of Binocular Vision. 201 



out about 7 n — in some a little less, in some a little more. 

 Beyond 3 inches there seems to be a slight progressive increase 

 rather than decrease; within 3 inches the action of the eyes was 

 irregular. 



I then adopted another method. I used the diagram of pa- 

 rallel lines (fig. 6) and inclined it at an angle of exactly 7° from 

 the perpendicular in the supposed direction of the horopter and 

 at the distance of 3 feet. In this position the verticals, of course, 

 all converge by perspective. I then brought together succes- 

 sively the lines 3 inches apart, then those 6 inches apart, then 

 those 9 inches, 12 inches, 15 inches, 18 inches, and so on even 

 to the last, which were 30 inches apart : in each case the lines 

 seemed to come together parallel', or at least the divergence, if any, 

 was so small that I could not be sure about it. Now in this 

 experiment the point of sight varied from 16^ inches to only 

 2*8 inches in distance, and yet the inclination of the horopteric 

 line seemed to be nearly the same for all, viz. 7°. If there was 

 any difference at all, it seemed to be in favour of greater inclina- 

 tion at greater distance. This result (which I arrived at, though 

 doubtfully, by experiment alone) would be the necessary result of 

 any residual difference between the real and apparent vertical, 

 or, in other words, any residual inclination of the vertical upon 

 the horizontal line of demarcation of the eye in its primary po- 

 sition, such as Helmholtz maintains and as I have supposed 

 possible. Still it by no means proves the existence of this resi- 

 dual difference. 



It must not be supposed, however, because the lines 3 inches, 

 6 inches, 9 inches, 12 inches, &c. apart are all brought into 

 coincidence at the same or nearly the same inclination, that 

 therefore the amount of rotation of the eye is the same for all. 

 The perspective convergence of the lines, of course, increases 

 with their distance apart, and therefore the rotation of the eye 

 necessary to bring them successively into coincidence increases 

 also. It is quite possible that the rotation should increase with 

 the optic convergence, and yet the inclination of the horopteric 

 line remain constant or even decrease with the convergence. 

 Whether the inclination of the horopteric line increases or de- 

 creases with distance would depend upon the law of increase of 

 rotation with increasing convergence. If it increases with dis- 

 tance, then it is possible that when we look at the ground before 

 us the horopter may be a line lying along the ground, as main- 

 tained by Helmholtz. 



I next tried the same experiments with the eyes inclined 

 downward 45°. The lines do not change at all their natural per- 

 spective convergence. In all the experiments made with eyes in 

 this position the inclination of the lines in the image was the 



Phil. Mag. S. 4. Vol. 38. No. 254. Sept. 1869. P 



