476 Dr Alison on Single and Correct Vision, by means of 



the phenomena of single and double vision are so invariable and 

 uniform in all men, and so exactly regulated by mathematical 

 rules, that I think we have good reason to conclude, that they 

 are not the effect of custom, but of fixed and immutable laws of 

 Nature."* 



In fact, it is a very imperfect and inaccurate expression of the 

 phenomenon in question, to speak of it merely as single vision re- 

 sulting from two images on the retinae. The precise expression 

 of the fact, as fully illustrated by Dr Re id, is, that when images 

 are formed on corresponding points of the retince, they appear as 

 one ; and in all other circumstances they appear as tivo, as they 

 really are ; and this general fact holds good, equally in the case 

 of those, in whom the experience of the sense of Touch habitually 

 opposes the inference drawn from Sight, as in that of those in 

 whom it habitually confirms, and has been thought to suggest 

 that inference. 



The difficulty which is presented by the inversion of the 

 images on the retina is, I think, most correctly expressed thus : 

 The sensations, both of Sight and of Touch, obviously differ from 

 one another in position ; and by doing so, both convey to us in- 

 timations of the situation of external objects. But the judg- 

 ments which we form of the relative position of objects, or of 

 the parts of an object, from the relative position of the im- 

 pressions which they make on the sensitive surface of the re- 

 tina, are just the reverse of those which we form of the relative 

 position of objects or their parts, from impressions made on the 

 sensitive surface of the skin. Thus, if two impressions are made 

 on the upper and lower portions of the eye-ball, and felt through 

 the fifth nerve, the inference immediately drawn is, that the up- 

 per impression is from a higher object, and the lower from a 

 lower ; but if two impressions are made on the upper and lower 

 part of the retina, and felt through the optic nerve, the inference 



* Inquiry into the Human Mind, &c. Sect. 17, ad Jin. 



