Prof. Emerson on the Perception of Relief. 127 



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 Professor Cima to be a sensation of 

 relief, will be observed : that it is not relief will be most mani- 

 fest by comparing it with a stereograph of the same scene. 



But the reader will very naturally inquire, How did Professor 

 Cima, and those who have unquestioningly quoted his experiment, 

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

 reasonable question we will endeavour 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 

 exists, if we try, no more than we can help hearing sounds or 

 seeing colours. 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 hears, 

 it hears intuitively if it is a perfect child, but learns as it grows 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 instantaneously 

 upon the retina ; but it may require a long education before the 

 child will have an intelligent idea of what it sees : indeed it may 

 go through life and never be able to give more than one name to 

 a great variety of very different colours, such as red, vermilion, 

 scarlet, orange, and crimson. It is unphilosophical to confound 

 a faculty with its use. We have the natural faculty of seeing 

 solidity ; but the acuteness with which it is employed depends 

 greatly upon the intelligent attention with which it is exercised. 



It is no answer to this to say that we can analyse the optical 

 conditions upon which the perception of relief depends. This 

 has been splendidly done by Wheatstone, Dove, and others, and 

 is beautifully illustrated by the stereoscope; but this has no 

 necessary connexion with the question before us. When I say 

 we hear intuitively, it is nothing, in the way of refutation, to 

 explain to me the acoustic conditions upon which hearing de- 

 pends, or to assert that Mozart had no intuitive perception of 

 melody or harmony because the laws are fixed by which a melody 

 ought to proceed, and harmony, to be such, must be according 

 to the formula of thorough bass, whereas the child Mozart could 

 not know all this. So with the matter in question ; all men see 

 solidity who have the proper use of their eyes : very few indeed 

 know how it is effected, or are able to distinguish acutely 

 between the perception of binocular relief and the perception of 

 mere perspective, or the appearance of distance without relief. 



The perception of relief depends upon the angle formed by the 

 rays which proceed from any object of sight to the right and 



