of Binocular Vision. 193 



It is certain, therefore, that the law of Listing is far from 

 being true in strong convergence. Evidently the reason is, that 

 in convergence muscles are used which are not used in simply 

 turning the eyes from side to side, as in the experiments used by 

 Helmholtz to prove this law (p. 180). That different muscles 

 are used in strong convergence is easily shown as follows : — It is 

 easy to turn either eye inward until it looks in the direction of 

 the root of the nose, provided the other eye moves parallel with 

 it, i. e. outward; but it is almost impossible to turn both eyes 

 at the same time so as to look at this point. Great strain is 

 experienced in producing convergence even much short of this. 

 The eyes are turned from side to side, parallel to each other, by 

 means of the interior and exterior recti muscles, while in con- 

 vergence the oblique muscles are also used. For this reason 

 Professor Helmholtz's experiments on spectra do not apply to 

 convergence. 



The law of Donders is equally untrue for strong convergence. 

 This law asserts that the position of the eye is rigorously con- 

 stant for every position of the visual line. But in the experi- 

 ment represented by fig. 9, the eye II, although the direction of 

 its visual line is unchanged, rotates on its axis when the visual 

 line of the other eye is turned from the direction I b to the di- 

 rection I a. 



The reason is, that as I turns toward a the oblique muscles 

 in both eyes begin to act. It is probable that the action of the 

 oblique muscles, and therefore the rotation of the eye, is consen- 

 sual with the two adjustments and with the contraction of the 

 pupil; and it is well known that, under the circumstances repre- 

 sented by the ligure, the pupil of the eye II would contract also, 

 although the direction of the visual line is unchanged. 



III. The Horopter. 



If we look intently at any point, the visual lines converge and 

 meet at that point. Its image is therefore impressed on exactly 

 corresponding points of the two retinae, viz. on the central spot 

 of each. A small object at this point is therefore seen single. 

 We have called this point the point of sight . All objects beyond 

 or on this side of the point of sight are seen double, for their 

 images do not fall on corresponding points of the two retina?. 

 But objects above or below, or to one side or the other of the 

 point of sight, may possibly be seen single also. The sum of all 

 the points which are seen single, while the point of sight remains 

 unchanged, is called the horopter. Or it may be expressed dif- 

 ferently thus : each eye projects its retinal images outward into 

 space, and therefore has its own field of view crowded with its 



