SINGLE VISION WITH TWO EYES. STEREOSCOPE. 171 



sight, that is to say, when it is placed at the point at which 

 -the optical axes converge; but if the direction of one of 

 these axes is changed, from pressing lightly with the point of 

 the finger on the external angle of one of the eyes, for in- 

 stance, the object appears double, and the images are sepa- 

 rated more and more as the pressure is increased, and as it 

 changes the direction of the axis more and more. On the 

 other hand, two objects, one placed in front, and the other 

 beyond the point of convergence of the two axes, but in 

 the same direction, give but one impression, and we see but 

 one. 



Double or single vision with two eyes is explained by the 

 correspondence of the terminal divisions of the retina in each 

 eye. These are called the identical points. When the rays 

 of light strike corresponding divisions in each eye, the sensa- 

 tion is single, but when they strike portions which do not 

 correspond, it is double. This correspondence of the parts 

 of the retina is shown by pressing lightly with the fingers on 

 the closed eyes. If the internal or external angle of the eyes 

 be pressed simultaneously, luminous images will be formed at 

 the points directly opposite those which are pressed, and if 

 the pressure be applied to the internal angle of one eye and 

 the external angle of the other, or if we press the upper por- 

 tion of one and the lower portion of the other, we see but a 

 single image. From this we conclude that in the first ex- 

 periment the two points pressed upon do not coincide because 

 we see two distinct images, and that in the second they do 

 correspond because we see but one. According to Miiller, 

 if we consider the retina as a sphere, the pole of which is the 

 middle of the membrane or at some point in the same direc- 

 tion and at the same distance from the middle, the corres- 

 ponding or identical points, on a section of this sphere, 

 occupy the same meridian and the same parallel. Thus, in 

 seeing with two eyes, the two images cause but a single 

 sensation when they are formed on the corresponding por- 

 tions of the retina, and consequently we receive a double 

 sensation when they are placed on divisions which are not 

 identical. 



Stereoscope. From what has been already stated it would 



