatt5$ vault1 ! reason = ! app>5t decrease 9 ! size ( ! sun1 ! moon1 or ! con/ellations1 z !y approa* ! z5i?1 is sugge/$ by ! diagram4 ,x has also be5 sugge/$ t su* illusions z ^? %[n 9 ,figs4 #aj & #ai are associat$ ) t ( app>5t enl>gem5t ( heav5ly bodies ne> ! horizon4 ,x w be left to ! read] to decide :e!r or n !re is any simil>ity or relation4 ,zo? appe>s to h prov$1 to 8 own satisfaction at lea/1 t ! *ief factors are n a]ial p]spective1 ! app>5t curvature or =m ( ! sky1 & ! comp>ison ( ! sun or moon ) objects ( kn[n size4 ,he ma9ta9$ t ! illusion ( app>5t decrease 9 size z ^! bodies increase 9 altitude is due to ! necess>y elevation ( ! eye4 ,no available exp]im5tal evid5ce seems to refute 8 /atem5t4 ,9 fact1 ,guttman's exp]im5ts seem to confirm x to "s ext5t4 ,! latt] f\nd t !re 0 an app>5t dim9ution 9 ! size ( objects ( sev]al p] c5t1 9 objects slightly m ?an a foot di/ant f ! eyes1 z !y 7 rais$ s t ! l9e ( vision *ang$ f horizontal to an angle ( =ty degrees4 ,! magnitude ( ?is dim9ution is n suffici5t to promote ! acceptance ( elevation ( ! eyes z a prim>y cause ( ! illusion 9 respect to ! heav5ly bodies4 ,not)/&+ argum5ts to ! contr>y1 x is difficult to elim9ate a]ial p]spective & ! app>5t =m ( ! sky z important factors4 ,t no explanation ( ?is illusion has be5 g5]ally accept$ indicates ! complexity ( ! causes4 ,c]ta9ly ! r$di% coloration ( ! sun & moon ne> ! horizon & ! contra/ ) ! mi/y atmosph]e comb9$ ) ! g5]al vague aspect ( ! atmosph]e contribute some?+ if no m ?an a deep5+ ( ! my/]y4 ,v>iations 9 ! transp>5cy & brightness ( ! air mu/ play "s "p4 ,9 discuss+ ! great illusions ( nature1 x appe>s appropriate to introduce ! mirage4 ,?is is n due to an error ( s5se ( judgm5t4 ,! eye sees :at is pres5t$ b ! inv]sions & o!r peculi> effects are due to v>iations 9 ! refractive index ( ! atmosph]e4 ,^! v>iations acc\nt = ! appe>ance ( lakes 9 arid des]ts1 ( ! inv]t$ images ( %ips & iceb]gs on ! sea & ( pools ( wat] on pavem5ts4 ,! refractive index ( ! atmosph]e is cont9ually *ang+1 b ! *anges are *iefly ( two types3 7#a7 ^? due to irregul> heat+ & 7#b7 ^? due to normal v>iation ) altitude4 ,! =m] type are p>ticul>ly responsible = mirages4 ,illu/ration3 ,fig4 #ha4--;,explanation ( a common mirage4 ,a common type ( mirage is illu/rat$ 9 ,fig4 #ha4 ,?is is (t5 visible on des]ts :]e ! hot s& causes ! adjac5t lay] ( air to exp& & !re=e1 ! refractive index to increase4 ,?is lay] ( air !n may be consid]$ to op]ate l an inv]t$ prism4 ,! rays ( light close to ! e>? are b5t convex to ! e>? & ! curvature ( ^? high] up may be rev]s$4 ,! reason t an object may appe> d\ble1 or z if mirror$ by ! surface ( a ne>by pond1 is cle>ly %[n 9 ! illu/ration4 ,simil> atmosph]ic conditions are f\nd sometimes ov] pavem5ts & ov] bodies ( wat]4 ,z "o rides along 9 an automobile asc5d+ an incl9e1 if he closely obs]ves at ! mom5t ! l9e ( sight is j on ! level ( ! pavem5t1 he w (t5 be rew>d$ by ! sight ( a mirage4 ,an approa*+ p$e/rian may h no feet 7!y are replac$ by a bit ( sky7 & ! di/ant pavem5t w appe> to conta9 pools ( wat] on its surface4 ,sometimes on des]ts1 ov] ice fields1 or on nor!rn seas1 mirages are ( ! inv]t$ type4 ,a horseman or %ip may appe> susp5d$ 9 ! air 9 an inv]t$ position4 ,:5 ! d5sity ( ! air is great en\gh s t only ! upp] rays rea* ! eye1 ! object w be se5 inv]t$ & f> above ! surface ^u :i* no?+ is se54 ,_m modifications ( ^! types are possible ?r\gh v>iations 9 ! refractive indices ( v>i\s /rata ( air4 ,sometimes ! air is /ratifi$ horizontally & ev5 v]tically1 :i* results 9 magnification z well z o!r peculi> effects4 ,z "o rides ov] ! des]t 9 a rapidly mov+ tra9 or automobile ^! vag>ies ( nature are sometimes v /rik+1 because ! spe$ ( motion w make ! effects ( ! v>y+ refractive indices m m>k$4 ,a di/ant foo?ill may appe> to float 9 ! air or to *ange its %ape v rapidly4 ,an isl& surr\nd$ by quiet air & wat] may appe> l a huge mu%room b>ely support$ by a /em4 ,arctic mirages are no less wond]ful ?an ^? ( ! hot b>r5 des]ts4 ,:ile travel+ along ov] ! ice & sn[ di/ant :ite peaks may assume ! mo/ fanta/ic %apes4 ,at fir/ !y may appe> flatt5$ l a table-l& & !n sudd5ly !y may /ret* upw>d l spires4 ,!y may %r9k !n spread l huge mu%rooms support$ by ! /alk-l bases & /ret*+ out lat]ally4 ,sudd5ly !y may %oot upw>d into ano!r s]ies ( p9nacles z if ano!r range _h sudd5ly aris54 ,su* antics may g on = h\rs z "o travels along a froz5 valley4 ,ev5 a *ange ( position ( ! eyes accompany+ a *ange f erect to ly+ d[n may cause rem>kable contortions ( ! di/ant m\nta9s & "o is rem9d$ ( ! psalmi/'s qu]y1 ,:y hop ye s1 ye hills8 ,al?\gh n an illusion b a physical reality1 x is ( int]e/ 9 pass+ to note ! color$ halo or aureole surr\nd+ ! %ad[s ( objects ca/ by ! sun aga9/ a cl\d1 fog1 or jet ( /eam4 ,! mo/ wond]ful effects are se5 by ! a]ial travel] ov] a bank ( cl\ds :5 ! upp] sky is cle>4 ,= example1 ! %ad[ ( ! aircraft ca/ by ! sun ^u a d5se lay] ( cl\ds is surr\nd$ by a halo or aureole ( ! colors ( ! ra9b[4 ,! ph5om5on is purely optical1 involv+ diffraction ( light4 ,a well-kn[n example ( ?is is ! ,spectre ( ! ,brock54 ,,xii ,,pa9t+ ,,& ,,decoration ,9 ! arts :]e colors1 brightnesses1 contra/s1 l9es1 =ms1 & p]spectives mean s mu*1 x is obvi\s t visual illusions are important4 ,sometimes !y are evils :i* mu/ be suppress$2 9 "s cases !y are boons to ! arti/ if he is equal to ! task ( h>ness+ !m4 ,(ttimes !y appe> unh]ald$ & unexpect$4 ,! exi/5ce ( visual illusions is suffici5t to ju/ify ! arti/'s pride 9 8 eye & 8 dep5d5ce ^u 8 visual judgm5t r ?an ^u :at he kn[s to be true4 ,h[ev] true ?is may be1 k is z useful to ! arti/ z to anyone else4 ,! arti/1 if he is to produce art1 is confront$ ) ! trem5d\s task ( p]fect+ an imp]fect nature & he is h&icapp$ ) tools inf]ior to ^? :i* nature has at h] disposal4 ,he mu/ deal ) reflect$ lights f e>?ly mat]ials4 ,nature has ^! besides ! great prim>y light-s\rces--! sun1 ! moon1 ! />s1 &1 we might say1 ! sky4 ,%e also has ! advantage ( ov]:elm+ magnitudes4 ,^! are only a few ( ! disadvantages "u :i* ! arti/ works1 b !y indicate t he mu/ grasp any advantage "h & !re :i* he may4 ,k _c fail him2 /ill1 if he fe>s t x w take him out ( 8 dream _w & ta9t him ) e>?l9ess1 let him pond] ov] da ,v9ci1 ,rembr&t1 & su* m54 ,^! m5 knew _m ?+s4 ,!y possess$ mu* k &1 aft] all1 ! latt] is no?+ m nor less ?an sci5ce :5 its facts are arrang$ 9 an ord]ly mann]4 ,if ! arts are to speak a noble & expressive language despite ! h&icaps ( ! arti/1 k _c be drawn ^u too deeply4 ,p]haps 9 no o!r art are ! workm5 z little acqua9t$ ) _! h&icaps & ) ! sci5tific facts :i* w\ld aid !m z 9 pa9t+4 ,pa9t]s1 ( c\rse1 may n agree z to ?is /atem5t1 b if !y wi% to see h[ mu* ( ! sci5ce ( light1 color1 light+1 & vision !y are unacqua9t$ )1 let !m invade ! book-%elves4 ,if !y ?9k !y "k ! facts ( nature let !m pa9t a giv5 sc5e & !n inquire ( ! sci5ti/ reg>d+ ! relative values 7brightnesses7 9 ! actual sc5e4 ,!y w usually be amaz$ to le>n t !y _c pa9t ! lights & %ad[s ( nature except+ 9 ! feeble/ mann]4 ,! range ( contra/ repres5t$ by _! entire palette is _m ?\s& times less ?an ! range ( values 9 nature4 ,9 fact exclusive ( nature's prim>y light-s\rces1 su* z ! sun1 %e sometimes exhibits a range ( brightness 9 a l&scape a million times great] ?an ! pa9t] c produce ) black & :ite pigm5ts4 ,?is sugge/s t ! arti/ is ju/ifi$ 9 us+ any available means = ov]com+ ! h&icap & among 8 tools1 visual illusions are p]haps ! mo/ p[]ful4 ,a pa9t+ 9 ! broade/ s5se is an illusion1 = x /rives to pres5t ! ?ree-dim5sional _w ^u plane areas ( two dim5sions4 ,?r\gh repres5tation or imitation x creates an illusion4 ,if ! arti/'s s5sibility has be5 capable ( adequate selection1 8 art w transmit1 by means ( & ?r\gh ! tru?s ( sci5ce1 f ! region ( p]ception to ! region ( emotion4 ,sci5ce consi/s ( kn[+2 art consi/s ( do+4 ,if ! arti/ is famili> ) ! facts ( light1 color1 light+1 & vision1 he w possess k t c aid him 9 ov]com+ ! great ob/acles :i* are ev]-pres5t4 ,a glimpse ( visual illusions %\ld /r5g!n him 9 8 resolution to dep5d ^u visual p]ception1 b he c utilize ^! v illusions4 ,he c f9d a use = facts z well z anyone4 ,facts z well z exp]i5ce w prep>e him to d 8 "w be/4 ,! arti/ may sugge/ brilliant sunlight by means ( deep %ad[4 ,! old pa9t]s ga9$ color at ! exp5se ( light & !re=e l[]$ ! scale ( color 9 _! repres5tations ( nature4 ,x is int]e/+ to see h[ increas+ k1 z c5turies pass$1 direct$ pa9t]s z x did o!rs onw>d t[>d ! tru?4 ,turn] 0 "o ( ! fir/ to ab&on ! old] me?ods 9 an attempt to raise ! scale ( 8 pa9t+s t[>d a brilliance m resembl+ nature4 ,by do+ ?is he 0 able to put color 9 %ad[s z well z 9 lights4 ,gradually pa9t+s became m brilliant4 ,monet1 ,claude1 & o!rs work$ t[>d ?is goal until ! brightnesses ( pa9t+s rea*$ ! limits ( pigm5ts4 ,! impressioni/s1 9 _! desire to pa9t nature's light1 introduc$ some?+ :i* 0 no?+ m nor less ?an sci5ce4 ,all ?is "t ! true creative arti/ 0 introduc+ sci5ce--9 fact1 illusions--to produce ! p]fect illusion :i* 0 8 goal4 ,a survey ( any repres5tative pa9t+s' gall]y %[s ! result ( ! application ( m & m k1 z ! art ( pa9t+ progress$ ?r\gh ! c5turies4 ,surely we _c g back to ! br[n %ad[s & sombre l&scapes ( ! pa/4 ,9 ! e>lie/ art1 9 ! ef=ts ( *ildr51 9 ! wall-pa9t+s ( ! ,egyptians1 & 9 ,japanese repres5tation ( nature1 ! process is selective & n imitative4 ,c]ta9 ?+s are *os5 & ev]y?+ else is disc>d$4 ,9 su* art selection is c>ri$ to ! extreme4 ,mu* ( ?is simplicity 0 due to a lack ( k4 ,light & %ade1 or %ad+1 0 n introduc$ until sci5ce discov]$ & organiz$ its facts4 ,q 9 ! same mann] l9e> & a]ial p]spective made _! appe>ances until 9 our pres5t art ! process ( selection is complex4 ,9 our pa9t+s ( today objects are model$ by light & %ade2 !y are relat$ by p]spective2 backgr\nds & surr\nd+s are c>efully consid]$2 ! prop] emphasis ( light1 %ade & color are giv5 to c]ta9 details4 ,! pres5t complexity provides unprec$5t$ opportunities = ! application ( k p]ta9+ to illusions b x %\ld be und]/ood t ?is application t5ds only t[>d realism ( ext]nal ?+s4 ,idealism 9 art & realism ( "* & expression are accompli%$ by ! same tools--pigm5ts & bru%es--z realism ( objective details is atta9$ & !re is no?+ my/]i\s 9 ! ma/]pieces ( ?is k9d4 ,my/]y 9 art z 9 o!r activities is m]ely lack ( und]/&+ due to inadequate k4 ,my/]ies ( today become facts tomorr[4 ,sci5ce moves ) c]ta9ty into ! unkn[n1 reap+ & b9d+ ! facts & dropp+ !m beh9d :]e !y may be utiliz$ by ^? :o w4 ,! pa9t] c imitate a]ial p]spective al?\gh _m c5turies elaps$ be=e mank9d 0 ke5 en\gh to note its pres5ce 9 nature4 ,! atmosph]ic haze dim9i%es ! brightness ( v bright objects & increases t ( d>k objects4 ,x blurs ! di/ant details & adds a t+e ( blue or violet to ! di/ance4 ,9 pa9t+ x is a p[]ful illusion :i* ! pa9t] has le>n$ to employ4 ,! pa9t] c accurately imitate ma!matical or l9e> p]spective b ! art ( e>ly c5turies does n exhibit ?is feature4 ,9 a pa9t+ a trem5d\sly p[]ful illusion ( ! ?ird dim5sion is obta9$ by dim9i%+ ! size ( objects z !y are repres5t$ 9 ! di/ance4 ,conv]g+ l9es & ! o!r manifold details ( p]spective are aid+ ! arti/ 9 8 ef=ts t[>d ! production ( ! great illusion ( pa9t+4 ,! pa9t] _c imitate focal p]spective or b9ocul> p]spective4 ,he c try to imitate ! def9ition 9 ! c5tral portion ( ! visual field & ! increas$ blurr+ t[>d ! p]iph]y4 ,focal p]spective is n ( mu* importance 9 pa9t+1 because x is sc>cely p]ceptible at ! di/ances at :i* pa9t+s are usually view$4 ,h[ev] ! abs5ce ( b9ocul> p]spective 9 pa9t+ does decrease ! effectiv5ess ( ! illusion v m>k$ly4 ,= ?is reason a pa9t+ is a m successful illusion :5 view$ ) "o eye ?an ) two eyes4 ,( c\rse1 9 "o ( nature's sc5es ! conv]se is true because :5 view+ x ) bo? eyes all ! =ms ( p]spective cooep]ate to ! f9al end--! true impression ( ?ree dim5sions4 ,! pa9t] may imitate ! light & %ade ( solid =ms & !reby app>5tly model !m4 ,9 ?is respect a rem>kable illusion ( solid =m or ( dep? may be obta9$4 ,= example1 a pa9t$ column may be made to appe> circul> 9 cross-section or a circle :5 prop]ly %ad$ w appe> to be a sph]e4 ,bo? ( ^!1 ( c\rse1 are pure illusions4 ,"s /age pa9t+s are rem>kable illusions ( dep?1 & _! success dep5ds *iefly ^u l9e> p]spective & %ad[s4 ,h[ev]1 ! illusion :i* 0 s complete at a di/ance q disappe>s at close range4 ,! inadequate range ( brightnesses or values obta9able by means ( pigm5ts has already be5 discuss$4 ,! sky 9 a l&scape may be ?