148 ANNUAL OF SCIENTIFIC DISCOVERY. 



All these effects depend on the principle, that one of the two prim- 

 itive pictures is seen by one eye, and the other by the other eye, and 

 that the axes are so converged by looking at the index or mark on 

 the upper screen, that those separate images fall on the points in the 

 eye which produce single vision. To a person who has perfect volun- 

 tary control over the axes of his eyes, the upper screen and index are 

 unnecessary. Such an observer can at any time look two contiguous 

 persons into one, or superimpose the image of one upon the image of 

 the other. 



We find in a letter from Professor Locke one or two additional ex- 

 periments described. He says : " I took the figure or picture of a 

 person about two inches in height, and, having cut its outline from 

 the paper, and cut off the head, I placed the body to the left on the 

 lower screen, and the head to the right on a level with its proper po- 

 sition, and directed my eyes to the index of the movable screen, 

 when the body appeared to move in from the left and the head from 

 the right till they were apparently reunited, and an entire figure pre- 

 sented itself to view. But, from a little unsteadiness of the eye in a 

 forced position, the head had a small motion, sometimes reaching for- 

 ward in the attitude of earnestness, then drawing back with an expres- 

 sion of dignity. I found, too, that my eyes were not always mates. At 

 one time the body, which was seen by the right eye, appeared bright, 

 while the head, seen by the other eye, was dim and confused. After 

 a little these conditions were reversed, and the left eye gave the 

 brighter image. When the images of two colored objects are opti- 

 cally superimposed by the phantascope, say a blue and red wafer, the 

 phantom will sometimes be purple, again it will be red, when, on re- 

 versing it, the blue will predominate ; showing that one eye is more 

 sensitive to colors than the other, and that the double-imaged phantom 

 will appear of that color which falls on the stronger eye. Throwing 

 aside the machinery of the phantascope, F crossed the axes of my eyes, 

 and looked at the window of my room, increasing the convergence un- 

 til the two images of the window lay side by side, the right-hand 

 side of one image lying along the left-hand side of the other. These 

 edges did not appear to be parallel, the lower ends being apart while 

 the upper ends were in apparent contact. From this it appears that 

 the eyes did not rotate in the same horizontal plane. On throwing 

 the head back as far as possible, and making the same convergence of 

 the axes, the perpendicular objects preserved their parallelism, and the 

 two sides of the two images of the window coincided throughout. 

 Whether this is true of the e'yes of all persons is doubtful. These 

 experiments on^ binocular vision are not so amusing as several others 

 in optics, and to some persons the effort to distort the optical axes is 

 painful, like looking at a double impression in printing. The strug- 

 gle between the knowledge of where the primitive picture really is, 

 and the optical impression of the phantom, is sometimes quite painful ; 

 but, as soon as the imagination realizes the place of the phantom, it is 

 contemplated with as much ease as a real object. There is a math- 

 ematical ratio in the several quantities concerned. For example, 

 the distance from the eye to the phantom is to the distance from the 



