the double images beyond B must be homonymous. In this 

 experiment (as also in a less degree in the last), since the eyes 

 are converged, the distance between the points L and B. on the 

 plane^ must be a little less than the interocular distance. 



In both the above cases the visual result is the drawing 

 rotated about the point jof sight in opposite directions through 

 one-half the visual angle ; and for convenience I have spoken of 



3 eyes as accomplishing this rotation. But this is not strictly 

 true. The fields of view of both eyes are habitually and nor- 

 mally shifted one-half interocular space right and left heterony- 

 mously, when the eye is in a passive state. Under these con- 

 ditions the visual results of figs. 15 and 17 would be figs. 19 

 and 20. But in converging the eyes upon A or upon B, the 

 median lines 7i h a, n'h'a' turning upon n and n' as fixed points 

 approach each other, meet and cross at the point of sight so as 

 to form the result already given in figs. 16 and 18. 



Lastly — if without using the plane, three objects A, B and 

 C, be placed in the median line of sight, and the eyes be fixed 

 upon the middle one B (fig. 21), then by the usvxil mode of 

 representation the position of the double images of A and C 

 will be a' a, c c' ; the lines ,here representing of course not 

 visible lines but only lines of direction. But if using the 

 plane we draw a visible line in the direction of the median line, 



the eyes upon B, the falseness of the former mode of represen- 

 tation becomes at once evident. The visual result of this 

 experiment is shown in fig. 23. Every line of this somewhat 

 complex figure can be distinctly seen. By comparing the two 

 modes of representation (figs. 21 and 23), it will be seen that the 

 parallactic position of the double images, or their position 

 when referred to the plane of B, is the same (the dots represent 

 these positions in fig. 23), but only the second mode (fig. 23) 

 represents trulv their apparent distance. 



Thus there are two apparent movements of the visual fields 

 accomplished by the eye (or the mind) in binocular vision : 1st, 

 in a passive state, a shifling of each field one-half interocular 

 space to the opposite side, so as to bring the two visual Imes 

 together to form a true median visual line ; this movement is 

 involuntary and habitual : 2d, in ocular convergence a rotation of 

 each field m a direction opposite to the motion of the eye, on a 

 line passing through the eye normal to the visual plane ; this 

 takes place in every voluntary act of sight In all other move- 

 ments of the eyes, as when we look from side to side or up- 

 ward or downward, objects seem stationary, and we are con- 

 scious of the movement of the visual lines over them ; but 

 when the optic axes move toward each other as in convergence, 



