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XXV. On Single and Double Vision produced hy viewing 

 objects iioith both eyes ; and on an Optical Illusion mth regard 

 to the distance of objects. By Joum Locke*. 



I HAD commenced the investigation of this subject so early 

 as 1816, while I was a student of medicine in Yale Col- 

 lege, and I have occasionally turned my attention to it up to 

 the present time. 



Although I have been fairly anticipated in the publication 

 .of some of my results, perhaps most of them, by the late in- 

 vestigations of Prof. Wheatstonef and Sir David Brewster J, 

 yet 1 deem it not useless to give you an account of the history 

 of my own experiments and conclusions, especially as some of 

 them are not, so far as I know, contained in the publications 

 of either of the distinguished philosophers who have just 

 written upon the subject. It is a well-known phaenomenon, 

 that with both eyes open we can see a single object either 

 single or double, according as the axes of the eye^ are made 

 to converge and meet either at the object, or at a point nearer 

 than that object. Having acquired the power of voluntary con- 

 vergence of the optical axes to an extreme degree without the 

 aid of viewing near objects, such as the nose, or a finger held 

 near to the eyes, I commenced my experiments as follows : — 



Experiment I. — I viewed a burning candle at the distance 

 of about eight feet, the axes of the eyes being " crossed " or 

 extremely converged. Two images were of course seen, the 

 distance between which could be varied at pleasure by the 

 amount of that convergence. The two images of the candle 

 being thus seen, I suddenly closed one of my eyes, when the 

 image on the same side of the closed eye vanished. Thus on 

 closing the right eye, the right image disappeared ; and on 

 closing the left eye, the left image became extinct. 



Inferences, — 1st. As the axis of the right eye was directed 

 to the left of the object, and the image which disappeared on 

 closing that eye was to the right of it, that image must have 

 been an oblique one, seen as v/e see lateral objects to which 

 the eyes are not directed. It appears, too, that while the axes 

 were converged upon vacancy, the oblique image in the right 

 eye took the place of an image formed directly in the axis of 

 the left eye, and the same relatively of the left eye; thus each 

 eye appeared to have an image in its axis, which image was 

 really in the opposite eye§. 



* From Silliman's Journal for January 1849. 

 t I have not seen bis paper. % Phil. Mag., May 1847. 



§ This is not always the condition of strabismus, for one eye may be so 

 directed that the axis shall be on the object while the other is oblique. 



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