1864.] Human Eye in relation to Binocular Vision. 



193 



wards, and we are therefore unable to bring our eyes into such positions. 

 But it is a well known fact, that when we look at stereoscopic pictures, and 

 increase the distance of the pictures by degrees, our eyes follow the motion 

 of the pictures, and that we are able to combine them into an apparently 

 single object, although our eyes are obliged to turn into diverging direc- 

 tions. Professor Bonders, as well as myself, has found that when we 

 look to a distant object, and put before one of our eyes a prism of glass 

 the refracting angle of which is between 3 and 6 degrees, and turn the 

 prism at first into such a position before the eye that its angle looks to the 

 nose and the visual lines converge, we are able to turn the prism slowly, 

 so that its angle looks upwards or downwards, keeping all this time the 

 object apparently single at which we look. But when we take away the 

 prism, so that the eyes must return to their normal position before they 

 can see the object single, we see the object double for a short time — one 

 image higher than the other. The images approach after some seconds of 

 time and unite at last into one. 



By these experiments it is proved that we can move both eyes outward, 

 or one up and the other down, when we use them under such conditions 

 that such a position is required in order that we may see the objects single 

 at which we are looking. 



I have sometimes remarked that I saw double images of single objects, 

 when I was sleepy and tried to keep myself awake. Of these images one 

 was sometimes higher than the other, and sometimes they were crossed, 

 one of them being rotated round the visual line. In this state of the 

 brain, therefore, where our will begins to lose its power, and our muscles 

 are left to more involuntary and mechanical impulses, an abnormal rota- 

 tion of the eye round the visual line is possible. I infer also from this 

 observation, that the rotation of the eye round the visual axis cannot be 

 effected by our will, because we have not learnt by which exertion of our 

 will we are to effect it, and that the inability does not depend on any 

 anatomical structure either of our nerves or of our muscles which limits 

 the combination of motion. We should expect, on the contrary, that, if 

 such an anatomical mechanism existed, it should come out more distinctly 

 when the will has lost its power. 



"We may ask, therefore, if this peculiar manner of moving the eyes, 

 which is determined by the law of Listing, is produced by practical exer- 

 cise on account of its affording any advantages to visual perceptions. And 

 I believe that certain advantages are indeed connected with it. 



We cannot rotate our eyes in the head, but we can rotate the head with 

 the eyes. When we perform such a motion, looking steadily to the same 

 point, we remark that the visible objects turn apparently a little round 

 the fixed point, and we lose by such a motion of our eye the perception of 

 the steadiness of the objects at which we look. Every position of the visual 

 line is connected with a determined and constant degree of rotation, accord- 



VOL. XIII. Q 