\s&s ( times bright] ?an a deep %ad[ or a hole 9 ! gr\nd4 ,a cumulus cl\d 9 ! sky may be a hundr$ ?\s& times bright] ?an ! deepe/ %ad[4 ,h[ev]1 ! arti/ mu/ repres5t a l&scape by means ( a palette ^: :ite is only ab\t ?irty times bright] ?an its black4 ,if ! sun is consid]$ we may h 9 a l&scape a range ( brightness repres5t$ by millions4 ,?is illu/rates ! pitiable weakness ( pigm5ts alone z repres5tative m$ia4 ,w n light transmitt$ ?r\gh m$ia "s day be utiliz$ to ov]come ?is inh]5t h&icap ( reflect+ m$ia8 ,to :at ext5t is ! success ( /a9$ glass w9d[s due to a less5+ ( ?is h&icap8 ,! range ( brightness 9 ?is case may be repres5t$ by a black 7non-transmitt+7 portion to ! brightness ( ! backgr\nd 7artificial or sky7 z se5 ?r\gh an area ( cle> glass4 ,transp>5cies h an inh]5t advantage ov] ord9>y pa9t+s 9 ?is respect & _m effective results may be obta9$ ) !m ev5 9 photography4 ,x is int]e/+ to /udy ! effect ( greatly increas+ ! range ( values or brightnesses 9 pa9t+s by utiliz+ non-uni=m di/ributions ( light4 ,let u take a giv5 l&scape pa9t+4 ,if a light-s\rce be s plac$ t x is close to ! bright] areas 7p]haps cl\ds & sky ne> ! sun7 x w illum9ate ?is bright] portion sev]al times m int5sely ?an ! m di/ant d>k] portions ( ! picture 7=egr\nd ( trees1 und]bru%1 deep %ad[s1 etc474 ,! addition to ! effectiv5ess ( ! illusion is q p]ceptible4 ,?is effect ( non-uni=m light+ may be c>ri$ to ! extreme = a pa9t+ by mak+ a positive lant]n-slide 7r contra/y7 ( ! pa9t+ & project+ ?is slide ^u ! pa9t+ 9 accurate sup]position4 ,n[ if ! pa9t+ is illum9at$ solely by ! lant]n-slide ! range ( contra/ or brightness w be enorm\sly increas$4 ,! lighte/ portions ( ! picture w n[ be illum9at$ by light pass+ ?r\gh ! almo/ totally transp>5t portions ( ! slide & ! d>ke/ portions by light greatly r$uc$ by pass+ ?r\gh ! ne>ly opaque portions ( ! slide4 ,! orig9al range ( contra/ 9 ! pa9t+1 p]haps tw5ty to "o1 is n[ increas$ p]haps to m ?an a ?\s& to "o4 ,?is demon/ration w be surpris+ to anyone & w emphasize a v important po9t to ! pa9t]4 ,! pa9t] has at 8 disposal all ! sci5tific facts ( light1 color1 & vision4 ,_m ( ^! h be5 pres5t$ else:]e1#i & ^? p]ta9+ to illusions h be5 discuss$ 9 prec$+ *apt]s4 ,^! ne$ n be repeat$ "h except+ a few = ! purpose ( rem9d+ ! read] ( ! weal? ( mat]ial available to ! pa9t] & decorator4 ,_m tricks may be int]ject$ into ! =egr\nd = _! effect ^u ! backgr\nd & vice v]sa4 ,= example1 a bran* ( a tree droop+ 9 ! =egr\nd app>5tly close to ! obs]v]1 if done well1 w give a rem>kable dep? to a pa9t+4 ,model+ ( =m may be effect$ to "s ext5t by a judici\s use ( ! retir+ & advanc+ colors4 ,?is is "o way to obta9 ! illusion ( dep?4 ,aft]-images play _m subtle p>ts 9 pa9t+4 ,= example1 9 a pa9t+ :]e a gray-blue sky meets ! horizon ( a blue-gre5 body ( wat]1 ! involunt>y eye-movem5ts may produce a p9ki% l9e j above ! horizon4 ,?is is ! aft]-image ( ! blue-gre5 wat] creep+ upw>d by eye-movem5ts4 ,_m vivid illusions ( ?is "* may be delib]ately obta9$ by ! arti/4 ,"s ( ! peculi> re/less effects obta9$ 9 impressioni/ic pa9t+ 7/ippl+ ( small areas ) relatively pure hues7 are due to contra/s & aft]-images4 ,a pa9t+ came to ! au?or's notice 9 :i* sev]al aft]-images ( ! sun1 besides ! image ( ! sun itself1 7 dispos$ 9 v>i\s positions4 ,_! colors v>i$ 9 ! same mann] z ! aft]-image ( ! sun4 ,d\btless ! pa9t] /rove to give ! impression :i* "o has on gaz+ at ! sun4 ,:e!r or n ?is attempt 0 successful does n matt] b x 0 gratify+ to see ! attempt made4 ,!re are _m int]e/+ effects obta9able by judici\s exp]im5tation4 ,= example1 if a gray m$ium be spray$ ^u a l&scape 9 su* a mann] t ! mat]ial dries 9 a v r\gh or diffus+ surface "s rem>kable effects ( fog & haze may be produc$4 ,:ile exp]im5t+ 9 ?is mann] a v f9ely et*$ cle> glass 0 plac$ ov] a l&scape & ! comb9$ effect ( diffusely reflect$ light & ( ! slight blurr+ 0 rem>kable4 ,by sep>at+ ! et*$ glass f ! pa9t+ a slight di/ance1 a v good imitation porcela9 0 produc$4 ,! optical prop]ties ( v>ni%es v>y & _! effect v>ies consid]ably1 dep5d+ ^u ! mode ( application4 ,^! & _m o!r details are available to ! pa9t] & decorator4 ,an int]e/+ example among _m is a cellulose lacqu] dy$ ) an ord9>y yell[ dye4 ,! solution appe>s yell[ by transmitt$ light or x w color a surface yell[4 ,by spray+ ?is solution on a metallic object su* z a nickel-plat$ piece1 9 a mann] t leaves ! m$ium r\gh or diffus+1 ! effect is no long] m]ely a yell[ b a rem>kable lu/re resembl+ gilt4 ,q 9 ! same mann] _m effects ( ri*ness1 dep? ( color1 haz9ess1 etc41 are obta9able by ! arti/ :o is /riv+ to produce a great illusion4 ,all ! means = success :i* ! pa9t] possesses are also available to ! decorator2 h[ev]1 ! latt] may utilize "s ( ! illusions ( l9e1 =m1 irradiation1 etc41 :i* ! ar*itect enc\nt]s4 ,! decorator's field may be consid]$ to include almo/ all ( ! pa9t]'s & mu* ( ! ar*itect's4 ,?is be+ ! case1 little space w be giv5 to ?is phase ( ! subject because pa9t+ & ar*itecture are sep>ately treat$4 ,! decorator %\ld beg9 to realize m fully ! great pot5tiality ( light+ 9 creat+ moods or 9 giv+ expression to an int]ior4 ,! psy*ology ( light & ! use ( light+ z a mode ( expression h b>ely be5 drawn ^u by ! decorator4 ,light+ has already be5 discuss$ s x w be pass$ by at ?is po9t4 ,! practice ( hang+ pictures on walls :i* are brilliantly color$ is op5 to criticism4 ,!re are gall]ies 9 exi/5ce :]e pa9t+s are hung on brilliant gre5 or rose walls4 ,! *anges 9 ! appe>ance ( ! object due to ^! highly color$ environm5ts are easily demon/rat$ by view+ a piece ( :ite pap] p9n$ ^u ! wall4 ,on ! gre5 wall1 ! :ite pap] appe>s p9ki%2 on ! rose wall1 x appe>s blui% or gre5i%4 ,a portrait or a picture 9 :i* !re are areas ( :ite or delicate t9ts is subject to consid]able di/ortions 9 ! appe>ance ( its colors4 ,simil>ly1 if a woman mu/ h a color$ backgr\nd1 x is well to *oose "o :i* w induce ! m desirable t9ts 9 h] appe>ance4 ,! design] ( g[ns c]ta9ly mu/ recognize ^! illusions ( color :i* may be desirable or undesirable4 ,! light+ ( a picture has already be5 m5tion$1 b ! discussion 0 conf9$ solely to di/ribution ( light4 ,! quality ( ! light 7its spectral "*7 may h an enorm\s influ5ce ^u ! pa9t+4 ,9 fact ) ! same pa9t+ _m illusions may be produc$ by light+4 ,9 g5]al1 pa9t+s are pa9t$ 9 daylight & !y are n ! same 9 appe>ance "u ord9>y artificial light4 ,= ?is reason ! arti/ is usually entitl$ to ! pres]vation ( ! illusion z he complet$ x4 ,by us+ artificial daylight :i* has be5 available = "s ye>s1 ! pa9t+ appe>s z ! arti/ gave x 8 la/ t\*4 ,( c\rse1 x is q legitimate to v>y ! quality ( light 9 case ! own] desires to d s1 b ! purpose "h is to emphasize ! fact t ! quality ( light is a p[]ful influ5ce ^u ! appe>ance ( ! pa9t+4 ,! influ5ce is n g5]ally en\gh recogniz$ & its magnitude is appreciat$ by relatively few p]sons4 ,all o!r consid]ations aside1 a pa9t+ is be/ hung ^u a colorless backgr\nd & black velvet = ?is purpose yields rem>kable results4 ,gray velvet is bett]1 :5 ! appe>ance ( ! room is tak5 into consid]ation1 z x mu/ be4 ,h[ev]1 ! influ5ce ( d>k surr\nd+s t[>d enhanc+ ! illusion is well wor? recogniz+4 ,9 ! case ( a special picture or a special occasion1 a pa9t+ may be exhibit$ 9 a boo?--a huge %ad[-box n unlike a %[-w9d[ 9 :i* ! light-s\rces are conceal$4 ,su* exp]im5ts yield _m int]e/+ data p]ta9+ to ! illusions :i* ! pa9t] /rives to obta94 ,illu/ration3 ,fig4 #hb4--;,illu/rat+ ! app>5t di/ortion ( a picture frame 9 :i* ! gra9 ( ! wood is visible4 ,incid5tally on view+ "s picture frames 9 :i* ! gra9 ( ! wood 0 noticeable1 ! frames did n appe> to be /rictly rectangul>4 ,! illusions 7 s /rong t only by measur+ ! frames c\ld "o be conv9c$ t !y 7 truly rectangul> & possess$ /raight sides4 ,two ( ^! are repres5t$ 9 ,figs4 #hb & #hc4 ,9 ! =m]1 ! horizontal sides appe> b5t upw>d 9 ! middle & ! two v]tical sides appe> b[$ t[>d ! "r4 ,9 ,fig4 #hc1 ! frame appe>s consid]ably n>r[] at ! left end ?an at ! "r4 ,bo? ^! frames 7 repres5t$ 9 ! orig9al draw+s by true rectangles4 ,_m illusions are to be se5 9 furniture & 9 o!r woodwork 9 :i* ! gra9 is conspicu\s4 ,?is appe>s to ! au?or to be an objection 9 g5]al to ?is k9d ( f9i%4 ,9 ,fig4 #hd !re is reproduc$ a photograph ( ! end ( a bo>d :i* 0 plane or /raight not)/&+ its w>p$1 or b[$1 appe>ance4 ,! orig9al photographs 7 plac$ s z to be relat$ z %[n 9 ! figure4 ,v>i\s degrees ( ! illusion are evid5t4 ,! read] w p]haps f9d x necess>y to conv9ce himself ( ! /raightness ( ! horizontal edges by apply+ a /raight edge4 ,^! are examples ( ! same illusion z %[n 9 ,figs4 #cg to #dj4 ,illu/ration3 ,fig4 #hc4--;,ano!r example simil> to ,fig4 #hb4 ,illu/ration3 ,fig4 #hd4--;,f actual photographs ( ! end-gra9 ( a bo>d4 ,p]haps a brief /atem5t reg>d+ ! mod]n isms 9 art may be ( int]e/4 ,9 consid]+ "s ( ! extreme examples1 we mu/ revise our idea t art is or %\ld be always beautiful4 ,! _m def9itions ( art w\ld lead u too f> afield to discuss !m "h b 9 its mo/ ext5d$ & popul> s5se1 art may be consid]$ to mean ev]y?+ :i* we di/+ui% f nature4 ,c]ta9ly art ne$ n be beautiful1 al?\gh x does seem t ! _w w\ld welcome ! beautiful & w\ld get along cont5t$ly )\t art t is ugly or repulsive4 ,! mod]n isms mu/ be view$ ) consid]ation1 = !re are _m impo/ors conceal+ _! inabilities by flock+ to ^! less und]/ood fields4 ,h[ev]1 !re are _m s9c]e work]s--rese>* arti/s--9 ! mod]n isms & _! works may be/ be describ$ at pres5t z exp]im5ts 9 ! psy*ology ( light1 %ade1 & color4 ,!y h ca/ aside or r$uc$ 9 importance "s ( ! m famili> compon5ts su* z realism & are /riv+ m deeply to utilize ! psy*ology ( light & color4 ,"s ( !m admit t !y /rive to pa9t ?r\gh *ild's eyes & m9d--free f exp]i5ce1 prejudice1 & imitation4 ,^! ne$ all ! sci5tific k :i* is available--& maybe m4 ,9 clos+ ?is *apt]1 x appe>s necess>y to rem9d ! arti/ & o!rs t x is f> f ! au?or's int5tion to subord9ate ! arti/'s s5sibility to ! sci5tific facts or tools4 ,art _c be manufactur$ by means ( =mulae4 ,?is w\ld n be true if we knew a great deal m ?an we d p]ta9+ to ! sci5ce ( light1 color1 & vision4 ,! arti/'s f9e s5sibility w always be ! dom9at+ necessity 9 ! production ( art4 ,he mu/ possess ! ability to compose exquisitely2 he mu/ be able to look at nature ?r\gh a special temp]am5t2 he mu/ be gift$ 9 eye & 9 h&2 he mu/ be ma/] ( unusual visual & intellectual processes4 ,b k w aid him z well z ^? 9 o!r activities4 ,a sup]ior acqua9tance ) sci5tific facts lift$ pa/ ma/]s above _! fell[s & :at help$ ,leon>do da ,v9ci1 ,rembr&t1 ,velasquez1 ,turn]1 ,claude1 ,monet1 & o!r ma/]s w help arti/s ( today4 ,:at w\ld n ^? pa/ ma/]s h accompli%$ if !y _h available 9 _! "t ! great] k ( ! pres5t6 ,,xiii ,,ar*itecture ,_m illusions are f\nd 9 ar*itecture &1 /rangely en\gh1 _m ( ^! 