476 Dr Aurson 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 retine. The precise expression 
of the fact, as fully illustrated by Dr Rerp, is, that when images 
are formed on corresponding points of the retine, they appear as 
one; and in all other circumstances they appear as two, 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 7e- 
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, &e. Sect. 17, ad fin. 
Se 
