left retains exactly the same appearance it had 



r ranee of solidity simply because it had 

 ;^er join the two halves together and cl( 



E. Emerson on the Perception of Relief . 313 



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 experi- 

 ment 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 Prof. Cima's experiment there is 

 no real perception of relief. All that is really seen is the per- 

 spective, which is mistaken for relief or solidity. To prove this 

 —let the observer, while looking at the two half-pictures in the 

 mode alledged to produce the effect of solidity, close one eye, the 

 right for instance, the right half of the picture disappears, but the 

 left retains exactlv the same aooearance it had before ; it loses no 

 Or, let the 



... ^_ „ ^ . closely and continu- 

 ously observe them with one eye, the effect will be the same as 

 in Prof. Cima's experiment. Or, to vary the test, take a single 

 photographic picture, for instance the right hand side of a stereo- 

 graph, cut it in two by a vertical line through the centre and 

 place the halves the proper distance apart in a stereoscope, so as 

 to unite them readily into a whole, the same effect, claimed by 

 Prof. C. to be a sensation of relief will be observed: that it is 

 Dot relief will be most manifest by comparing it with a stereo- 

 h of the same scene. -, . -r, 



the reader will very naturally inquire—' How did Prof. 

 Cima, and those who have unquestioningly quoted his experi- 

 inent fall into this error with regard to the presence of relief?' 

 This reasonable question we will endeavor now to answer. 

 , The ability to perceive relief, or solidity, is a natural one. To 

 those who have the proper use of their eyes, and can walk, it is 

 an intuitive faculty, we cannot help seeing solidity, where it ex- 

 ists, if we try, no more than we can help hearing sounds or see- 

 ing colors. The common idea that this faculty is the result of 

 experience and is, therefore, acquired, is opposed by the whole 

 analogy of our being. The infant does not learn to hear; it 

 ^ears, it hears intuitively, if it is a perfect child, but learns as it 

 g^ows to know what it hears ; it feels a blow but may be too 

 young and feeble to know what that blow is; so it has but to 

 open its eyes and the scene enters, it is painted properly and in- 

 stantaneously upon the retina, but it may require a long educa- 

 :^'on before the child will have an intelligent idea of what it sees; 

 "^deed it may go through life and never be able to give more 

 ^-^n one nar^e^to a greal variety of very different colors, such as 

 Vermillion, scariet, orange and crimson. It is unphilosophi- 

 confound a faculty with its use. We have the natural 



}' of seeing solidity; but the acuteness with which it is 



"1^ 