7 recogniz$ long be=e pa9t+ develop$ beyond its primitive /ages4 ,! ar*itecture ( classic ,greece displays a highly develop$ k ( _m geometrical illusions & ! ar*itects ( ^? f>-(f c5turies c>efully work$ out details = c\nt]act+ !m4 ,draw+s reveal _m illusions to ! ar*itect1 b _m are n pr$ict$ by !m4 ,! ev]-*ang+ relations ( l9es & =ms 9 ar*itecture z we v>y our viewpo9t introduce _m illusions :i* may appe> & disappe>4 ,no view ( a gr\p ( build+s or ( ! compon5ts ( a s+le /ructure c be free f optical illusions4 ,we nev] see 9 ! reality ! same relations ( l9es1 =ms1 colors1 & brightnesses z indicat$ by ! draw+s or blue-pr9ts4 ,p]haps ?is is "o ( ! be/ reasons = ju/ify+ ! con/ruction ( exp5sive models ( our m pret5ti\s /ructures4 ,no detail$ acc\nt ( ! _m ar*itectural illusions w be attempt$1 = x is easy = ! read] to see _m ( ! possibilities sugge/$ by prec$+ *apt]s4 ,h[ev]1 a few w be t\*$ ^u to reveal ! magnitude ( ! illusory effect & to aid ! obs]v] 9 look+ = or recogniz+ !m1 or purely = hi/orical int]e/4 ,9 ar*itecture ! eye _c be :olly satisfi$ by su* tools z ! level1 ! squ>e1 & ! plumb-l9e4 ,! eye is satisfi$ only :5 ! appe>ance is satisfactory4 ,= ! purpose ( %[+ ! ext5t ( c]ta9 ar*itectural illusions1 ! comp5satory measures appli$ by ! ,greeks are excell5t examples4 ,^! also reveal ! rem>kable application ( sci5ce to ar*itecture z comp>$ ) ! scanty application 9 pa9t+ ( ! same p]iod4 ,dur+ ! be/ p]iod ( ,grecian art _m ref9em5ts 7 appli$ 9 ord] to correct optical illusions4 ,x w\ld be int]e/+ to "k to :at ext5t ! magnitude ( ! illusions z !y appe>$ to _m p]sons 7 actually /udi$4 ,! ,p>!non ( ,a!ns af=ds an excell5t example ( ! magnitude ( ! corrections :i* ! design] ?\ght necess>y 9 ord] to satisfy ! eye4 ,! long l9es ( ! ar*itrave--! beam :i* surm\nts ! columns or ext5ds f column to column--w\ld appe> to sag if x 7 actually /raight4 ,?is is also true ( ! /ylobate1 or sub/ructure ( a colonnade1 & ( p$im5ts & o!r features4 ,^! l9es 7 (t5 convex in/ead ( be+ /raight z ! eye desires to see !m4 ,9 ! ,p>!non1 ! /ylobate has an upw>d curvature ( m ?an f\r in*es on ! sides ( ! edifice & ( m ?an two & a half in*es on ! ea/ & we/ fronts4 ,v]tical features 7 made to incl9e inw>d 9 ord] to correct ! common appe>ance ( lean+ outw>d at ! top4 ,9 ! ,p>!non1 ! axes ( ! columns are n v]tical1 b !y are incl9$ inw>d ne>ly ?ree in*es4 ,!y are said also to be incl9$ t[>d ea* o!r to su* a degree t !y w\ld meet at an altitude ( "o mile above ! gr\nd4 ,! elev5-foot frieze & ar*itrave is incl9$ inw>d ab\t "o & "o-half in*es4 ,9 ,fig4 #he1 a repres5ts ! front ( a temple z x %\ld appe>2 b repres5ts its appe>ance 7exagg]at$7 if x 7 actually built l a )\t comp5sations = optical illusions2 c repres5ts x z built & %[+ ! physical corrections 7exagg]at$7 9 ord] t x may appe> to ! eye z a does4 ,tall columns if !y are actually /raight are likely to appe> some:at %runk5 9 ! middle2 !re=e !y are sometimes made slightly swoll5 9 ord] to appe> /raight4 ,?is outw>d curvature ( ! pr(ile is t]m$ an entasis & 9 ! ,p>!non column1 :i* is ?irty-f\r feet 9 height1 am\nt$ to ab\t ?ree-f\r?s ( an in*4 ,9 "s e>ly ,grecian works1 x is said t ?is correction 0 ov]done b t its omission entirely is q unsatisfactory4 ,"s au?orities appe> to believe t an excell5t compromise is f\nd 9 ! ,p>!non columns4 ,illu/ration3 ,fig4 #he4--;,exagg]at$ illusions 9 ar*itecture4 ,"o ( ! conditions :i* is responsible = c]ta9 illusions & has be5 comp5sat$ = on occasions is repres5t$ 9 ,fig4 #hf4 ,on ! left are a s]ies ( squ>es ( equal size plac$ 9 a v]tical r[4 ,if ^! are l>ge s t !y might repres5t /ories 9 a build+ !y w appe> to decrease 9 size f ! bottom upw>d1 because ( ! decreas+ projection at ! eye4 ,?is is obvi\s if ! eye is consid]$ to be at ! po9t :]e ! incl9$ l9es meet4 ,9 ord] to comp5sate = ! v>iation 9 visual angle1 !re mu/ be a s]ies ( rectangles increas+ consid]ably 9 height t[>d ! top4 ,! correction is %[n 9 ! illu/ration4 ,x is /at$ t an inscription on an anci5t temple 0 writt5 9 lett]s arrang$ v]tically1 & 9 ord] to make !m appe> ( equal size !y 7 actually increas$ 9 size t[>d ! top accord+ to ! law repres5t$ 9 ,fig4 #hf4 ,obvi\sly a giv5 correction w\ld be correct only = "o di/ance 9 a giv5 plane4 ,illu/ration3 ,fig4 #hf4--;,illu/rat+ ! influ5ce ( visual angle ^u app>5t v]tical height4 ,9 ,*apt] ,,viii ! ph5om5on ( irradiation 0 discuss$ & v>i\s examples 7 pres5t$4 ,x ex]ts its influ5ce 9 ! arts z else:]e4 ,columns view$ aga9/ a backgr\nd ( :ite sky appe> ( small] diamet] ?an :5 !y are view$ aga9/ a d>k backgr\nd4 ,?is is illu/rat$ 9 ,fig4 #hg :]e ! :ite & ! black columns are suppos$ to be equal 9 diamet]4 ,! c>eful obs]v] w f9d numb]less optical illusions & occasionally he w recognize an attempt on ! "p ( ! ar*itect to apply an illusory effect to 8 advantage4 ,9 ,fig4 #hh "s commonplace illusions are pres5t$1 n = :at !y are wor?1 b to sugge/ h[ preval5t !y may be4 ,:]e ! pole or column int]sects ! ar*es or circle1 !re is an app>5t *ange 9 ! direction ( ! curv$ l9es4 ,! diff]5t types ( ar*es %[ diff]5t degrees ( ! illusion4 ,x may be ( int]e/ = ! read] to ref] to prec$+ *apt]s & to asc]ta9 :at types ( illusions are involv$4 ,illu/ration3 ,fig4 #hg4--;,irradiation 9 ar*itecture4 ,if a high wall ends 9 a s]ies ( long horizontal /eps at a slightly incl9$ sidewalk1 ! /eps are n likely to appe> horizontal4 ,"s rem>kable illusions ( dep? or ( solid =m are giv5 to flat surfaces :5 sn[ is driv5 aga9/ !m s z to adh]e 9 decreas+ am\nts simil> to %ad+4 ,a sugge/ion ( augm5t$ height may be giv5 to a l[ t[] by decreas+ ! size ( its successive portions m rapidly ?an dem&$ by p]spective alone4 ,! same pr9cipal c be appli$ 9 _m ways4 ,= example1 9 ,fig4 #hi ! ro( appe>s q ext5sive :5 view$ s t ! end-walls ( ! /ructure are n se54 ,su* illusions f9d applications 9 ! mov+-picture /udio :]e ext5sive int]iors1 great =tresses1 & ev5 villages mu/ be erect$ )9 small areas4 ,incid5tally ! cam]a aids to create ! illusion ( magnitude 9 photographs because x usually magnifies p]spective1 !reby caus+ sc5es to appe> m ext5sive 9 ! photographs ?an 9 ! reality4 ,illu/ration3 ,fig4 #hh4--,"s simple geometrical-optical illusions 9 ar*itecture4 ,balance 9 ar*itecture is subject to illusions & might be consid]$ an illusion itself4 ,= example1 our judgm5t ( balance is bas$ l>gely ^u me*anical laws4 ,a composition mu/ appe> to be /able2 t is1 a l>ge compon5t su* z a t[] mu/ n be situat$ too f> f :at we take z a c5t] ( gravity1 to appe> capable ( tipp+ ! rema9d] ( ! /ructure4 ,9 physics we w\ld apply ! t]m mom5t4 ,ea* mass may be multipli$ by its di/ance f ! c5t] ( gravity1 ?us det]m9+ its mom5t4 ,= a build+ or o!r composition to appe> /able ! sum ( ^! mom5ts mu/ be z]o2 t is1 ^? t5d+ to turn ! figure 9 "o direction mu/ be c\nt]balanc$ by ^? t5d+ to turn x 9 ano!r direction4 ,9 apprais+ a composition1 our intellect summates ! effects ( diff]5t p>ts some:at 9 ?is mann] & if satisfactory1 balance is consid]$ to h be5 atta9$4 ,! colors ( ! v>i\s compon5ts ex]t an influ5ce 9 ?is respect1 s x is se5 t illusions may h mu* to d ) ! satisfactor9ess ( ar*itectural compositions4 ,illu/ration3 ,fig4 #hi4--;,by decreas+ ! expos$ l5g? ( %+les t[>d ! top a great] app>5t expanse is obta9$4 ,v>i\s illusions ( height1 ( ceil+1 compos$ness1 etc41 may be obta9$ by ! color ( ! ceil+4 ,a d>k cornice 9 an int]ior may appe> to be unsupport$ if ! walls bel[ are light 9 color1 )\t any app>5t v]tical supports = ! cornice4 ,we are !n subject$ to ! illusion ( in/ability or incongruity4 ,d>k beams ( ceil+s are n s obtrusive because our intellect tells u t !y are supports pass+ ov] ! top ( ! walls & are !re=e able to support !mselves4 ,color & brightness 9 su* cases are v important4 ,! ar*itectural details on ext]iors evolv$ "u daylight+ outdoors s t _! =m has be5 det]m9$ by ! %ad[s desir$4 ,! ar*itect leads 8 lights & %ad[s ar\nd ! build+ model+ x z he desires4 ,an (fset "h & a depression !re models ! ext]ior 9 light & %ade4 ,! =ms mu/ be p[]ful en\gh to resi/ ! oblit]at+ effect ( ov]ca/ skies b not)/&+ all precautions ! expression ( an ext]ior v>ies consid]ably ) nature's light+4 ,indoors ! ar*itect has a p[]ful controllable m$ium 9 artificial light :i* he may draw ^u = produc+ v>i\s expressions or moods 9 rooms4 ,! effect ( %ad[s is int]e/+ :5 view+ "s /ructures flood-light$ at night4 ,9 ^? cases :]e ! light is direct$ upw>d !re is a rev]sal ( %ad[s :i* is sometimes v unsatisfactory4 ,x is int]e/+ to exp]im5t ) v>i\s ornam5tal objects light$ f v>i\s directions4 ,= example1 a ,cor9?ian capital light$ f bel[ may produce an unpleasant impression ^u ! obs]v]4 ,we d n l to h ! dom9ant light f bel[1 p]haps because x is annoy+ to ! eyes4 ,possibly ?is is an in/9ct acquir$ by exp]i5ce 9 sn[-fields or on ! des]t1 or x may be a h]itage ( ance/ral exp]i5ce ga9$ "u ^! gl>+ conditions4 ,?is dislike manife/s itself :5 we appraise %ad[-effects & !re=e our f9al impression is temp]$ by x4 ,all sculptur$ objects dep5d = _! appe>ance ^u ! light+1 & !y are greatly influ5c$ by x4 ,9 sculpture1 9 a /rict s5se1 illusions play a less] "p ?an 9 o!r arts4 ,p]haps 9 ^? ( v l>ge proportions v>i\s corrections h be5 appli$4 ,a m9or detail ( int]e/ is ! small cavity 9 ! eye1 correspond+ to a rev]s$ cornea4 ,?is depression cat*es a %ad[ :i* gives consid]able expression to ! eye4 ,,xiv ,,mirror ,,magic ,/rictly speak+ !re are few] illusions f\nd 9 ! practice ( ! magician ?an is g5]ally suppos$2 t is1 ! eye usually deliv]s correctly to ! intellect1 b ! judgm5t errs = v>i\s reasons4 ,! illusion is due to false assumptions1 to ! di/ract+ words1 to unduly acc5t$ sup]flu\s movem5ts ( ! magician2 or 9 g5]al to d[nright trick]y4 ,mu* ( ! magician's success is due to glibness ( tongue & deftness ( f+]s1 b _m ( ! m notable tricks 7 ^? involv+ ! use ( mirrors & ! control ( light4 ,black curta9s1 black5$ assi/ants1 & controll$ light h play$ prom95t p>ts 9 ! old] magic1 b ! pr9ciples ( ^! are easily und]/ood4 ,h[ev]1 ! mirror p]haps has done m to a/\nd ! audi5ce ?an any o!r device employ$ by ! magician4 ,= ?is reason1 & because its effects are commonly t]m$ illusions1 "s repres5tative examples w be pres5t$4 ,9 a previ\s *apt] att5tion 0 call$ to ! simple b usually ov]look$ fact t1 = example1 ! image ( a face 9 a mirror is rev]s$ z to "r & left4 ,:5 ?is fact is ov]look$ we may be a/oni%$ at ! *ang$ expression ( an intimate fri5d z we view ! face 7rev]s$7 9 ! mirror4 ,simil>ly our own features are rev]s$ z to "r & left & we are acqua9t$ ) ?is rev]s$ image r ?an ! appe>ance ( our face z x is4 ,inasmu* z faces are n accurately symmetrical & _m are q unsymmetrical ! effects ( ! mirror are sometimes />tl+4 ,x might be ( int]e/ = ! read] to /udy 8 face 9 ! mirror & note t ! "r e> is ! left e> ( ! image :i* he sees4 ,he w also f9d x ( int]e/ to comp>e ! face ( a fri5d z view$ directly ) ! appe>ance ( its image 9 ! mirror4 ,if he desires to see himself z o!rs see him1 he c arrange two mirrors v]tically almo/ at a "r-angle4 ,by a little rese>* he w f9d an image ( 8 own face1 :i* is n rev]s$2 t is1 an image ^: "r e> is really 8 "r e>4 ,a fam\s illusion :i* a/\nd$ audi5ces 0 ! sph9x illu/rat$ 9 ,fig4 #ij4 ,! box 0 plac$ ^u a table & :5 op5$ !re 0 reveal$ a ,sph9xian head1 b :y x 0 call$ a ,sph9x is clo!d 9 my/]y because ^u "s occasions x talk$4 ,z a matt] ( fact x belong$ to a body :i* ext5d$ d[nw>d f ! table-top & ?is kneel+ human be+ 0 conceal$ f ! audi5ce by two v clean plate-glass mirrors ,m %[n 9 ! accompany+ diagram4 ,! table actually appe>$ to h ?ree legs b ! audi5ce if x notic$ ?is at all assum$ ! f\r? leg 0 obscur$ by ! =emo/ leg4 ,! walls1 floor1 & ceil+ ( ! box-l recess 9 :i* ! table 0 plac$ 7 cov]$ ) ! same mat]ial4 ,x is se5 by ! diagram t ! mirrors ,m reflect$ images ( ! side walls ,w & ^! images 7 tak5 by ! audi5ce to be portions ( ! re> wall ,w4 ,?us ! table appe>$ to be op5 und]nea? & ! possibilities ( ! app>atus are evid5t4 ,illu/ration3 ,fig4 #ij4--;,an example ( a mirror illusion4 ,! magician ) a f9e fl[ ( language c\ld dwell at l5g? ^u ! com+ to life ( ! head ( an anci5t /atue :i* he _h 9 ! box 9 8 h&4 ,walk+ to ! table he c\ld place ! box ov] a trap-door & by ! "t he _h unlat*$ ! door ( ! box1 ! assi/ant kneel+ "u ! table c\ld h 8 head ?ru/ upw>d ?r\gh ! trap-door ( ! table-top into ! box4 ,aft] a few impressive words1 suppos$ to be ,h9doo b 9 reality 7 ,hoodoo1 pre/o6 & ! ,sph9x 0 reveal$4 ,x conv]s$ aft] a p]iod ( sil5ce ext5d+ back to ! days ( ,rameses :5 a wra?ful god condemn$ an un=tunate k+ to imprisonm5t 9 ! /one /atue4 ,! orig9al trick aw$ audi5ces = _m nights & defi$ explanation until "o night a ke5 obs]v] not$ f+]-pr9ts on :at prov$ to be a mirror4 ,d\btless a c>eless accomplice lo/ 8 job1 b ! damage _h be5 done1 = ! trick 0 reveal$4 ,?is illusion is s effective t x1 or v>iations ( x1 are /ill 9 use4 ,illu/ration3 ,fig4 #ia4--;,ano!r example ( mirror magic4 ,ano!r simple case is illu/rat$ 9 ,fig4 #ia4 ,a l>ge plate-glass mirror ,m 0 plac$ at an angle ( approximately #de degrees f ! floor4 ,?r\gh a hole 9 x an assi/ant's head & %\ld]s project$ & ! edge ( ! op5+ 0 cov]$ ) a drap$ clo?4 ,! audi5ce saw ! image ( ! ceil+ ,c ( ! alcove reflect$ by ! mirror b be+ ignorant ( ! pres5ce ( ! mirror1 assum$ ?is image to be ! re> wall4 ,?is trick 0 effective = _m ye>s4 ,obvi\sly ! mirrors mu/ be spotlessly clean & ! illum9ations ( ! walls1 ceil+1 & 9 "s cases1 ! floor mu/ be v uni=m4 ,fur!rmore1 no l>ge conspicu\s patt]n c\ld be us$ = l9+ ! box-l recess4 ,! =ego+ examples illu/rate ! pr9ciples involv$ 9 ! appe>ance ( gho/s on ! /age & ( a skeleton or o!r gruesome object 9 place ( a human be+4 ,! possibilities ( mirrors 9 su* fields are endless & !y c be /udi$ on a small scale by anyone int]e/$4 ,! pseudoscope :i* produces effects opposite to ^? ( ! /]eoscope is an int]e/+ device4 ,! =ego+ is ! fa9te/ glimpse ( ! use ( ! mirror1 b x does n appe> advisable to dwell fur!r ^u its use1 = aft] all ! results are n visual illusions 9 ! s5se ( ! t]m z usually employ$ ?r\gh\t ?is book4 ,,xv ,,cam\flage ,illusions play$ _m roles 9 ! sci5ce & art ( deception dur+ ! ,_w ,w>1 b !y s]v$ mo/ prom95tly 9 ! lat] /ages ( ! w> ^u ! sea4 ,inasmu* z ! /ory ( ! sci5ce ( cam\flage is n g5]ally available1 x appe>s wor? :ile to pres5t x briefly4 ,besides be+ ( int]e/1 x w reveal to ! read] ! "p t ! sci5ce ( light1 color1 light+1 & vision play$ 9 deception4 ,fur!rmore1 ! read] w s5se ! numb]less illusions :i* are wov5 into cam\flage z develop$ 9 nature1 & 9 human activities4 ,! ^w cam\flage by orig9 does n include all k9ds ( deception2 h[ev]1 by ext5sion x may & w "h signify almo/ ! entire art & sci5ce ( deception z f\nd 9 nature & z practic$ 9 ! ,_w ,w>4 ,t]re/rial ,cam\flage4--,cam\flage is an art :i* is ! natural outgr[? ( our in/9ct = concealm5t & deception :5 pitt+ our wits aga9/ ^? ( a crafty prey or enemy4 ,x is an art old] ?an ! human race1 = its beg9n+s may be trac$ back to ! obscurity ( ! e>ly ages ( ! evolution ( animal life4 ,! "n 0 co9$ by ! ,fr5* to apply to a def9ite art :i* develop$ dur+ ! ,great ,w> to a high /ate1 z _m o!r arts develop$ by draw+ deeply ^u ! res\rces ( sci5tific k4 ,) ! introduction ( ?is specific ^w to cov] a va/ field ( activity 9 sci5tifically conceal+ & deceiv+1 _m are l$ to believe t ?is is a new art1 b su* is n ! case4 ,h[ev]1 l _m o!r arts1 su* z t ( fly+1 ! exig5cies ( mod]n w>f>e h provid$ an impetus :i* has result$ 9 a highly develop$ art4 ,sci5ti/s h recogniz$ = _m ye>s1 & p]haps m or less vaguely = c5turies1 t ,nature exhibits wond]ful examples ( concealm5t & deception4 ,! survival ( ! fitte/1 z ,d>w9 express$ 8 doctr9e1 includ$ ^? individuals ( a species :o 7 be/ fitt$ by _! m>k+s & p]haps by peculi> habits to survive 9 ! environm5t 9 :i* !y liv$4 ,naturally1 m>k+s1 habits1 & environm5t became m & m adapt$ to ea* o!r until ! species became 9 equilibrium ) ,nature suffici5tly to insure its p]petuity4 ,if we look ab\t u ^u animal life we see on e h& examples ( conceal+ coloration & attitudes design$ to deceive ! prey or enemy4 ,! rabbit is mottl$ because ,nature's inf9ite v>iety ( highlights1 %ad[s1 & hues dem& v>iety 9 ! m>k+s ( an animal if ! latt] is to be securely hidd54 ,solid color does n exi/ 9 ,nature's l&scapes 9 l>ge areas4 ,! rabbit is light] und]nea? to comp5sate = ! l[] int5sity ( illum9ation receiv$ on ^! portions4 ,z w9t] approa*es1 animals 9 rigor\s climates ne$ w>m] coats1 & ! hairs gr[ long]4 ,9 _m cases ! color ( ! hairs *anges to gray or :ite1 provid+ a bett] coat+ = ! w9t] environm5t4 ,animals are kn[n to mimic inanimate objects = ! sake ( safety4 ,= example1 ! bitt]n w /& rigid ) its bill po9t$ skyw>d = _m m9utes if x suspects an enemy4 ,non-poison\s snakes resemble poison\s ones 9 g5]al *>act]i/ics & get along 9 ! _w on ! reputation ( _! h>mful relatives4 ,! drone-bee has no /+1 b to ! casual obs]v] x is a bee & bees g5]ally /+4 ,"s animals h v contra/+ patt]ns :i* are conspicu\s 9 %ape1 yet ^! v features disguise ! fact t !y are animals4 ,close obs]vation ( fi%es 9 _! natural environm5t provides /rik+ examples ( conceal+ coloration4 ,va/ works h be5 writt5 on ?is subject by sci5ti/s1 s x w only be t\*$ ^u "h4 ,!re are _m examples ( mobile cam\flage to be f\nd 9 ,nature4 ,seasonal *anges h be5 cit$ 9 a =ego+ p>agraph4 ,! *ameleon *anges its color f mom5t to mom5t4 ,! fl\nd] *anges its color & patt]n to suit its environm5t4 ,x w ev5 /rive to imitate a black & :ite *eck]bo>d4 ,9 look+ at a bird1 animal1 insect1 or o!r liv+ ?+ x is necess>y to place x 9 its natural environm5t at lea/ 9 ! imag9ation1 be=e analyz+ its coloration4 ,= example1 a male mall>d duck hang+ 9 ! m>ket is a v gaudy object1 b place x 9 ! pond among ! we$s1 ! gre5 leaves1 ! highlights1 & ! %ad[s1 & x is surpris+ly inconspicu\s4 ,! zebra 9 ! zoo appe>s to be m>k$ = ! purpose ( h]ald+ its pres5ce any:]e 9 ! range ( vision1 b 9 its re$y1 bu%y1 grassy environm5t x is suffici5tly inconspicu\s = ! species to survive 9 ,nature's cont9u\s w>f>e4 ,?us /udies ( ,nature reveal ! importance ( g5]al hue1 ! necessity = brok5 color or patt]n1 ! fact t black spots simulate %ad[s or voids1 ! comp5sation = l[] illum9ation by c\nt]-%ad+1 & _m o!r facts4 ,! arti/ has aid$ 9 ! developm5t ( cam\flage1 b ! def9ite & work+ basis ( all bran*es ( cam\flage are ! laws & facts ( light1 color1 & vision z ! sci5ti/ kn[s !m4 ,j z l[] animal life has unconsci\sly surviv$ or evolv$ by be+ fitt$ to d s1 mank9d has consci\sly1 or at lea/ in/9ctively1 appli$ cam\flage ( v>i\s k9ds to fool 8 prey or 8 enemy4 ,_m ( u 9 hunt+ ducks h conceal$ ! b[ ( our sneak-boat ) mud & we$s1 or 9 ! season ( float+ ice1 ) a :ite clo?4 ,9 our que/ ( wat] f[l we use decoys & grass suits4 ,! ,esquimau /alks 8 game beh9d a piece ( ice4 ,9 fact1 on e h& we f9d evid5ces ( ?is natural in/9ct4 ,! ,indian pa9t$ 8 face & body 9 a v>iety ( colors & patt]ns4 ,did he d ?is m]ely to be hide\s8 ,x seems v possible t ! same in/9ct :i* made him ! supreme ma/] ( wood-craft caus$ him to reap "s ( ! advantages ( concealm5t due to ! pa9t+ ( 8 face & body4 ,9 pa/ w>s !re is pl5ty ( evid5ce t concealm5t & deception 7 practic$ to ! full ext5t ju/ifiable by ! advantages or necessity4 ,9 ! ,_w ,w> ! adv5t ( ! airplane plac$ ! ?ird dim5sion 9 reconnaissance & call$ = ! application ( sci5ce 9 ! greatly ext5d$ necessity = concealm5t & deception4 ,) ! adv5t ( ! airplane1 a]ial photography became a m important factor ?an visual obs]vation 9 mu* ( ! reconnaissance4 ,?is necessitat$ t cam\flage 9 ord] to be successful _h to meet ! requirem5ts ( ! photographic eye1 z well z t ( ! human eye4 ,9 o!r words1 ! special *>act]i/ics ( ! colors us$ _h to be simil> to ^? ( ,nature's colors4 ,= example1 *lorophyl1 ! gre5 color+ matt] ( vegetation1 is a peculi> gre5 z comp>$ ) gre5 pigm5ts4 ,:5 exam9$ ) a spectroscope x is se5 to reflect a b& ( deep r$ light n reflect$ by ord9>y pigm5ts4 ,9 consid]+ ?is aspect x is well to be> 9 m9d t ! eye is a syn!tic app>atus2 t is does n analyze color 9 a spectral s5se4 ,an arti/ :o views color subjectively & is r>ely famili> ) ! spectral basis may mat* a gre5 leaf p]fectly ) a mixture ( pigm5ts4 ,a photographic plate1 a visual filt]1 or a spectroscope w reveal a diff]5ce :i* ! unaid$ eye does n4 ,"s "t be=e ! ,great ,w> began1 x occurr$ to ! writ] t color$ filt]s c\ld be utiliz$ 9 aid+ vision by increas+ ! contra/ ( ! object to be view$ aga9/ its surr\nd+s4#i ,/udies 7 made ( v>i\s filt]s1 made ) ! object ( ! exp]im5t 9 m9d1 9 view+ ! uni=ms ( v>i\s armies4 ,fur!r developm5ts 7 made by apply+ ! same pr9ciples to color$ lights & pa9t$ pictures4 ,_m ( ^! h be5 describ$ else:]e4 ,) ! developm5t ( ! sci5ce ( cam\flage1 filt]s came into use = ! detection ( cam\flage4 ,z a result ( ! dem& = avoid+ detection by photographic plates & by v>i\s color$ filt]s1 "s pa9ts provid$ = ! cam\fleur 7 develop$ accord+ to ! spectral requirem5ts4 ,_m o!r applications ( sci5ce 7 develop$ s t cam\flage c n[ be call$ an art bas$ ^u s\nd sci5tific pr9ciples4 ,natural light+ is s v>iable t x is (t5 impossible to provide cam\flage :i* w rema9 satisfactory f day to day2 !re=e1 a broad k ( ,nature's light+ is necess>y 9 ord] to provide ! be/ compromise4 ,!re are two s\rces ( light 9 ! daytime1 namely1 ! sun & ! sky4 ,! relative am\nts ( light contribut$ by ^! two s\rces is cont9ually *ang+4 ,! sky on cl\dless days contributes f "o-t5? to "o-?ird ( ! total light receiv$ by a horizontal surface at noon4 ,light f ! sky & light reflect$ f ! surr\nd+s illum9ate ! %ad[s4 ,^! %ad[s are diff]5t 9 color ?an highlights1 al?\gh ^! f9] di/9ctions may be ignor$ 9 mo/ cam\flage because color becomes less conspicu\s z ! di/ance ( obs]vation increases4 ,9 g5]al1 ! di/ribution ( brightness or light & %ade is ! mo/ important aspect to be consid]$4 ,! cam\fleur worries ov] %ad[s m ?an any o!r aspect g5]ally4 ,on ov]ca/ days cam\flage is g5]ally mu* m successful ?an on sunny days4 ,obvi\sly1 c\nt]-%ad+ is resort$ to 9 ord] to elim9ate %ad[s1 & :]e ?is is unsuccessful confusion is resort$ to by mak+ m %ad[s4 ,! %ape & ori5tation ( a build+ is v important to ^? *>g$ ) ! problem ( r5d]+ x inconspicu\s to ! enemy1 b little att5tion has be5 paid to ^! aspects4 ,= example1 a hang> pa9t$ a v satisfactory dull gre5 w be di/+ui%able by its %ape z indicat$ by its %ad[ & %ad$ sides4 ,9 ?is zone a hang>1 = example1 w\ld be m readily conceal$ if its l5g? lay nor? & s\?4 ,its sides c\ld be br\ght ) a gradual curve to ! gr\nd & its re>1 :i* is dur+ mo/ ( ! day 9 %ad[1 c\ld be effectively treat$ to conceal ! %ad[4 ,a little ?