E. Emerson on the Perception of Relief. 



Art. XXYl.— On the Perception of Relief; by Prof. Edwin 

 Emerson", of the Troy University. 



Prof. Cima, of Turin, has sent us the description (says the 

 editor of the Cosmos) of a stereoscopic experiment which is not 

 without interest. He takes the picture of a front view of a hu- 

 man head, executed either in crayon or lithograph or copper-plate, 

 and which is three or four centimetres in height ; this he cuts into 

 two parts along a line which coincides with the vertical axis of 

 the nose; he takes one of these halves in each hand, and hold- 

 ing them in the same perpendicular plane, he brings them before 

 the eyes at a distance which is less than that of distinct vision; 

 he then allows the optic axes to converge, and thus causes the 

 drawings to approach or recede until he is able to see two pic- 

 tures of each half, and until the two middle ones overlap, so that 

 they make the impression of an entire countenance. When one 

 makes this experiment for the first time, says Prof Cima, he will 

 see with astonishment that the full face which is produced by the 

 overlappmg of tlie two halves makes, in a hic^h* degree, the im- 

 pression of a solid body ; the half tones melt and mix together 

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

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

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

 assumes, in a remarkable degree, a living expression. The 

 necessary distance of the two half-pictures from each other and 

 also the proper distance from the eyes of the observer for the 

 production 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 

 m Pogg. Annalen, bd. cii, p. 319 ; in II Nuovo Cimento, vi, 18^ i 

 m Die Theorie des Schens und raumlichhen Vorsielkn, bei Vt. 

 Cornelius, Halle, 1861 ; and in Monographic du Stereoscope, V^,^ 

 Blanchdre, Paris, 1862. Seemingly endorsed by such a high 

 authority as Moigno, the alleged fact passes through scientitic 

 treatises unquestioned, and is now apparently regarded as estab- 

 lished. We consider it, therefore, important to refute the con- 

 clusions involved in the experiment as described by Prof Cma. 

 and at the same time point out some analogous mistakes as to 

 the perception of relief , 



When the experiment of Prof Cima is carefully performea 

 and analyzed it will be found that the right eye sees the ng^] 

 half of the middle picture, and the left eye the"lefl; half, now a^ 

 these two dissimilar masses are not superposed upon each other, 

 aa IS the case with the dissimHar complementary figures m oroi- 



