126 Prof. Emerson on the Perception of Relief. 



as in a modelled figure ; the nose rises well from the face ; the 

 eyebrows, lips, and chin stand out very well; and the entire 

 figure raises itself from the ground upon which it is drawn, and 

 assumes in a remarkable degree a living expression. The neces- 

 sary distance of the two half-pictures from each other, and also 

 the proper distance from the eyes of the observer for the pro- 

 duction of the greatest effect, can only be ascertained by trial. 

 The more steadily one gazes at the pictures, the more the sensa- 

 tion of relief is strengthened "*. 



The foregoing extract from the Cosmos has been reproduced 

 in PoggendorfFs Annalen, vol. cii. p. 319; in II Nuovo Cimento, 

 vol. vi. p. 185 ; in Die Theorie des Sehens und raumlichen Vorstellen, 

 bei Dr. Cornelius, Halle, 1861 ; and in Monographic du Stereo- 

 scope, par Blanchere, Paris, 1862. Seemingly endorsed by such 

 a high authority as Moigno, the alleged fact passes through 

 scientific treatises unquestioned, and is now apparently regarded 

 as established. We consider it, therefore, important to refute 

 the conclusions involved in the experiment as described by Pro- 

 fessor Cima, and at the same time point out some analogous mis- 

 takes as to the perception of relief. 



When the experiment of Professor Cima is carefully performed 

 and analysed, it will be found that the right eye sees the right 

 half of the middle picture, and the left eye the left half. Now, as 

 these two dissimilar masses are not superposed upon each other, 

 as is the case with the dissimilar complementary figures in ordi- 

 nary vision, but are merely joined together at the line passing 

 through the centre of the resultant picture, it is evident that, if 

 such an effect is realized as that " the nose rises well from the 

 face/' or that there is any " sensation of relief," we have here an 

 experiment which refutes the established theory of binocular 

 vision, and leaves the effects of the stereoscope without any 

 adequate explanation. 



The fact is, however, that in Professor Cima's experiment 

 there is no real perception of relief. All that is really seen is 

 the perspective, which is mistaken for relief or solidity. To 

 prove this, let the observer, while looking at the two half-pictures 

 in the mode alleged to produce the effect of solidity, close one 

 eye, the right for instance ; the right half of the picture disap- 

 pears, but the left retains exactly the same appearance it had 

 before ; it loses no appearance of solidity, simply because it had 

 none. Or let the observer join the two halves together, and 

 closely and continuously observe them with one eye; the effect 

 will be the same as in Professor Cima's experiment. Or, to vary 

 the test, take a single photographic picture, for instance the 

 right-hand side of a stereograph, cut it in two by a vertical line 

 * Cosmos, vol. ii. p. 353. 