\ght w conv9ce ! read] ( ! importance ( %ape & ori5tation4 ,brok5 color or patt]n is ano!r fundam5tal ( cam\flage :i*1 ( c\rse1 mu/ be adapt$ to its environm5t4 ,= our trucks1 cannon1 & _m o!r implem5ts ( w>1 d>k gre51 yell[1 d>k blue1 light gray1 & o!r colors h be5 us$ 9 a jumble ( l>ge patt]ns4 ,a f9al ref9em5t is t ( ! bl5d+ ( ^! colors at a di/ance1 :]e ! eye no long] resolves ! individual pat*es1 to a color :i* simulates ! g5]al hue ( ! surr\nd+s4 ,= example1 r$ & gre5 pat*es at a di/ance bl5d to yell[2 yell[ & blue pat*es bl5d to a neutral gray if suitably balanc$1 b if n1 to a yell[-gray or a blue-gray2 r$1 gre51 & blue if prop]ly balanc$ w bl5d to a gray2 black1 :ite & gre5 pat*es w bl5d to a gre5 %ade1 & s on4 ,^! facts are simple to ^? :o are famili> ) ! sci5ce ( light & color1 b ! arti/1 ^: k is bas$ ^u ! mixture ( pigm5ts1 sometimes errs 9 consid]+ ?is aspect ( color-bl5d+ by di/ance4 ,= example1 x is n uncommon = him to /ate t at a di/ance yell[ & blue pat*es bl5d to make gre51 b ! addition ( lights or ( juxtapos$ colors is q diff]5t 9 result f ! addition ( pigm5ts by intimately mix+ !m4 ,9 con/ruct+ su* a patt]n ( v>i\s colors x is also desirable to h ! f9al mean brightness approximate t ( ! g5]al surr\nd+s4 ,?is problem c be solv$ by means ( ! photomet] & a =mula provid$1 :i* /ates1 = example1 t a c]ta9 p]c5tage ( ! total area be pa9t$ 9 gray1 ano!r p]c5tage 9 gre51 & s on4 ,! photomet] has play$ an important role 9 e/abli%+ ! sci5tific basis ( cam\flage4 ,! size ( ! patt]n mu/ be gov]n$ by ! di/ance at :i* x is to be view$1 = obvi\sly if too small ! effect is t ( solid color1 & if too l>ge x w r5d] ! object conspicu\s1 :i* is a disadvantage rank+ next to recognizable4 ,:]e ! arti/ is conc]n$ ) a backgr\nd :i* does n include ! sky1 t is1 :]e he deals only ) illum9at$ objects on ! e>?1 8 tra9$ eye is valuable provid$ ! colors us$ meet ! dem&s made by photographic plates & visual color-filt]s4 ,9 o!r words1 ! sky z a backgr\nd gives tr\ble to all :o are unfamili> ) sci5tific measurem5ts4 ,! brightnesses ( sky & cl\ds are outside ! scale ( brightnesses ord9>ily enc\nt]$ 9 a l&scape4 ,_m int]e/+ in/ances ( ! arti/'s mi/akes 9 deal+ ) ^! backgr\nds c\ld be pres5t$2 h[ev]1 ! arti/'s tra9$ eye has be5 a great aid 9 con/ruct+ patt]ns & v>i\s o!r types ( cam\flage4 ,"o ( ! mo/ conspicu\s aspects ( ! e>?'s surface is its texture4 ,f great heights x appe>s flat1 t is1 roll+ l& is iron$ out & ! g5]al cont\r ( ! gr\nd is flatt5$4 ,h[ev]1 ! elem5t ( texture always rema9s4 ,?is is ! *ief reason = ! ext5sive use ( nett+ on :i* dy$ raffia1 foliage1 pieces ( color$ clo?1 etc41 are ti$4 ,su* network has conceal$ _m guns1 headqu>t]s1 ammunition dumps1 communication tr5*es1 roadways1 etc4 ,:5 ?is has be5 well done ! concealm5t is p]fect4 ,"o ( ! greate/ annoyances to ! cam\fleur is ! lack ( dullness or flatness ( ! pa9ts1 fabrics1 & "s ( ! o!r m$ia us$4 ,:5 view$ at "s angles ! gl9t ( highlights due to specul> reflection r5d]s ! "w v conspicu\s4 ,= ?is reason natural foliage or su* mat]ial z dy$ raffia has be5 v successful4 ,sy/ems ( network & v]tical scre5s h be5 ext5sively employ$ on roadways ne> ! front1 n = ! purpose ( conceal+ f ! enemy ! fact t ! roadways exi/1 b to make x necess>y to %ell ! entire roadway cont9ually if x is hop$ to prev5t its use4 ,al?\gh ! cam\fleur is provid$ ) a va/ am\nt ( mat]ial = 8 "w1 _m ( 8 requirem5ts are met by ! mat]ial at h&4 ,obvi\sly1 ! mo/ conv5i5t me?od ( provid+ concealm5t = a giv5 environm5t is to use ! mat]ials ( ! environm5t4 ,h5ce1 rubbi% f ru9$ build+s or villages supplies cam\flage = guns1 huts1 etc41 9 t environm5t4 ,9 woods ! mat]ial to simulate ! woods is at h&4 ,_m ( ^! aspects are s obvi\s to ! read] t space w n be giv5 to _! consid]ation4 ,! color ( ! soil is important1 = if x is conspicu\s ! cam\fleur mu/ provide scre5s ( natural turf4 ,9 ?is great game ( hocus-pocus _m deceptions are resort$ to4 ,replicas ( l>ge guns & tr5*es are made2 dummy soldi]s are us$ to foil ! snip] & to make him reveal 8 location1 & papi]-ma*e horses1 trees1 & o!r objects conceal snip]s & obs]v]s & af=d li/5+ po/s4 ,gunn]s h be5 dress$ 9 summ] 9 gre5 fl[+ robes4 ,9 w9t] :ite robes h be5 utiliz$4 ,h[ f> away f mod]n w>riors are all ! usual glitt] & glam\r ( milit>y imp$im5ta 9 ! pa/ p>ades ( peace "t6 ,! armies n[ dig 9 = concealm5t4 ,! artill]y is no long] invisible beh9d yond] hill1 = ! eyes ( ! a]ial obs]v] ( ! cam]a reveal its position unless cam\flag$ = ! ?ird dim5sion4 ,9 ! =ego+ only ! highlights ( a va/ art h be5 view$1 b ! art is /ill va/]1 = x ext5ds into o!r fields4 ,s\nd mu/ sometimes be cam\flag$ & ?is c only be done by us+ ! same m$ium--s\nd4 ,9 ^! days ( sci5tific w>f>e x is to be expect$ t ! positions ( enemy guns w\ld be detect$ by o!r means ?an employ$ 9 ! pa/4 ,a notable me?od is ! use ( velocity ( s\nd4 ,records are made at v>i\s /ations ( ! fir+ ( a gun & ! explosion ( ! %ell4 ,by trigonometric laws ! position ( ! gun is asc]ta9$4 ,x is said t ! ,g]mans fir$ a numb] ( guns simultane\sly ) ! #ge-mile gun 9 ord] to cam\flage its location4 ,! airplane & subm>9e w\ld gladly employ s\nd cam\flage 9 ord] to foil ! s\nd detector if practicable solutions 7 propos$4 ,! =ego+ is a brief /atem5t ( "s ( ! fundam5tal pr9ciples ( l& cam\flage4 ,let u n[ briefly consid] ! eyes ( ! enemy4 ,( c\rse1 mu* concealm5t & deception is devis$ to foil ! obs]v] :o is on ! gr\nd & fairly close4 ,! proc$ure is obvi\s to ! av]age imag9ation2 h[ev]1 ! read] may n be acqua9t$ ) ! a]ial eyes f :i* concealm5t is v important4 ,z "o asc5ds 9 an airplane to view a l&scape he is impress$ ) ! inadequacy ( ! eyes to obs]ve ! va/ numb] ( details & ( ! m9d to reta9 !m4 ,field glasses _c be us$ z satisfactorily 9 an airplane z on solid gr\nd1 ow+ to vibration & o!r movem5ts4 ,! diff]5ce is n z great 9 ! huge fly+ boats z x is 9 ! ord9>y airplane4 ,! cam]a c record _m details ) high] accuracy ?an ! eye4 ,at an altitude ( "o mile ! l5s c be us$ at full ap]ture & ?us v %ort exposures are possible4 ,?is t5ds to avoid ! difficulty due to vibration4 ,:5 ! plates are develop$ = detail & enl>gem5ts are made1 _m m9ute details are di/+ui%able4 ,fur!rmore1 ow+ to ! fact t ! spectral s5sibilities ( photographic emulsions diff] f t ( ! eye1 contra/s are br\ght out :i* ! eye w\ld n see4 ,?is applies also to cam\flage :i* is devis$ m]ely to suit ! eye4 ,individual footpr9ts h be5 di/+ui%$ on pr9ts made f negatives expos$ at an altitude ( #fjjj feet4 ,by means ( photography1 daily records c be made if desir$ & ^! c be comp>$4 ,a slight *ange is readily not$ by su* comp>ison by skill$ int]pret]s ( a]ial photographs4 ,! disappe>ance ( a tree f a clump ( trees may ar\se suspicion4 ,sometimes a wilt$ tree has be5 not$ on a photograph :i* naturally attracts att5tion to ?is position4 ,x has be5 said t ! bellig]5ts resort$ to transplant+ trees a %ort di/ance at a "t f day to day 9 ord] to provide cle>ance = newly plac$ guns4 ,by pa?s conv]g+ t[>d a c]ta9 po9t1 x may be conclud$ f ! photographs t an ammunition dump or headqu>t]s is locat$ !re ev5 ?\gh ! position itself 0 well cam\flag$4 ,cont9u\s photographic records may reveal di/urbances ( turf & lead to a m c>eful inspection ( ! region = sapp+ op]ations1 etc4 ,by ^! few details x is obvi\s t ! airplane is responsible = mu* ( ! developm5t ( cam\flage on l&1 ow+ to ! necessity :i* x creat$ = a mu* m ext5sive concealm5t4 ,! entire /ory ( l& cam\flage w\ld ov]fl[ ! conf9es ( a volume1 b x is hop$ t ! =ego+ w aid ! read] 9 visualiz+ ! magnitude ( ! art & ! sci5tific basis ^u :i* t]re/ial cam\flage is f\nd$4 ,m>9e ,cam\flage4--,at ! "t ( ! ,spani%-,am]ican w>1 our battle%ips 7 pa9t$ :ite1 app>5tly ) little ?\ght ( atta9+ l[ visibility4 ,lat] ! s-call$ battle%ip gray 0 adopt$1 b x has be5 app>5t to close obs]v]s t ?is gray is 9 g5]al too d>k4 ,app>5tly x is a mixture ( black & :ite4 ,! %ips ( ! ,briti% navy 7 at "o "t pa9t$ black1 b prec$+ ! ,great ,w> _! coats 7 ( a w>m d>k gray4 ,g]many adopt$ d>k gray be=e ! close ( ! la/ c5tury & ,au/ria adopt$ ! ,g]man gray at ! outbreak ( ! w>4 ,! ,fr5* & ,italian fleets 7 also pa9t$ a w>m gray4 ,?is developm5t t[>d gray 0 ! result ( an aim t[>d atta9+ l[ visibility4 ,o!r *anges 7 necessitat$ by subm>9e w>f>e :i* w be discuss$ lat]4 ,9 ! e>ly days ( unre/rict$ subm>9e w>f>e _m s*emes = modify+ ! appe>ance ( vessels 7 submitt$4 ,_m ( ^! 7 m]ely wild fancies ) no e/abli%$ reason+ beh9d !m4 ,"h aga9 sci5ce came to ! rescue & ?r\gh rese>* & consultation1 f9ally /raight5$ out matt]s4 ,! "q ( l[ visibility = vessels c\ld be ?or\ghly /udi$ on a laboratory scale1 because ! seascape & natural light+ conditions c\ld be reproduc$ v closely4 ,ev5 ! g5]al wea!r conditions c\ld be simulat$1 al?\gh1 ( c\rse1 ! exp]im5ts c\ld be prosecut$ outdoors ) small models1 z inde$ !y 74 ,mr4 ,l4 ,a4 ,jones#aj c>ri$ out an inve/igation on ! %ore ( ,lake ,ont>io1 & laboratory exp]im5ts 7 conduct$ by o!rs ) ! result t mu* light 0 %$ on ! que/ions ( m>9e cam\flage4 ,?is "w confirm$ ! conclusion ( ! au?or & o!rs t our battle%ip gray 0 too d>k4 ,( c\rse1 ! color be/ adapt$ is t :i* is ! be/ compromise = ! extreme v>iety 9 light+ & wea!r conditions4 ,^! v>y 9 diff]5t p>ts ( ! _w1 s naturally ^? 9 ! w> zone 7 ( prim>y importance4 ,all cam\flage g5]ally mu/ aim to be a compromise be/ suit$ = av]age or dom9at+ conditions4 ,= example1 9 foggy wea!r a c]ta9 pa9t may r5d] a %ip ( l[ visibility1 b on a sunny day ! %ip might be pla9ly visible4 ,h[ev]1 if %ips are r5d]$ ( l[ visibility = ev5 a portion ( ! "t x is obvi\s t an advantage has be5 ga9$4 ,cl\d9ess increases g5]ally f ! equator nor?w>d1 z indicat$ by meteorological annals4 ,9 ord] to /udy l[ visibility a scale ( visibility mu/ be e/abli%$1 & x is ess5tial to beg9 ) ! fundam5tals ( vision4 ,we di/+ui% objects by contra/s 9 brightness & 9 color & we recognize objects by ^! contra/s :i* mold _! =ms4 ,9 rese>*es 9 vision x is cu/om>y to devise me?ods by :i* ^! contra/s c be v>i$4 ,?is is done by increas+ or decreas+ a veil ( lum9osity ov] ! object & its surr\nd+s & by o!r means4 ,mu* "w has be5 done 9 pa/ ye>s 9 /udy+ ! m9imum p]ceptible contra/1 & x has be5 f\nd to v>y ) hue1 ) ! magnitude ( brightness1 & ) ! size ( ! image1 t is1 ) ! di/ance ( an object ( giv5 size4 ,9 su* problems z ?is "o mu* sci5tific "w c be drawn ^u4 ,a simple1 ?\gh r\gh1 scale ( visibility may be made by us+ a s]ies ( photographic scre5s ( diff]5t d5sities4 ,a photographic scre5 is slightly diffus+1 /ill ! object c be view$ ?r\gh x v well4 ,su* me?ods h be5 employ$ by v>i\s inve/igators 9 ! /udy ( visibility4 ,ow+ to ! curvature ( ! e>?1 ! di/ance at :i* a vessel c be se5 on a cle> day is limit$ by ! height ( ! obs]v] & ( ! %ip's sup]/ructure4 ,= an obs]v] 9 a c]ta9 position ! visibility range v>ies z ! squ>e root ( ! di/ance ( ! object f him4 ,su* data are easily available1 s !y w n be giv5 "h4 ,s f> we h consid]$ ! %ip itself :51 z a matt] ( fact1 on cle> days ! smoke cl\d emitt$ by ! %ip is usually visible long be=e a %ip's sup]/ructure appe>s on ! horizon4 ,?is l$ to ! prev5tion ( smoke by bett] combu/ion1 by us+ smokeless fuels1 etc4 ,! irregul> skyl9e ( a %ip is p]haps "o ( ! mo/ influ5tial factors :i* t5d to increase its visibility4 ,_m sugge/ions p]ta9+ to ! modification ( ! sup]/ructure h be5 made1 b ^! are g5]ally impracticable4 ,false "w suff]s 9 heavy seas & high w9ds4 ,aft] adopt+ a suitable gray z1 a l[-visibility pa9t = %ips1 p]haps ! next ref9em5t 0 c\nt] %ad+2 t is1 %ad[s 7 pa9t$ a light] color1 or ev5 :ite4 ,! sup]/ructure 0 pa9t$ 9 "s cases a light blue1 ) ! hope t x w\ld fade into ! di/ant horizon4 ,h[ev]1 ! effectiv5ess ( ! subm>9e dem&$ new exp$i5ts because )9 its range ( effectiv5ess no +5uity c\ld r5d] its int5d$ prey invisible4 ,! effective gun-fire f subm>9es is sev]al miles & torp$oes c be effective at ^! di/ances4 ,h[ev]1 ! subm>9e pref]s to dis*>ge ! torp$o at ranges )9 a mile4 ,x is obvi\s t1 9 av]age wea!r1 l[ visibility ceases to be v effective aga9/ ! subm>9e4 ,! movem5t ( a t>get is ( mu* less importance 9 ! case ( gun-fire ?an 9 ! case ( ! torp$o ) its relatively l[ velocity4 ,! subm>9e gunn] mu/ h ! range1 spe$1 & c\rse ( ! t>get 9 ord] to fire a torp$o ) any hope ( a hit4 ,!re=e1 any unc]ta9ties t c\ld be introduc$ p]ta9+ to ^! factors w\ld be to ! advantage ( ! subm>9e's prey4 ,= example1 l[ visibility gave way to confusibility 9 ! discussions ( def5ce aga9/ ! subm>9e & ! slogan1 ,a miss is z good z a mile 0 adopt$4 ,! =ego+ factors _c be det]m9$ ord9>ily ) high accuracy1 s t x appe>$ possible to add some:at to ! difficulties ( ! subm>9e comm&]4 ,_m optical illusions h be5 devis$ & /udi$ by sci5ti/s4 ,9 fact1 "s ( ^! tricks are well kn[n to ! g5]al read]4 ,/raight l9es may appe> brok51 conv]g5t1 or div]g5t by provid+ c]ta9 patt]ns or l9es int]m+l$ ) !m4 ,_m ( ^! 