August 16, 1901.] 



SGIENGE. 



265 



perception of distance is more accurate than 

 our perception of the visual angle. Owing to 

 the fact that we must commonly ignore 

 the visual angle in order to get useful per- 

 ceptions, our interest in it is small and our 

 knowledge vague. The moon, for example, 

 always subtending the same angle, looks much 

 larger at the horizon than at the zenith. If 

 the reader is asked whether the little finger if 

 held before the moon will cover it, he will 

 probably say 'No,' or else hesitate to answer. 

 As a matter of fact, the little finger covers the 

 moon, and when it is removed we have a fur- 

 ther illustration of the subject in the fact that 

 the moon appears decidedly reduced in size, 

 owing doubtless to a vague comparison with the 

 finger. 



Fig. 2. The upper and lower areas are of the same 

 size and shape. 



I have made a few experiments on the accu- 

 racy with which the retinal area or visual angle 

 can be judged under different conditions, and 

 on the relation between the geometrical magni- 

 tude and the perception, 

 and hope to continue 

 them, though it is diffi- 

 cult to obtain quantita- 

 tive results. The phe- 

 nomena can be studied 

 to advantage with the 

 aid of mirrors, and it ap- 

 pears that data of inter- 

 est can be secured by a 



consideration of photographs and of paintings 

 and drawings. This I may illustrate by repro- 

 ducing a figure from Professor Miinsterburg's 

 ' Pseudoptics,' in which the figures of the man 

 and boy are of the same objective size. 



While our ability to compare the retinal 

 magnitude of objects at diflfereut distances and 

 in different directions is very defective, the 

 perception of visual magnitude when objects 

 are side by side is perhaps the most accurate 

 of all the senses, a difference of one-hundredth 

 being noticeable. Yet even in this case the 

 retinal area yields readily to suggestion as 

 is shown by the accompanying figure, in which 

 the two areas are of exactly the same size. 



In a recent paper, "^ I have described what 

 seems to me one of the most striking disparities 

 between our perceptions and the j)hysical world 

 — no less than the perception of order in time 

 as extension in space. If, for example, first a 

 green and then a red surface pass rapidljj^ be- 

 hind a narrow slit, one does not see green fol- 

 lowed by red, but simultaneously a surface 

 with green below, white in the middle and red 

 fading into black above. Physiologically we 

 have a retinal process, consisting of a certain 

 commotion caused by green light, lasting say 

 1/20 sec, then a mingling of this process with 

 that excited by red, then that excited by red 

 gradually subsiding. But this time series is 

 perceived as a spatial continuum, and the field 

 is, perhaps, three times as large as the window 

 through which it is seen. If three squares, as 

 shown on the left of the figure, are seen as 

 they pass the window WW'., they do not ap- 

 pear one after the other, but are seen simulta- 

 neously as indicated to the right. 



This conversion of a time into a space order 

 is also what always happens in ordinary vision. 

 If the eye moves so that the line of sight moves 



Fig. 3. The squares at the left appear as shown at 

 the right when seen as they pass the window ( WW) 

 of the size indicated, 



* 'On the Eelation of Time and Space in Vision,' 

 The Psychological Review, 7 : 325-343, 1900. 



