1 68 THE HUMAN BODY. 



less than the diameter of the pupil, that is, not more than one- 

 twelfth of an inch from each other, then look through these 

 two apertures at a small object on a bright ground, at a black 

 point, for example, on a sheet of white paper. At a certain 

 distance the point is single, but if the head be moved back- 

 ward or forward it will appear double. 



In the first two experiments the eye is compelled to adapt 

 itself to the distance, in order to see distinctly and succes- 

 sively two objects at unequal distances, and of which the 

 images are not distinct, except when the apex of the cones 

 formed by the refracted rays of light exactly corresponds to 

 the surface of the retina, that is, when the retina is exactly in 

 the focus. And also, the experiment of looking through a 

 pierced card at an object, and seeing it distinctly, that is, 

 looking at it through an immovable artificial pupil, seems to 

 prove that the movements of the pupil are not necessary to 

 accommodation. 



The third experiment proves, that in order to see a single 

 image the retina must be in focus. In this case, in fact, the 

 rays coming from the external object converge and meet on 

 the same retinal divisions, hence there is but a single sensa- 

 tion; if the eye approaches or recedes, they reach the retina 

 either before their convergence is effected, or not until, 

 having converged, they cross each other, and diverge beyond 

 the focus, in either case in such a manner as to fall upon 

 different divisions of the retina, and in consequence produce 

 a double sensation. 



The accommodation of the eye therefore seems to be in- 

 contestable, in spite of the want of accord in the opinions of 

 savants upon its mechanism. A very little attention enables 

 us to recognize the effort which accompanies it, especially if 

 the adaptation is prolonged without variation at a short dis- 

 tance, as in looking through a microscope. Then, in fact, 

 the eye loses sometimes for several hours the faculty of 

 adapting itself to great distances; it becomes myopic for a 

 certain time. Persons who are in the habit of using a glass 

 for one eye, as watchmakers and engravers for example, are 

 generally myopic in that eye; and this effect is very marked 

 in infants, who acquire the habit of looking at objects near 