7 appli$ to models 9 laboratory exp]im5ts & x has be5 %[n t confusion results z to ! c\rse ( ! vessel4 ,! application ( ^! on vessels has result$ 9 ! grotesque patt]ns to be se5 on %ips dur+ ! latt] /age ( ! w>4 ,x is well kn[n t ^! illusions are mo/ effective :5 ! greate/ contra/s are us$1 h5ce black & :ite patt]ns are common4 ,color has n be5 utiliz$ z def9itely z patt]n 9 confusibility1 al?\gh !re is a second>y aim ( obta9+ l[ visibility at a great di/ance by prop]ly balanc+ ! black1 :ite1 & o!r colors s t a blue-gray results at di/ances too great = ! individual patt]ns to be resolv$ by ! eye4 ,color c\ld be us$ = ! purpose ( increas+ ! conclusion by app>5tly alt]+ ! p]spective4 ,= example1 blue & r$ patt]ns on ! same surface d n usually appe> at ! same di/ance1 ! r$ appe>+ clos] ?an ! blue4 ,illu/ration3 ,fig4 #ib4--;,a prim>y /age 9 ! evolution ( ! use ( geometrical-optical illusions on %ips4 ,su* app>5tly grotesque patt]ns aim$ to di/ort ! l9es ( ! %ip & to w>p ! p]spective by :i* ! c\rse is e/imat$4 ,?is 0 ! f9al type ( m>9e cam\flage at ! close ( ! w>4 ,besides rely+ ^u ^! illusions1 %ips zigzagg$ on be+ attack$ & aim$ 9 o!r ways to confuse ! enemy4 ,no g5]al attempt 0 made to disguise ! b[1 because ! b[-wave 0 g5]ally visible4 ,h[ev]1 attempts h be5 made to increase x app>5tly & ev5 to provide "o at ! /]n4 ,9 fact1 +5uity 0 heavily drawn ^u & _m exp$i5ts 7 tri$4 ,aft] l[-visibility 0 ab&on$ 9 favor ( ! optical illusion = fru/rat+ ! torp$o-attack by ! subm>9e1 !re 0 a p]iod dur+ :i* m]ely a mottl$ patt]n 0 us$ = vessels4 ,gradually ?is evolv$ t[>d su* patt]ns z %[n 9 ,fig4 #ib4 ,9 ?is illu/ration x is se5 t ! optical-illusion idea has tak5 def9ite =m4 ,dur+ ! p]iod ( unc]ta9ty z to ! c\rse ! patt]n %\ld take1 a regul>ity ( patt]n 0 tri$1 su* z illu/rat$ 9 ,figs4 #ic & #id4 ,f9ally1 :5 x dawn$ m or less simultane\sly ^u v>i\s sci5tific m51 :o 7 /udy+ ! problems ( protect+ vessels ^u ! seas1 t ! geometrical-optical illusion 9 its well-kn[n =ms 0 directly adaptable1 r5ew$ impetus 0 giv5 to inve/igation4 ,! sci5tific lit]ature yield$ _m facts b ! problems 7 also /udi$ directly by means ( models4 ,! latt] /udy is illu/rat$ by ,figs4 #ie & #if1 ! orig9als hav+ be5 furni%$ by ,mr4 ,e4 ,l4 ,w>n]1#aa :o among o!rs prosecut$ a /udy ( ! application ( illusions to vessels4 ,! f9al results 7 gratify+1 z %[n to "s ext5t 9 ,figs4 #ig & #ih1 also k9dly furni%$ by ,mr4 ,w>n]4 ,x is se5 t ^! patt]ns are really deceiv+ z to ! c\rse ( ! vessel4 ,illu/ration3 ,figs4 #ic & #id4--;,attempts at di/ortion ( outl9e :i* prec$$ ! adoption ( geometrical-optical illusions = %ips4 ,illu/ration3 ,figs4 #ie & #if4--;,illu/rat+ ! use ( models by ! ,navy ,dep>tm5t 9 develop+ ! geometrical-illusion = %ips4 ,! convoy sy/em is well kn[n to ! read]4 ,?is sav$ _m vessels f de/ruction4 ,vessels ( ! same spe$ 7 gr\p$ toge!r & /eam$ 9 flocks across ! ,atlantic4 ,anyone :o has _h ! extreme pleasure ( look+ d[n f an airplane ^u ^! convoys l$ by de/roy]s & att5d$ by *as]s is /rongly impress$ ) ! old adage1 ,9 unity !re is /r5g?4 ,be=e ! w> began1 a ,brazilian battle%ip laun*$ 9 ?is c\ntry 0 provid$ ) a sy/em ( blue lights = use :5 ne> ! enemy at night4 ,blue 0 adopt$ d\btless = its l[ range comp>$ ) light ( o!r colors4 ,we "k t ! sett+ sun is r$ because ! atmosph]ic du/1 smoke1 & moi/ure h scatt]$ & absorb$ ! blue & gre5 rays m ?an ! r$ & yell[ rays4 ,9 o!r words ! p5etrat+ p[] ( ! r$ & yell[ is great] ?an t ( ! blue rays4 ,?is c\ntry made use ( ?is exp$i5t to "s ext5t4 ,( c\rse1 all o!r lights 7 ext+ui%$ & por?oles 7 clos$ 9 ocean travel dur+ ! subm>9e m5ace4 ,illu/ration3 ,figs4 #ig & #ih4--;,examples ( ! geometrical-optical illusion z f9ally appli$4 ,naturally smoke-scre5s 7 adopt$ z a def5sive measure on sea z well z on l&4 ,de/roy]s bel* d5se smoke f _! /acks 9 ord] to scre5 battle%ips4 ,_m types ( smoke-boxes h be5 devis$ or sugge/$4 ,! smoke f ^! is produc$ *emically & ! app>atus mu/ be simple & safe4 ,if a m]*antman 7 attack$ by a subm>9e imm$iately smoke-boxes w\ld be dump$ ov]bo>d or "s :i* 7 in/all$ on deck w\ld be put into op]ation & ! %ip w\ld be /e]$ 9 a zigzag c\rse4 ,^! exp$i5ts 7 likely to r5d] %ell-fire & obs]vations inaccurate4 ,?is mode ( def5se is obvi\sly be/ suit$ to un>m$ vessels4 ,9 ! use ( smoke-boxes ! direction & velocity ( ! w9d mu/ be consid]$4 ,! writ] is unacqua9t$ ) any attempts made to cam\flage subm>9es "u wat]1 b t ?is c be done is evid5t f a]ial obs]vations4 ,:5 look+ ov] ! wat] f a po9t n f> above x1 z on a pi]1 we are unable to see into ! wat] except at po9ts ne> u :]e our direction ( vision is n v oblique to ! surface ( ! wat]4 ,! brightness ( ! surface ( wat] is due to mirror$ sky & cl\ds ord9>ily4 ,= a p]fectly smoo? surface ( wat]1 ! reflection factor is #b p] c5t = p]p5dicul> incid5ce4 ,?is increases only slightly z ! obliquity increases to an angle ( ab\t #fj degrees4 ,f ?is po9t ! reflection-factor ( ! surface rapidly increases1 becom+ #ajj p] c5t at #ij degrees incid5ce4 ,?is acc\nts = ! ease ) :i* we c see into ! wat] f a position directly ov]head & h5ce ! airplane has be5 an effective hunt] ( subm]g$ subm>9es4 ,! dep? at :i* an object c be se5 9 wat] dep5ds1 ( c\rse1 ^u its cl>ity4 ,x may be surpris+ to _m to le>n t ! brightness ( wat]1 su* z riv]s1 bays1 & oceans1 z view$ p]p5dicul>ly to its surface1 is l>gely due to light diffus$ )9 x4 ,?is po9t became /rik+ly evid5t dur+ ! progress ( "w 9 a]ial photometry4 ,a subm]g$ subm>9e may be invisible = two reasons3 7#a7 ,x may be deep en\gh to be effectively veil$ by ! lum9osity ( ! mass ( wat] above x 7includ+ ! surface brightness7 or1 7#b7 ,x may be ( ! prop] brightness & color to simulate ! brightness & color ( ! wat]4 ,x is obvi\s t if x 7 :ite x w\ld h to atta9 concealm5t by subm]g+ deeply4 ,if x 7 a fairly d>k gre5i%-blue x w\ld be invisible at v small dep?s4 ,9 fact1 x w\ld be ( v l[ visibility j bel[ ! surface ( ! wat]4 ,by ! use ( ! writ]'s data on hues & reflection-factors ( e>? & wat] areas x w\ld be easy to cam\flage subm>9es effectively f enemies ov]head4 ,! visibility ( subm>9es is well exemplifi$ by view+ l>ge fi% su* z %>ks f air%ips at l[ altitudes4 ,!y appe> z m9iature subm>9es d>k gray or almo/ black amid gre5i%-blue surr\nd+s4 ,incid5tally1 ! color ( wat] v>ies consid]ably f ! dirty yell[i%-gre5 ( %all[ inl& wat]s conta9+ mu* susp5d$ matt] to ! gre5i%-blue ( deep cle> ocean wat]s4 ,! latt] z view$ v]tically are ab\t "o-half ! brightness ( ! =m] "u ! same conditions & are decid$ly blu]4 ,! ,visibility ( ,airplanes4--,9 ! ,great ,w> ! airplane made its debut 9 w>f>e & 9 a %ort "t made a wond]ful record1 yet :5 ho/ilities ceas$ a]ial cam\flage _h n be5 put on a sci5tific basis4 ,no nation _h develop$ ?is g5]al aspect ( cam\flage sy/ematically or to an ext5t comp>able ) ! developm5ts on l& & sea4 ,"o ( ! *ief difficulties 0 t sci5tific data :i* 7 applicable 7 lack+4 ,dur+ ! au?or's activities z ,*airman ( ! ,committee on ,cam\flage ( ! ,national ,rese>* ,c\ncil he complet$ an ext5sive inve/igation#ab ( ! fundam5tals ^u :i* ! atta9m5t ( l[ visibility = airplanes mu/ be bas$4 ,solutions ( ! problems enc\nt]$ 9 r5d]+ airplanes ( l[ visibility result$ & v>i\s recomm5dations 7 made1 b ! exp]i5ces & data w be drawn ^u "h only 9 a g5]al way4 ,9 ?is g5]al review details w\ld consume too mu* space1 = ! int5tion has be5 to pres5t a broad view ( ! subject ( cam\flage4 ,! visibility ( airplanes pres5ts "s ( ! mo/ int]e/+ problems to be f\nd 9 ! developm5t ( ! sci5tific basis = cam\flage4 ,! g5]al problem may be subdivid$ accord+ to ! type ( airplane1 its field ( op]ation1 & its activity4 ,= example1 patrol craft :i* fly l[ ov] our own l9es w\ld prim>ily be cam\flag$ = l[ visibility z view$ by enemies above4 7,see ,fig4 #ii47 ,high-fly+ craft w\ld be r5d]$ ( l[ visibility z view$ prim>ily by ! enemy bel[4 ,airplanes = night use pres5t o!r problems & ! visibility ( seaplanes is a di/9ct problem1 ow+ to ! fact t ! important backgr\nd is ! wat]1 because seaplanes are n ord9>ily high-fly+ craft4 ,9 all ^! consid]ations x w be not$ t ! activity ( ! airplanes is ( prim>y importance1 because x det]m9es ! l9es ( proc$ure 9 r5d]+ ! craft ( l[ visibility4 ,?is aspect is too complicat$ to discuss ?or\ghly 9 a brief resume4 ,illu/ration3 ,fig4 #ii4--,repres5tative e>? backgr\nds = an airplane 7uncam\flag$7 z view$ f above4 ,! same fundam5tals ( light1 color1 & vision apply 9 ?is field z 9 o!r fields ( cam\flage1 b diff]5t data are requir$4 ,:5 view+ aircraft f above1 ! e>? is ! backgr\nd ( mo/ importance4 ,cumulus cl\ds on sunny days are g5]ally at altitudes ( #djjj to #gjjj feet4 ,cl\ds are n always pres5t & besides !y are ( su* a diff]5t ord] ( brightness f t ( ! e>? t !y _c be consid]$ 9 cam\flage design$ = l[ visibility f above4 ,9 o!r words1 ! compromise 9 ?is case is to accept ! e>? z a backgr\nd & to "w on ?is basis4 ,we are confront$ ) seasonal *anges ( l&scape1 b inasmu* z ! summ] l&scape 0 ( greate/ importance g5]ally1 x 0 ! dom9at+ factor 9 consid]+ l[ visibility f above4 ,on look+ d[n ^u ! e>? "o is impress$ ) ! def9ite types ( areas su* z cultivat$ fields1 woods1 b>r5 gr\nd & wat]4 ,diff]5t l&scapes conta9 ^! areas 9 v>i\s proportions1 :i* fact mu/ be consid]$4 ,_m ?\s& det]m9ations ( reflection-factor & ( approximate hue 7 made = ^! types ( areas1 & ^u ! mean values cam\flage = l[ visibility z view$ f above 0 develop$4 ,a few values are giv5 9 ! accompany+ table1 b a m compreh5sive pres5tation w be f\nd else:]e4#ab ,mean ,reflection-,factors 7,f ?\s&s ( measurem5ts made by view+ v]tically d[nw>d dur+ summ] & fall f v>i\s altitudes47 ,p] ,c5t ,woods #d4c ,b>r5 gr\nd #ac4j ,fields 7graz+ l& & gr[+ crops7 #f4h ,inl& wat] 7riv]s & bays7 #f4h ,deep ocean wat] #c4e ,d5se cl\ds #gh4j ,wood$ areas are ! d>ke/ g5]al areas 9 a l&scape & possess a v l[ reflection-factor4 ,f above "o sees ! deep %ad[s int]sp]s$ among ! highlights4 ,^! %ad[s & ! trapp+ ( light are l>gely responsible = ! l[ brightness or app>5t reflection-factor4 ,?is is be/ illu/rat$ by means ( black velvet4 ,if a piece ( c>dbo>d is dy$ ) ! same black dye z t us$ to dye ! velvet1 x w diffusely reflect #b or #c p] c5t ( ! incid5t light1 b ! black velvet w reflect no m ?an #j4e p] c5t4 ,! velvet fib]s provide _m light traps & ca/ _m %ad[s :i* r$uce ! relative brightness or reflection-factor f> bel[ t ( ! flat c>dbo>d4 ,cultivat$ fields on :i* !re are gr[+ crops are ne>ly twice z bright z wood$ areas1 dep5d+1 ( c\rse1 ^u ! d5s5ess ( ! vegetation4 ,b>r5 sunbak$ l&s are g5]ally ! brighte/ l>ge areas 9 a l&scape1 ! brightness dep5d+ ^u ! "* ( ! soil4 ,wet soil is d>k] ?an dry soil1 ow+ to ! fact t ! pores are fill$ ) wat]1 ?us r$uc+ ! reflection-factor ( ! small p>ticles ( soil4 ,a dry :ite blott+ pap] :i* reflects #ge p] c5t ( ! incid5t light w reflect only ab\t #ee p] c5t :5 wet4 ,inl& wat]s :i* conta9 mu* susp5d$ matt] are ab\t z bright z graz+ l& & cultivat$ fields4 ,%all[ wat] p>takes some:at ( ! color & brightness ( ! b$1 & deep ocean wat] is some:at d>k] ?an wood$ areas4 ,quiet /agnant pools or small lakes are sometimes exce$+ly d>k2 9 fact1 !y appe> l pools ( ink1 ow+ to ! fact t _! brightness z view$ v]tically is almo/ entirely due to surface reflection4 ,if x is due entirely to reflection at ! surface1 ! brightness w be ab\t #b p] c5t ( ! brightness ( ! z5i? sky4 ,t is1 :5 view+ su* a body ( wat] v]tically "o sees an image ( ! z5i? sky r$uc$ 9 brightness to ab\t #b p] c5t4 ,! e>? patt]ns 7 ext5sively /udi$ ) ! result t def9ite conclusions 7 =mulat$ p]ta9+ to ! be/ patt]ns to be us$4 ,al?