CASES IN WHICH WE SEE DOUBLE. 



Let o be an object, such as the flame of a candle or lamp, seen 

 at the distance of about 40 feet, so that the lines of direction L o 

 and K, o converging upon it from the centres of the eyes 19 

 may be regarded as practically parallel, the distance 

 being about 200 times greater than the distance L E, 

 between the eyes. The object o will therefore be seen in 

 the same direction by both eyes, and being at a distance 

 from the two eyes practically equal, will have the same 

 apparent magnitude, form, colour, and intensity of illu- 

 mination, and, consequently, will be seen single. 



Let a small white rod be held at the distance A, of 

 about 8 inches from the left eye L, and in the line L o, 

 so as to intercept the view of the object o from the left 

 eye. The left eye will then see the rod at A, and not 

 the object o ; but the right eye will still see the object o, 

 as before. Now, if the attention be earnestly directed 

 to the object o, the object A will not be perceived ; but 

 if the attention be directed to the object A, it will be 

 perceived distinctly, but the object o will be seen 

 through it as if it were transparent. 



Now, since the object o cannot be seen by the left eye 

 under the circumstances here supposed, the perception we have of it 

 must be derived from the right eye ; nevertheless it is seen in the 

 line L A o, immediately beyond the intercepting wand, and in the 

 same direction, and in the same manner precisely as it would be 

 seen by the left eye L if the intercepting wand were removed. It 

 follows, therefore, that the perception we obtain of the object by 

 the right eye is precisely the same as that which we should obtain 

 by the left eye if the right were closed, and the intercepting wand 

 A removed. This may therefore be taken as an experimental 

 proof of what, indeed, may seem sufficiently evident, d priori, 

 that an object, such as o, placed at a distance so great that lines 

 drawn to it from the centre of the eyes would be practically 

 parallel, produces precisely the same perception through the vision 

 of both eyes. 



But when the distance of an object from the eye is so small that 

 the line which separates the eyes bears a considerable proportion 

 to it, the directions in which such an object is seen by the two 

 eyes are different, and it is easy to show that in this case such 

 an object would be seen double. 



Let L, and R (fig. 13), as before, be the centres of the two eyes, 

 and let A B be a white screen placed vertically at a distance of 12 

 or 14 feet, having upon it a horizontal line on a level with th e 

 eye, upon which is marked a divided scale 1, 2, 3, 4, 5, 6, 7, 8, 9, 

 10. Let a black wand be held vertically at o, opposite the middle 



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