PROCEEDINGS OF THE SOCIETY. 487 



mena, of stereoscopic vision. It is not, I think, so mucli a question 

 of optics, as of the physiology of vision. If it was one of optics, I 

 should certainly not venture to put myself in antagonism with one 

 who is probably the greatest living master of the theory of the 

 Microscope. But I think I shall be able to show that it is essentially 

 a question of physiology, and in part also of psychology. Ever since 

 Wheatstone's invention of the Stereoscope, something like fifty years 

 ago, I have had the subject constantly before me : and from the first 

 introduction of the binocular Microscope, I have used it continually 

 for objects of suitable character. So completely, indeed, am I 

 accustomed to it, that when I look at some of the same objects under 

 the monocular Microscope, I scarcely know them again. 



The manner in which we form our visual conceptions from im- 

 pressions produced upon the retina, is a matter of both physiology 

 and psychology, lying on the border line between the two. Our visual 

 conceptions are formed by the process which is known as " suggestion " ; 

 that is, they do not necessarily conform to the visual impressions 

 produced upon the retina, but they are suggested to us by these visual 

 impressions; and it sometimes occurs that our conceptions are 

 erroneous. All who have given attention to the physiology of 

 vision, agree in considering our ordinary interpretations of the 

 solidity of an object placed before us, to be dependent upon a mental 

 co-ordination of our visual and tactile sensations. A child moves its 

 hands towards an object presented to its vision, and educates itself to 

 a conception of its form by the conjoint use of its sight and its touch. 

 It has happened that in some cases persons have obtained sight for 

 the first time, having been born blind, at an age when they have been 

 able to record their impressions of objects presented to their sight, 

 and to manifest their difficulties of interpretation. Many years ago 

 I had the opportunity of observing a child three years old, who had 

 been operated on for congenital cataract. He was too young to 

 describe his impressions to us, but we could observe when he was 

 guided by sight and when by touch, and it was very interesting to 

 watch him under these circumstances. In the lodging where he was 

 staying whilst under treatment, everything about him was strange, 

 and he used his sight and his touch conjointly in familiarizing himself 

 with them until he had learned to correlate the two impressions. 

 But when taken to his own home where the surroundings were per- 

 fectly familiar to him, he was for some time entirely guided by touch ; 

 he seemed to be quite puzzled by the sight of them, and often shut 

 his eyes in order to understand where he was. Many of you have 

 heard of the case recorded by the celebrated Cheselden, the subject 

 of which, being much older, could describe his own sensations. For 

 a long time after he could see distinctly, he could not distinguish 

 solid objects by vision alone from flat pictures. Not very many 

 years ago, the case was published of a young woman who from birth 

 had possessed enough sight to enable her to distinguish light from 

 darkness, but who could not see the form of any object about her. 

 She had been accustomed to work with her needle ; and her thread, 

 needle, scissors, balls of cotton, &c., were all perfectly well known to 