\gh x is out ( ! "q to pres5t a detail$ discussion ( ?is important phase 9 ?is resume1 att5tion w be call$ to ! mann] 9 :i* ! e>? patt]ns dim9i% ) increas+ altitude4 ,! ins]t 9 ,fig4 #ajj %[s ! actual size ( an image ( a #ej-;foot airplane f #j to #af1jjj feet bel[ ! obs]v] z comp>$ ) correspond+ images 7to ! same scale7 ( objects & areas on ! e>?'s surface #aj1jjj feet bel[ ! obs]v]4 ,= simplicity assume a cam]a l5s to h a focal l5g? equal to #aj in*es1 !n ! l5g? x ( ! image ( an object #ajj feet long w be relat$ to ! altitude h 9 ?is mann]3 x #ajj ----- ----- or xh #ajjj #aj h ,by sub/itut+ ! values ( altitude h 9 ! equation ! values ( ! l5g? x ( ! image are f\nd4 ,! foll[+ values illu/rate ! *ange 9 size ( ! image ) altitude3 ,altitude h 9 feet ,size ( image x 9 in*es #a1jjj #a4jj #b1jjj #j4ej #c1jjj #j4cc #d1jjj #j4be #aj1jjj #j4aj #bj1jjj #j4je ,x is se5 t ! image dim9i%es less rapidly 9 size z ! altitude increases4 ,= example1 go+ f #ajjj feet to #bjjj feet ! image is r$uc$ to "o-half4 ,! same r$uction takes place 9 asc5d+ f #aj1jjj to #bj1jjj feet4 ,by tak+ a s]ies ( photographs & kn[+ ! r$uction-factor ( ! l5s x is a simple matt] to /udy patt]n4 ,an airplane ( kn[n dim5sions c be plac$ 9 ! imag9ation at any altitude on a photograph tak5 at a kn[n altitude & ! futility ( c]ta9 patt]ns & ! advantages ( o!rs are at once evid5t4 ,illu/ration3 ,fig4 #ajj4--;,illu/rat+ ! /udy ( patt]n = airplanes4 ,! photograph 0 tak5 f an altitude ( #aj1jjj feet4 ,! ins]t %[s ! relative l5g?s 7v]tical scale7 ( an airplane ( #ej-;foot spread at v>i\s di/ances bel[ ! obs]v]4 ,x is impracticable to pres5t color$ illu/rations 9 ?is resume & values express$ 9 numb]s are mean+less to mo/ p]sons1 s a few g5]al rem>ks w be made 9 clos+ ! discussion ( l[ visibility z view$ f above 9 spr+1 summ] & fall4 ,a black craft is ( mu* l[] visibility ?an a :ite "o4 ,:ite %\ld n be us$4 ,! pa9ts %\ld be v d>k %ades4 ,! hues are approximately ! same = ! e>? areas z se5 at ! e>?'s surface4 ,inl& wat]s are a dirty blue-gre5 or blui%-gre51 & deep ocean wat] is a gre5i%-blue :5 view$ v]tically1 or ne>ly s4 ,mean hues ( ^! 7 det]m9$ approximately4 ,be=e consid]+ o!r aspects ( cam\flage x is well to consid] su* features z haze1 cl\ds & sky4 ,!re appe> to be two k9ds ( haze :i* ! writ] w arbitr>ily call e>? & high haze1 respectively4 ,! =m] consi/s *iefly ( du/ & smoke & usually ext5ds to an altitude ( ab\t "o mile1 al?\gh x occasionally ext5ds mu* high]4 ,its upp] limit is v di/9ct1 z se5 by ! false horizon4 ,?is horizon is us$ m by ! pilot :5 fly+ at c]ta9 altitudes ?an ! true horizon4 ,at ! top ( ?is haze cumulus cl\ds are commonly se5 to be pok+ out l ne>ly subm]g$ iceb]gs4 ,! upp] haze appe>s some:at :it] 9 color & appe>s to ext5d sometimes to altitudes ( sev]al or ev5 _m miles4 ,! fact t ! e>? haze may be se5 to end usually at ab\t #ejjj to #fjjj feet & ! upp] haze to p]si/ ev5 beyond #bj1jjj feet has l$ ! au?or to apply diff]5t names = conv5i5ce4 ,! upp] limit ( ! e>? haze is det]m9$ by ! height ( diurnal atmosph]ic convection4 ,haze aids 9 l[]+ ! visibility ( airplanes by provid+ a lum9\s veil1 b x also op]ates at "s altitudes to increase ! brightness ( ! sky1 :i* is ! backgr\nd 9 ?is case4 ,! sky g5]ally decreases consid]ably 9 brightness z ! obs]v] asc5ds4 ,! brightness ( ! sky is due to scatt]$ light1 t is1 to light be+ reflect$ by p>ticles ( du/1 smoke1 ?9ly diffus$ cl\ds1 etc4 ,by mak+ a s]ies ( measurem5ts ( ! brightness ( ! z5i? sky = v>i\s altitudes1 ! altitude :]e ! e>? haze ends is usually pla9ly di/+ui%able4 ,_m obs]vations ( ?is "* 7 accumulat$4 ,9 "s extreme cases ! sky 0 f\nd to be only "o-t5? z bright :5 obs]v$ at high altitudes ( #ae1jjj to #bj1jjj feet z se5 f ! e>?'s surface4 ,?is acc\nts p>tly = ! decrease 9 ! visibility ( an airplane z x asc5ds4 ,at #bj1jjj feet ! sky 0 f\nd to contribute z little z #d p] c5t ( ! total light on a horizontal plane & ! extreme h>%ness ( ! light+ is v noticeable :5 ! upp] sky is cl\dless & cle>4 ,d\btless1 x has be5 commonly not$ t airplanes are g5]ally v d>k objects z view$ f bel[ aga9/ ! sky4 ,ev5 :5 pa9t$ :ite !y are usually mu* d>k] ?an ! sky4 ,z !y asc5d ! sky above !m becomes d>k]1 al?\gh to ! obs]v] on ! gr\nd ! sky rema9s con/ant 9 brightness4 ,h[ev]1 9 asc5d+1 ! airplane is leav+ bel[ x m & m lum9\s haze :i* acts z a veil 9 aid+ to scre5 x until1 :5 x rea*es a high altitude1 ! comb9ation ( d>k sky beh9d x & lum9\s haze betwe5 x & ! obs]v] on ! gr\nd1 x becomes ( mu* l[] visibility4 ,ano!r factor :i* contributes some:at is its dim9i%+ size z view$ f a fix$ position at ! e>?4 ,! m9imum p]ceptible contra/ becomes l>g] z ! size ( ! contra/+ pat* dim9i%es4 ,inasmu* z !re is n en\gh light reflect$ upw>d f ! e>? to illum9ate ! l[] side ( an opaque surface suffici5tly to make x z bright z ! sky ord9>ily1 except+ at v high altitudes = v cle> skies1 x is necess>y1 9 ord] to atta9 l[ visibility = airplanes z view$ f bel[1 to supply "s additional illum9ation to ! l[] surfaces4 ,computations h %[n t artificial light+ is impracticable1 b measurem5ts on undop$ airplane fabrics indicate t on sunny days a suffici5t brightness c be obta9$ f direct sunlight diffus$ by ! fabric to increase ! brightness to ! ord] ( magnitude ( ! brightness ( ! sky4 ,on ov]ca/ days an airplane w ne>ly always appe> v mu* d>k] ?an ! sky4 ,t is1 ! brightness ( ! l[] sides c 9 no o!r mann] be made equal to t ( ! sky4 ,h[ev]1 l[ visibility c be obta9$ on sunny days :i* is an advantage ov] high visibility at all times1 z is ! case ) airplanes n[ 9 use4 ,_m obs]vations & computations ( ^! & o!r factors h be5 made1 s t x is possible to pr$ict results4 ,transp>5t m$ia h obvi\s advantages1 b no satisfactory ones are available at pres5t4 ,hav+ consid]$ l[ visibility ( aircraft z view$ f above & f bel[1 respectively1 x is ( int]e/ to discuss briefly ! possibility ( atta9+ bo? ( ^! simultane\sly ) a giv5 airplane4 ,frankly1 x is n practicable to d ?is4 ,an airplane to be ( l[ visibility aga9/ ! e>? backgr\nd mu/ be pa9t$ or dy$ v d>k %ades ( appropriate color & patt]n4 ,?is r5d]s x almo/ opaque & x w be a v d>k object :5 view$ aga9/ ! sky4 ,if ! l[] surfaces ( ! airplane are pa9t$ z :ite z possible ! airplane /ill rema9s a d>k object aga9/ ! blue sky & a v d>k object aga9/ an ov]ca/ sky1 except at high altitudes4 ,9 ! latt] cases ! contra/ is n z great z already expla9$4 ,a practicable me?od ( decreas+ ! visibility ( airplanes at pres5t z view$ f bel[ is to increase ! brightness by ! diffuse transmission ( direct sun-light on cle> days4 ,on ov]ca/ days cl\ds & haze mu/ be dep5d$ ^u to scre5 ! craft4 ,9 consid]+ ^! aspects x is well to recall t ! two s\rces ( light are ! sun & ! sky4 ,assum+ ! sun to contribute #hj p] c5t ( ! total light :i* rea*es ! upp] side ( an opaque horizontal diffus+ surface at midday at ! e>? & assum+ ! sky to be cl\dless & uni=m 9 brightness1 !n ! brightness ( ! horizontal upp] surface w equal #e ,,rb1 :]e ,r is ! reflection-factor ( ! surface & ,b is ! brightness 7diff]5t 9 ! two cases7 ( ! sky4 ,on a uni=mly ov]ca/ day ! brightness ( ! surface w\ld be equal to ,,rb4 ,n[ assum+ ,re to be ! mean reflection-factor ( ! e>?1 !n ! l[] side ( a horizontal opaque surface susp5d$ 9 ! air w\ld receive light 9 proportion to ,re,b4 ,if ?is l[] surface 7 a p]fect mirror or a p]fectly reflect+ & diffus+ surface its brightness w\ld equal #e ,re,b on ! sunny day & ,re,b on ! ov]ca/ day :]e ,b is ! value 7diff]5t 9 ! two cases7 ( ! brightness ( ! uni=m sky4 ,! surface c nev] be a p]fect reflector1 s on an ov]ca/ day its brightness w be a fraction 7,,rre7 ( ! brightness ,b ( ! uni=m sky4 ,inasmu* z ,re is a v small value x is se5 t l[ visibility ( airplanes z view$ f bel[ g5]ally _c be atta9$ on an ov]ca/ day4 ,x c be approa*$ on a sunny day & ev5 realiz$ by adopt+ ! exp$i5t already m5tion$4 ,fur!r computations are to be f\nd else:]e4#ab ,seasonable *anges pres5t no difficulties1 = f a practical /&po9t only summ] & w9t] ne$ be g5]ally consid]$4 ,if ! e>? is cov]$ ) sn[ an airplane cov]$ completely ) :ite or gray pa9t w\ld be fairly satisfactory z view$ f above1 & if a c]ta9 %ade ( a blue t9t be appli$ to ! l[] surfaces1 l[ visibility z view$ f bel[ w\ld result4 ,! :ite pa9t w\ld possess a reflection-factor ab\t equal to t ( sn[1 ?us provid+ l[ visibility f above4 ,inasmu* z ! reflection-factor ( sn[ is v high1 ! :ite l[] sides ( an airplane w\ld receive a great deal m light 9 w9t] ?an !y w\ld 9 summ]4 ,obvi\sly1 a blue t9t is necess>y = l[ visibility aga9/ ! sky1 b color has n be5 prim>ily consid]$ 9 ! prec$+ p>agraphs because ! *ief difficulty 9 a*iev+ l[ visibility f bel[ lies 9 obta9+ brightness ( ! prop] ord] ( magnitude4 ,9 w9t] ! b>r5 gr\nd w\ld be approximately ( ! same color & reflection-factor z 9 summ]1 s x w\ld n be difficult to take ?is into consid]ation4 ,seaplanes ^: backgr\nds g5]ally consi/ ( wat] w\ld be pa9t$ ( ! color & brightness ( wat] ) p]haps a slight mottl+4 ,! color w\ld g5]ally be a v d>k %ade1 approximat+ blue-gre5 9 hue4 ,aircraft = night use w\ld be treat$ 9 ! same mann] z aircraft = day use1 if ! moonlight is to be consid]$ a dom9ant factor4 ,?is is "o ( ! cases :]e ! judgm5t mu/ be bas$ on actual exp]i5ce4 ,x appe>s t ! great enemy ( night raid]s is ! se>*light4 ,if ?is is true ! obvi\s exp$i5t is to pa9t ! craft a dull jet black4 ,exp]im5ts indicate t x is m difficult to pick up a black craft ?an a gray or :ite "o & also x is m difficult to hold x 9 ! beam ( ! se>*light4 ,?is c be readily prov$ by ! use ( black1 gray1 & :ite c>ds 9 ! beam ( an automobile head-light4 ,! :ite c>d c be se5 9 ! outskirts ( ! beam :]e ! gray or black _c be se51 & ! gray c be pick$ up :]e ! black "o is invisible4 ,! sci5ce ( vision acc\nts = ?is z x does = _m o!r que/ions :i* arise 9 ! consid]ation ( cam\flage or l[ visibility4 ,"s attempts h be5 made to apply ! pr9ciple ( confusibility to airplanes z f9ally develop$ = vessels to circumv5t ! subm>9e1 b ! folly ( ?is appe>s to be evid5t4 ,air battles are conduct$ at t]rific spe$s & ) skillful maneuv]+4 ,trigg]s are pull$ )\t computations & ! :ole activity is almo/ lightn+-l4 ,to expect to confuse an oppon5t z to ! c\rse & position ( ! airplane is folly4 ,! cam\flage ( obs]vation balloons has n be5 develop$1 ?