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On Single and Double Vision, and an Optical Illusion. 69 



2d. 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. 



3d. The images appeared in snch 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 sin- 

 gle, 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 single 

 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, and from it in the left 

 e ye. 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 

 Teh n a of one eye correspond to cwtain specific parts of the re- 

 tina of the other eye 7 in such a manner that when identical pic- 

 tures fall on those corresponding parts, single vision is the re- 

 suit. Those corresponding parts lie inward in one eye, and out- 

 ward in the other, viz. : both to the right or both to the left. 

 From each of those corresponding parts of the retina, it is prob- 

 able that the fibres of the optic nerve proceed, and severally unite 

 a * 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 up- 

 w ard, 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 im- 

 pression 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 pro- 

 gressed, each pair of images receded gradually from the original 

 place of the single image until the two contiguous ones, the sec- 

 ond and third, approached, and finally coalesced into one, when 

 ^ree images only were in view. The same experiment may be 

 made with two letters or any other figures or objects which are 



equal in size and form, as lollows: 



In all castas the mind seems to make no allowance for distortion of the eye, 

 b ut refers the image to its true place were the eye in its natural position. 



