98 J. LeConte — Phenomena of Binocular Vision. 



too large, i. e. not farther apart or much farther apart than the 

 two eyes) by looking beyond the plane of the figures as in the 

 stereoscope, so as. to make phantom-images, which may seem 

 to the observer as distinct as the object itself. The combina- 

 tion by crossing is by far the easier, and it is this method there- 

 fore which we shall use in all the following experiments. As 

 in this case the phantom image is always at the point of sight, 

 i. e., where the optic axes meet, it is evident that by combining 

 first contiguous figures, and then by stronger convergence 

 those more and more distant from each other, the phantom- 

 plane is brought nearer and nearer to the eyes, until by ex- 

 treme convergence it may be brought within a fraction of an 

 inch of the root of the nose. A tessellated pavement or oil- 

 cloth may be thus brought up as phantoms successively through 

 many different planes (the number depending upon the size of 

 the figures) almost to contact with the face. Then by relaxing 

 the converging muscles, the phantom-image may be let fall and 

 caught on successively lower planes until finally it is dropped to 

 its natural place and becomes real. Of course since the angu- 

 lar diameter of the figures remain the same, the apparent size 

 becomes smaller as the phantom-image comes nearer. The 

 phantom may thus become the most exquisite miniature imagin- 

 able. If on the other hand we combine by looking beyond the 

 plane of the object as in the stereoscope, the phantom-figures are 

 proportionally enlarged. The phantom is usually represented 

 as a plane. Such it would be by geometric construction, but 

 such it is not in reality as we shall presently show. 



The foregoing experiments are to me as easy as natural 

 vision, and the resulting phantom as distinct as a natural object. 

 Most persons, however, will find some difficulty in making the 

 phantom clear ; because the eyes are accommodated to nearer 

 distance, viz : the point of sight, while the light comes from a 

 greater distance, viz : from the real object. In such cases the 

 use of slightly concave spectacles will remedy the difficulty. 



I recall these well-known facts to mind only that what fol- 

 lows may be readily understood. 



Experiment 1. — I sit in a chair in the middle of a regularly 

 tessellated floor and direct the eyes on the floor at an angle of 

 about 45°. By ocular convergence I now combine successively 

 the figures of the floor, stopping a little while after each com- 

 bination until the phantom-image becomes perfectly clear. 

 These phantom-floors are distinctly perceived to be not horizon- 

 tal as they ought to be by geometric construction, but inclined ; 

 dipping away from the observer at higher and higher angle in 

 proportion as, by greater convergence, the phantom-floor comes 

 nearer and nearer. I am sure I can easily make the apparent 

 slope 30°-40°. 