\gh exp]im5ts 7 be+ consid]$ 9 ?is direction z ! w> clos$4 ,inasmu* z !y are l[-altitude crafts x appe>s t !y w\ld be be/ cam\flag$ = ! e>? z a backgr\nd4 ,_! enemies p\nce d[n ^u !m f ! sky s t l[ visibility f above seems to be ! bett] *oice4 ,9 ! =ego+ x has be5 aim$ to give ! read] ! g5]al und]ly+ pr9ciples ( cam\flage & l[ visibility1 b at be/ ?is is only a resume4 ,9 ! foll[+ ref]5ces w be f\nd m ext5sive discussions ( v>i\s phases ( ! subject4 ,,ref]5ces #a4 ,a ,/udy ( ,zoelln]'s ,figures & ,o!r ,relat$ ,figures1 ,j4 ,ja/r[1 ,am]4 ,j\r4 ( ,psy*4 #ahia1 #d1 p4 #cha4 #b4 ,a ,/udy ( ,geometrical ,illusions1 ,c4 ,h4 ,judd1 ,psy*4 ,rev4 #ahii1 #f1 p4 #bda4 #c4 ,visual ,illusions ( ,dep?1 ,h4 ,a4 ,c>r1 ,psy*4 ,rev4 #aiji1 #af1 p4 #bai4 #d4 ,irradiation ( ,light1 ,f4 ,p4 ,boswell1 ,psy*4 ,bul4 #aije1 #b1 p4 #bjj4 #e4 ,retir+ & ,advanc+ ,colors1 ,m4 ,luckie%1 ,am]4 ,j\r4 ,psy*4 #aiah1 #bi1 p4 #ahb4 #f4 ,! ,language ( ,color1 #aiah1 ,m4 ,luckie%4 #g4 ,app>5t ,=m ( ! ,dome ( ! ,sky1 ,ann4 d4 ,physik1 #aiah1 #ee1 p4 #chg2 ,sci4 ,abs4 #aiah1 ,no4 #aadg4 #h4 ,c\rse on ,optics1 #agch1 ,rob]t ,smi?4 #i4 ,color & ,its ,applications1 #aiae & #aiba2 ,light & ,%ade & ,_! ,applications1 #aiaf1 ,m4 ,luckie%4 #aj4 ,report ( ,! ,subm>9e ,def5se ,association1 ,l4 ,t4 ,bates & ,l4 ,a4 ,jones4 #aa4 ,m>9e ,cam\flage ,design1 ,e4 ,l4 ,w>n]1 ,trans4 ,i4 ,e4 ,s4 #aiai1 #ad1 p4 #bae4 #ab4 ,! ,visibility ( ,airplanes1 ,m4 ,luckie%1 ,j\r4 ,frank4 ,in/4 ,m>* & ,april1 #aiai2 also ,a]ial ,photometry1 ,a/rophys4 ,j\r4 #aiai1 #di1 p4 #ajh4 #ac4 ,j\r4 ,am]4 ,opt4 ,soc41 ,e4 ,k>r]1 #aiba4 ,! =ego+ are only a few ref]5ces indicat$ 9 ! text4 ,hundr$s ( ref]5ces are available & obvi\sly x is impracticable to include su* a li/4 ,! mo/ fruitful s\rces ( ref]5ces are ! g5]al works on psy*ology4 ,e4 ,b4 ,tit*5]'s ,exp]im5tal ,psy*ology 7vol4 #a7 conta9s an excell5t li/4 ,a *apt] on ,space 9 ,william ,james' ,pr9ciples ( ,psy*ology 7vol4 ,,ii7 w be f\nd ( int]e/ to ^? :o wi% to delve deep] into visual p]ception4 ,o!r g5]al ref]5ces are ,elem5ts ( ,physiological ,psy*ology by ,ladd & ,woodwor?2 ! works ( ,helmholtz2 a contribution by ,h]+ 9 ,h]mann's ,h&b4 d4 ,phys4 ,bk4 ,,iii1 "p #a2 ,physiological ,psy*ology by ,wundt2 ,e4 ,b4 ,delab>re1 ,am]4 ,j\r4 ,psy*4 #ahih1 #i1 p4 #egc2 ,w4 ,wundt1 ,taeus*ung51 p4 #aeg & ,philos4 ,/ud4 #ahih1 #ad1 p4 #a2 ,t4 ,lipps1 ,raumae/hetik & ,zeit4 f4 ,psy*4 #ahif1 #ab1 #ci4 ,,index ,ab]ration1 #ai sph]ical1 #abb *romatic1 #ace ,a]ial p]spective1 #afe1 #ahc ,aft]-images1 #bd1 #be1 #ei1 #abh1 #ahf positive & negative1 #abi ,airplanes1 visibility (1 #bcc cam\flage = diff]5t types1 #bcd size ( image at v>i\s altitudes1 #bch cam\flage = v>i\s conditions1 #bdj ,alhaz51 #h ,angles1 influ5ce (1 #gf v>i\s effects (1 #ha cont\rs &1 #hg app>5t effect on l5g?1 #ia !ories1 #ih ,animals1 protective coloration1 #baa ,ar*itecture1 #aie balance 91 #bja ,arcs1 circul>1 illusions due to1 #hf ,areas1 juxtapos$1 illusions (1 #if ,arti/1 #agi ,att5tion1 fluctuation (1 #fe1 #ajf1 #ada1 #afd ,aub]t1 #di ,au]ba*'s indirect vision !ory1 #ajj ,aureole1 #agh ,balance 9 ar*itecture1 #bja ,bas-relief1 #adc ,battle%ips1 #bbb ,b9ocul> disp>ity1 #aje ,b9ocul> vision1 #bi1 #ca ,bl5d+ ( colors 9 cam\flage1 #baf ,bl9d spot1 #ba ,blue light on w>-vessels1 #bcj ,boswell1 v>ieties ( irradiation1 #abb ,brightness1 illusions due to v>iations 91 #ajg & color contra/s1 #aad app>atus1 #aae & hue1 #abe sky1 #bda ,brucke's !ory1 #cg ,brunot's mean di/ance !ory1 #aja ,cam\flage1 #baj t]re/rial1 #baj detection (1 #bae m>9e1 #bbb airplane1 #bcd ,c>r1 obs]vations on di/ance illusions1 #ajh ,*romatic ab]ration1 #ai1 #ace ,*lorophyl1 #bad ,circle1 #aa arcs (1 illusion1 #hf contract+ & exp&+ illusion1 #ih ,cl\ds1 #bce ,color1 #abd aft]-images1 #abh bl5d+ 9 cam\flage1 #baf contra/s & brightness1 #aad gr[? & decay ( s5sation1 #aca illusions (1 #aef retir+ & advanc+1 #ach saturation1 #aed s5sibility ( ret9a1 #ach w>m & cold1 #aeh ,confusability1 #bbf ,confusion !ory ( angul> illusions1 #ajj ,cont\r1 illusions (1 #eb & angles1 illusions1 #hg ,contract+ & exp&+ circles1 illusion (1 #ih ,contra/s1 illusions (1 #ec simultane\s1 #abd1 #aed app>atus =1 #aae1 #abe color1 #aad1 #aed1 #ahh brightness1 #aad ,conv]g5ce1 illusions (1 #ajh1 #aia ,cord1 twi/$1 illusion1 #hh ,daylight1 artificial1 #ahi ,decoration1 pa9t+ &1 #agi ,decorator1 #ahh ,demb]1 #afg ,dep? & di/ance1 illusions1 #ajb ,direction1 illusions ( ,zoelln]'s1 #gf ,wundt's1 #gi ,h]+'s1 #hj ,disk1 ,mason1 #acb ,di/ance & dep?1 illusions1 #ajb & size1 #ce1 #ajd1 #aff ,di/ance illusions1 ,c>r's obs]vations1 #ajh ,d\ble images1 #cg ,dynamic !ory ( angul> illusions1 #ii ,enl>gem5t ( sun & moon at horizon1 app>5t1 #afi ,equivocal figures1 #fd ,euclid1 #h ,ext5t1 int]rupt$1 illusions (1 #dh ,ext]nal image1 #ae1 #ag1 #cd ,eye1 physiology1 #ac position1 #cj adju/m5ts1 #cc defects1 #ai ,fatigue1 #abh ,field1 visual1 effect ( location 91 #dd ,figures1 equivocal1 #fd ,filt]s1 color1 #bad ,fluctuation ( att5tion1 #fe1 #ajf1 #ada1 #afd ,focus+1 #ad ,=m ( sky1 app>5t1 #aff ,fovea c5tralis1 #bb1 #bc1 #aci ,frames1 picture1 effect ( wood gra91 #aij ,geometrical illusions1 #dd ,gl>e1 #aai ,gra9 ( wood1 app>5t di/ortions due to1 #aij ,grecian art1 #aif ,gr[? & decay ( color s5sation1 #aca ,guttman1 #age ,halluc9ation1 #d1 #gb ,halo1 #agh ,haze1 illusions1 etc41 #ajc1 #aff1 #ahc e>? & high1 #bdj ,helmholtz1 #ac1 #gd ,h]+1 #gd illusion ( direction1 #hj ,hue & brightness1 #abe ,illusions1 geometrical1 #dd equivocal figures1 #fd influ5ce ( angles1 #gf ( dep? & di/ance1 #ajb irradiation & brightness contra/1 #aad color1 #abd light & %ad[s1 #add 9 nature1 #afd 9 pa9t+ & decoration1 #agi mirror1 #bje cam\flage1 #baj ,image aft]-1 #bd1 #be1 #ei1 #abh1 #abi1 #ahf d\ble1 #cg ext]nal1 #ae1 #ag1 #cd ret9al1 inv]sion (1 #af ( airplane1 size at v>i\s altitudes1 #bch ,indirect vision !ory1 ,au]ba*'s1 #ajj ,intaglio1 #adc ,int]rupt$ ext5t1 illusions (1 #dh ,iris1 #ae ,irradiation1 #abj & brightness contra/1 #aad v>ieties ( 7,boswell71 #abb 9 ar*itecture1 #aii ,james1 #agj ,ja/r[1 #hj ,jones1 ,l4 ,a41 #bbc ,judd1 #hf1 #ic ,judgm5t1 #a ,k>r]1 #afj ,kepl]1 #h ,light1 effect ( spectral "*1 #ahi ,light+1 illusions ( dep? & di/ance due to1 #ajb contra/s1 #aed diffusion1 effect (1 #ade direction1 effect (1 #add1 #aea end+ ( se>*light beam1 #afj w>m & cold colors1 #aeh ,lipps1 #aj1 #aa dynamic !ory (1 #ii ,location 9 visual field1 effect1 #dd ,mean di/ance !ory1 ,brunot's1 #aja ,me*anical1 e/hetic unity1 #aa ,magician1 #c ,magic1 mirror1 #bje ,m>9e cam\flage1 #bbb ,mason disk1 #acb ,mirage1 #c1 #agf ,mirror magic1 #bje ,miscellane\s color effects1 #adj ,moon1 app>5t size at horizon1 #h1 #cf1 #afi !ories (1 #agc app>5t radius ( cresc5t1 #aba ,muell]-,ly] illusion1 #ec1 #ic ,national ,rese>* ,c\ncil1 ,committee on ,cam\flage1 #bcd ,nature1 #afd ,neck]1 #gd ,oppel1 #i ,pa9t+ & decoration1 #agi ,pa9t]1 #b1 #agi1 #ahf ,p>allax1 #aje ,p>!non ( ,a!ns1 #aif ,p]si/5ce ( vision1 #aca ,p]spective1 #eh 9 ar*itecture1 #aih a]ial #afe1 #ahc !ory1 #ih ,photograph]1 #aeb ,photography1 use 9 detection ( cam\flage1 #bbj ,photomet]1 #aef1 #bag ,pigm5ts1 #ahd ,pogg5dorff illusion1 #he ,protective coloration1 animals1 #baa ,psy*ology1 #b1 #f1 #aeg ( light1 #aic ,purk9je ph5om5on1 #bf1 #aci ,reflection-factors1 #bcf ,ret9a1 #ad1 #bj in]tia1 #acj color s5sibility1 #ach ,ret9al rivalry1 #adj ,retir+ & advanc+ colors1 #acf ,rev]sal ( mirror image1 #bje ,rods & cones1 #ba ,s*ro$]'s /aircase1 #gj ,sculpture1 #bjd ,se>*light beam1 end+ (1 #afj ,s5sation1 color1 gr[? & decay1 #aca ,s5se1 #a ,%ad+1 c\nt]1 = vessels1 #bbd ,%ad[s1 importance 9 cam\flage1 #bae ,size & di/ance1 #ce1 #cf illusions (1 #ajd1 #aff ,sky app>5t =m (1 #aff brightness1 #bda ,skylight & sunlight1 relative proportions (1 #bae1 #bdc ,smi?1 ,rob]t1 #agc ,smoke-scre5s1 #bcj ,spectral "* ( light1 #ahi ,sph]e1 illusions1 #ade1 #aej1 #aea ,sph]ical ab]ration1 #ai ,sph9x illusion1 #bjf ,spiral illusions1 #ij ,spray+1 pa9t1 #ahg ,/]eoscope1 #ci1 #adb ,/]eoscopic vision1 #ch1 #da1 #ada ,subm>9es1 #bbe cam\flage =1 #bcb ,sun1 app>5t enl>gem5t at horizon1 #afi ,sunlight & skylight1 relative proportions 9 nature1 #bae1 #bdc ,t]re/rial cam\flage1 #baj ,!ory ( influ5ce ( angles1 #ih p]spective1 #ih dynamic1 #ii confusion1 #ajj indirect vision1 #ajj mean di/ance1 #aja ,?i]y's figure1 #ga ,?i]y's p]spective !ory1 #ii ,transp>5cies1 #ahe ,twi/$ cord illusions1 #hh ,uibe1 #afg ,v]tical vs4 horizontal di/ances1 #aa1 #cf1 #df ,visibility1 l[1 = vessels1 #bbb ( airplanes1 #bcc ,vision1 #bi p]si/5ce (1 #aca /]eoscopic1 #ch ,visual p]ception1 #cb1 #cc ,w>m & cold colors1 #aeh ,w>n]1 ,e4 ,l41 #bbg ,:eat/one1 #cg ,wood gra91 illusions caus$ by1 #aia ,_w ,w>1 #bac ,wundt1 #aj1 #aa1 #cb1 #gd illusion ( direction1 #gi ,yell[ spot1 #aci ,zoelln]'s illusion1 #fg1 #gf ,zo?1 #age ,,o!r ,,books ,,by ,m4 ,,luckie% ,,color ,,& ,,its ,,applications ,second ,edition1 ,revis$ & ,enl>g$4 #f x #i1 #aej illu/rations1 #d color plates1 #dca pages4 #d4ej ,! object ( ?is treatise is n only to discuss ! _m applications ( color1 b to e/abli% a s\nd sci5tific basis = ^! applications4 ,! book is au?oritative1 well illu/rat$1 & conta9s _m ref]5ces & a weal? ( new mat]ial4 ,x 0 writt5 by an inve/igator 9 ! g5]al field ( color & is !re=e n n>r[ly limit$ 9 scope4 ,x fills a di/9ct gap t has exi/$ on ! book %elves4 ,,light ,,& ,,%ade ,,& ,,_! ,,applications #f x #i1 #ace illu/rations1 #bgg pages #c4jj ,! book is a cond5s$ record ( sev]al ye>s' rese>* by ! au?or 9 ! sci5ce ( light & %ade4 ,x is ! fir/ publi%$ "w :i* deals ) ! sci5ce ( light & %ade 9 a complete & analytical mann]4 ,! au?or has ! faculty ( br++ =? sci5tific facts 9 su* a mann] z to be helpful to ^? int]e/$ 9 ! v>i\s arts4 ,! book is ( extremely wide int]e/ because x deals ) ! appe>ances ( objects & h5ce ) vision & ) light+4 ,x is well illu/rat$ & repres5ts ! fir/ elaborate attempt to =mulate ! sci5ce ( light & %ade & to correlate x ) v>i\s arts4 ,,ultraviolet ,,radiation1 ,its ,prop]ties1 ,production1 ,measurem5t & ,applications4 #f x #i1 #ab plates1 #bgj pages #c4ej ,x is ! prim>y aim ( ?is book to pres5t au!ntic data ( su* scope z to be useful to ! *emi/1 physici/1 eng9e]1 biologi/1 oph?almologi/ & physician & o!rs int]e/$ 9 ultraviolet radiation4 ,!ory has be5 subord9at$ to exp]im5tal facts because ! latt] are n affect$ by ! inevitable *anges 9 !ory4 ,mu* ( ! lit]ature on ?is subject is confus+ because ( ! lack ( c>e 9 ! *oice ( def9itions & ( limit$ value result+ f c>elessness 9 specify+ important factors4 ,f ?is mass ( isolat$ mat]ial ! au?or has coo]d9at$ mu* valuable data4 ,! _m ref]5ces :i* are includ$ increase ! usefulness ( ! book4 ,,light+ ,,! ,,home #e x #g-a_/b1 illu/rat$1 #bhi pages #b4jj ,?is is a pione] book4 ,x ranks ) books on int]ior decoration & furniture z a help t[>d trans=m+ a h\se into a home4 ,x is practical 9 t x (f]s advice on all sorts ( light+ problems & x is fasc9at+ read+ z well4 ,,artificial ,,light1 ,,its ,,influ5ce ,,on ,,civilization #f x #i1 illu/rat$1 #cff pages #c4jj ,?is /ory ( ! a*ievem5ts ( artificial light is writt5 especially = ! man 9 ! /reet :o is n int]e/$ 9 te*nical sci5tific t]ms & =mulae1 b :o looks ) admiration ^u ! huge signs :i* fla% & sp>kle above ! cr[ds on ! ,great ,:ite ,way1 :o m>vels at ! colors & brilliance ( a spectacul> !atrical production & desires to "k h[ x is accompli%$1 & :o takes a natural delight 9 he>+ ab\t sci5tific discov]ies :5 !y are expla9$ 9 ! simple1 vivid language he und]/&s be/4 ,,! ,,light+ ,,art1 ,,its ,,practice ,,& ,,possibilities #f x #i1 illu/rat$1 #bbi pages #b4ej ,?is book discusses light+ z eng9e]+ plus art1 & treats ! subject z a bran* ( int]ior & ext]ior decoration4 ,! te*nical aspect ( ! subject is n neglect$1 b ! ma9 emphasis is ^u ! :y & n m]ely ! h[ ( light+4 ,,! ,,language ,,( ,,color #f x #i1 illu/rat$1 #bhb pages #b4jj ,a practical volume on color1 ! v>i\s fields 9 :i* x is us$ & its importance 9 portray+ ! ideas t make = progress4 ,a book ( special int]e/ to all ^? :o deal 9 color s*emes & values4 ,,! ,,book ,,( ,,! ,,sky #f x #i1 illu/rat$1 #bcf pages #c4ej ,! beauties1 wond]s1 awesome spectacles1 inspir+ panoramas1 & ext5sive ranges ( vision :i* await ! a]ial travel]1 make ( cl\dl& a v]itable fairyl& if he w op5 8 consci\sness to !m4 ,aircraft h br\ght ?is new _w ( exp]i5ces )9 easy rea* ( mank9d & x is "o ( ! aims ( ?is volume to awak5 ^? :o fly1 or w\ld fly1 to ! v>iety ( int]e/ :i* air travel af=ds4 ,! ,lit]ature ( ,eng9e]+ & ! ,indu/ries ,on our %elves is ! mo/ complete /ock ( eng9e]+1 sci5tific1 te*nical & indu/rial books 9 ! ,unit$ ,/ates4 ,a l>ge numb] ( ^! we publi% & = an ev] increas+ numb] we are ! sole ag5ts4 ,all inquiries are *e]fully & c>efully answ]$ & complete catalogs z well z special li/s are s5t free on reque/4 ,d4 ,van ,no/r& ,company ,,publi%]s ,,& ,,booksell]s #h ,,w>r5 ,,/reet ,,new ,,york ,transcrib]'s ,notes3 ,passages 9 italics are indicat$ by italics4 ,subscript$ *>act]s are indicat$ by subscript4 ,! foll[+ mispr9ts h be5 correct$3 imgaes correct$ to images 7page #abh7 bove correct$ to above 7page #agg7 v]ticle correct$ to v]tical 7page #bci7 colo correct$ to color 7,index7 ,o!r ?an ! corrections li/$ above1 inconsi/5cies 9 spell+ & hyph5ation h be5 reta9$ f ! orig9al4 ,! inv]t$ #h & ,s *>act]s at ! top ( page #de _c be prop]ly repres5t$ 9 ?is text v]sion4