of Edinburgh , Session 187 0-7 1 . 359 
lions which regulate the expansion and contraction of the iris of 
the eyes. 
I do not believe in any partial crossing of the true visual nerve- 
fibres. The fact, however, of an entire crossing, or of no crossing 
at all, in no ways affects my theory, which I shall now, after a few 
necessary words of explanation regarding the functions of the 
retina, proceed to explain. 
The central point of the retina, the fovea centralis , is distin- 
guished from the rest of the retina by its peculiar anatomical struc- 
ture. It is also distinguished by its superior discriminating powers. 
It is the only part of the retina which takes minute cognisance of 
the forms of objects. We may satisfy ourselves of this by fixing 
the eyes on any word in a printed book held at the usual reading 
distance. While the eyes remain fixed on the middle of any 
word of, say six or seven letters, most persons will find that they 
are quite unable to perceive a single letter of the adjoining word. 
This proves how limited is the area of distinct vision on the 
retinas. 
When we fix the eyes on any distinct object in an extended 
landscape we turn the axis of each eye to the object especially 
examined, and the images of it fall on the fovece centrales , and 
appear single. All the other objects in the landscape are mapped at 
the same time around these central points, on corresponding parts of 
each retina, i.e., on parts which are correspondent in distance and 
direction, from the foveoe centrales; and these objects also, so far as we 
can see them, appear single. The remarkable circumstance, how- 
ever, is, that the slightest shift or displacement of the axis of one 
of the eyes, and of the image on it, disorders correct vision, and 
produces the perception of a duplicate impression of the landscape. 
This circumstance has led authors very generally to the con- 
clusion, as I have said, that either habit, or some inscrutable law 
of the retinas, causes single vision when corresponding parts of that 
organ are impressed, and double vision when non-corresponding 
parts of the two retinas are acted on. The writer maintains that 
these phenomena, and also the phenomenon of increased brightness 
obtained by the use of both eyes, can only be explained on the 
assumption or theory, that the retinal impulses of both eyes are 
united in a central cerebral sensor ium. He, therefore, suggests 
