782 THE POPULAR SCIENCE MONTHLY. 



than one muscle is involved. To sum up, the eye, owing to the laws 

 of its mechanism, follows a line much more easily in the central than 

 in the peripheral parts of the field, and in the central parts it follows 

 a vertical line more easily than an oblique, and an horizontal more easi- 

 ly than a vertical. 



It would seem to follow, from these conditions of facile movement 

 in monocular vision, that in the case of binocular vision movements 

 with parallel axes will be easier than movements with convergent axes. 

 And this is proved by observation, for, as Wundt points out, infants 

 instinctively move their eyes in the former way. Combined move- 

 ments with convergent axes constantly involve an extra element of 

 muscular tension, namely, that which is required to counteract the 

 natural tendency to parallelism.* For the rest, it is to be noted that, 

 with respect to " movements of convergence " (which cause the axes 

 to approach one another, or vice versa), the symmetrical movements, 

 which would be executed in following a receding line in the medium 

 plane of the body, have so far a natural superiority over asymmetrical 

 ones that, in the former case, the movements of the two eyes are ex- 

 actly similar, in the latter case not so. The greater sense of ease which 

 accompanies such symmetrical movements is probably explained, in 

 l^art at least, by the constant need of executing such movements in 

 passing the eyes from near to distant points lying in this medium 

 plane. 



Let us now pass to the subjective aspects of ocular movement. 

 Although there is still a good deal of uncertainty respecting the exact 

 composition of the feelings of movement, it may be taken as fairly 

 proved that they include an active element or " feeling of innerva- 

 tion," which is correlated with the central excitation of motor fibers, 

 and a passive element or tactual sensation which is connected with a 

 reflex excitation of sensory fibers, consequent on certain differences in 

 the tensions and mutual pressure of various parts of the skin which 

 result from the movement, f The recognition of this twofold element 

 in the feelings of movement may help us in understanding the plea- 

 sures of ocular movement. 



It will, I think, be admitted as a truth, which is both borne out by 

 direct experience and deducible from more general principles, that 

 every movement of an organ is accompanied by at least a slightly plea- 

 suralDle feeling, provided it has an appreciable duration and rapidity, 

 and on the other hand is not excessive, whether as violently rapid, or 

 as unduly prolonged in time, or repeated, or, finally, as unduly pro- 



* It is to be added, however, that in the case of movements with convergent axes, 

 directed to a point immediately in front of the two eyes, the contrast between horizontal 

 and vertical movements, pointed out in the case of monocular vision, seems to be some- 

 what modified, though hardly obliterated. 



f It is probable that this passive element includes the mental concomitant of an ex- 

 citation of the sensory fibers which are known to run to the muscles themselves. 



