196 Mr. J. Locke on Single and Double Vision^ 



2nd. The two oblique images on the retina must have been 

 formed on points nearer to the nose, or nearer to the medial 

 line of the body than the principal axis of perfect vision. 



3rd. The images appeared in such position as objects 

 should have been to produce pictures on the same parts of the 

 retina, the axes being at the same time parallel, or nearly so. 



4th. As with both eyes we see a lateral object ordinarily 

 single, especially when at the same distance as the principal 

 object viewed, it is inferred that the two pictures, one in each 

 eye, must fall on parts of" the retina not correspondent to the 

 medial line of the body in order to produce single vision. For 

 example, on looking at a person standing ten yards in front, 

 the image of a person standing two yards or more to the right 

 will appear siiigle though not well-defined. The picture of 

 this second person must in such case be formed to the left side 

 of the retina of both eyes, towards the nose in the right eye, 

 Six\(\from it in the left eye. In these two situations on the 

 retina, and in no other, will the two oblique pictures present 

 a single image to the mind. 



5th. All this establishes the principle, that certain parts of 

 the retina of one eye correspond to certain specific parts of the 

 retina of the other eye, in such a manner that "when identical 

 pictures fall on those correspond itig parts, single visioti is the 

 7'esult. Those corresponding parts lie inward in one eye and 

 outward in the other, viz. both to the right or both to the left. 

 From each of those corresponding parts of the retina it is pro- 

 bable that the fibres of the optic nerve proceed, and severally 

 unite at the point of anatomical communication where the 

 optic nerves cross before entering the brain ; hence the single 

 impression or single image. 



It may be added to this experiment (I.), that if the finger 

 be pushed against the under part of the eyeball so as to roll it 

 upward, the image in that eye will appear to descend, and 

 double vision will thus be the result. Here the eye being 

 rolled upward, the image falls on the upper part of the retina; 

 hence the impression of a lower object would form a picture 

 on that part, were the eye not distorted*. 



Experiment II. — Two candles of equal size and height were 

 placed side by side on the table, and by converging the axes 

 of the eyes, four images were produced. As the convergence 

 progressed, each pair of images receded gradually from the 

 original place of the single image until the two contiguous 

 ones, the second and third, approached, and finally coalesced 



* In all cases the mind seems to make no allowance for distortion of the 

 eye, but refers the image to its true place were the eye in its natural po- 

 sition. Uij 



