68 On Single and Double 



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Art. VI. — On Single and Double Vision produced by viewing 

 objects xoith both eyes : and on an Optical Illusion icith regard 

 to the distance of objects ; by John Locke. 



I had commenced the investigation of this subject so early as 

 1816, while I was a student of medicine in Yale College, 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 investiga- 

 tions of Prof. Wheatstone,* and Sir David Brewster,f yet I 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 sub- 

 ject. It is a well known phenomenon, that with both eyes open 



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

 the left of the object and the image which disappeared on clos- 

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

 an oblique one, seen as we see lateral objects to which the eyes 

 are not directed. It appears too, that while the axes were con- 

 verged 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 oppo- 

 site eye. J 





we can see a single object either single or double, according as I 



the axes of the eyes are made to converge and meet either at the { 



object, or at a point nearer than that object. Having acquired 

 the power of voluntary convergence 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 ex- 

 periments as follows: 



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



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

 tremely converged. Two images were of course seen, the dis- 

 tance 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. 











* I have not seen his paper. t Phil. Mag., May, 1847. 



+ This is not always the condition of strabismus, for one eye may be so directed j 



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





