of Binocular Vision. 187 



which I bring together just as plainly as if they were black and 

 white threads. While watching them I control their motions almost 

 as perfectly as if I was sliding with my hands two frames with 

 white and black threads stretched across them. There is not the 

 shadow of a doubt, therefore, that in my own case the eyes in con- 

 vergence rotate slightly outward, and that the amount of rota- 

 tion increases with the degree of convergence. 



I next proceeded to determine the amount of rotation for dif- 

 ferent distances of the point of sight. In the diagram in which the 

 inclination of the lines was 5 degrees, the distance of the image 

 was only 2 to 2^ inches; for the lines inclined 2\ degrees, the 

 distance of the image was 4 inches; for lines inclined 1^ degree 

 the distance was 7 inches ; and for 40 minutes the distance was 

 about 12 to 14 inches. 1 am able by great strain to obliterate, 

 or nearly obliterate, the common field of view of the two eyes. In 

 this case, of course, the eyes both look at the root of the nose. 

 In this extreme convergence I find that lines coincide which 

 make with each other an angle of 22°, or 11° with the vertical. 

 This would seem, therefore, the extreme rotation for my eyes. 

 The distance of the image in this case is nearly at the root of 

 the nose. 



If, however, in extreme convergence rotation on the optic axes 

 takes place to the extent of 1 1°, this rotation ought to be de- 

 tectable by means of ocular spectra, or even by direct observa- 

 tion of the eye itself. I determined to try these also. My 

 method of experimenting with ocular spectra is as follows : — 

 Standing in a somewhat obscure room, I gaze with the left eye 

 (the other being shut) at a vertical crevice in a closed window 

 until a distinct spectrum is obtained. Placing myself now op- 

 posite a vertical line on the wall of the room, with my right side 

 toward the wall, I turn my head until my left eye II (fig. 9), look- 

 Fig. 9. 



ing across the root of my nose, n, can see the vertical line. I 

 now gaze at a point very near the vertical line, and, by inclining 

 my head to one side or the other, bring the spectrum exactly 

 parallel to the vertical line. In this position, if the wall be at 



02 



