616 



SCIENCE. 



[N. S. Vol. VII. No. 175. 



yond any reasonable doubt, and is proved 

 by the fact that all instruments which 

 really produce this appearance of depth, 

 however much they may differ in other 

 respects, must furnish some systematic 

 differences in the two pictures to be viewed. 

 It is evident, however, that this aid to the 

 perception of depth will differ considerably 

 according as the object represented is near 

 or far away. For near objects the differ- 

 ences in the retinal images will be quite 

 marked, while for distant ones the images 

 will be more nearly alike. To magnify the 

 perception of depth in distant views Helm- 

 holtz devised the telestereoscope, which acts 

 by practical^ spreading the distances be- 

 tween the eyes, and which, in combination 

 with lenses, finds a useful application in 

 stereoscopic field glasses. The processes 

 of convergence and accommodation accom- 

 pany these differences in the retinal images ; 

 and these, too, are more active in the per- 

 ception of near objects than of distant ones. 

 In order to determine which of the two fac- 

 tors, convergence or difference in the retinal 

 images, is the more essential it is necessary 

 to produce one more or less independently 

 of the other. This can be done, first, by 

 viewing in the ordinary stereoscope two 

 views which are precisely alike and which 

 are superimposed by means of convergence ; 

 and, again, two views which differ as the 

 two retinal images differ and which are 

 combined with a minimum of convergence 

 by means of devices described further on. 

 The result is unmistakable and shows that 

 convergence is only an added element and 

 that the difiference in the retinal images is 

 the all-important factor. 



But apart from these factors, which may 

 be expressed in physiological terms— that 

 is, in terms of what goes on on the retina 

 and within the eye and eye-muscles — there 

 are psj'chological factors in the perception 

 of depth which materially influence the re- 

 sult. While the former are either simple 



sensations or the inferences from them, the 

 latter involve more complicated forms of 

 interpretation on the basis of perceptions 

 which are the result of a varied experience. 

 First among these is the distribution of 

 light and shade. This factor is so impor- 

 tant in most of our experience in the inter- 

 pretation of depth that it alone frequently 

 determines the visual result and overrules 

 the influence of all other factors. For ex- 

 ample, it is not difficult to illuminate an 

 intaglio in such a way that it can be mis- 

 taken for a cameo. In the illusion of depth 

 which the artist produces this factor is obvi- 

 ously of supreme importance. A second psy- 

 chological factor, likewise invaluable to the 

 artist, arises from our constant tendency to 

 interpret outlines and contours as the rep- 

 resentations of three-dimensional objects. 

 As a result of our general experience, we 

 are quite prepared to interpret all lines in 

 a painting or an etching or a photograph as 

 representing certain views of objects. We 

 know, of course, that the pictures are flat, 

 but we see them as solid. Especially when 

 this tendency is combined with the inter- 

 pretation of lights and shadows do we have 

 an appearance of depth which, when skill- 

 fully portrayed, seems hardly less real than 

 the reality. A third factor equally opera- 

 tive in pictures and in reality is that 

 summed up in the term perspective, which 

 involves in the main the diminution in the 

 apparent size of the object as its distance 

 from the eye increases. Figures in the 

 foreground and in the background are in- 

 terpreted not according to their real size^ 

 that is, not the number of millimetres that 

 they occupy on the retina, or of inches on 

 the canvas — but according to these as modi- 

 fied by the estimated distance between the 

 object and the point of view of the beholder. 

 The familiarity of objects is, of course, a 

 great aid in the proper estimation of such 

 distances. If two men are represented 

 upon a picture, and the one representation 



