VISION, 565 
the lens and the retina, and in many of the illustrations to fol- 
low this will be done. The experiments referred to above will 
convince the student that such is the case; and we may here 
state that, while the various principles involved in the physiol- 
ogy of vision may be illustrated in great perfection by elab- 
orate experiments, we shall endeavor to supply the -student 
with accounts of very simple methods of convincing himself 
by personal observation, such as may be readily repeated at a 
future time, which is more than can be said for those that in- 
volve expensive apparatus. 
ACCOMMODATION OF THE EYE. 
Using the material already referred to, the student may 
observe that, with the natural eye of the albino rabbit, its lens 
(or better that-of a bullock’s eye, being larger), or a biconvex 
lens of glass, there is only one position of the instruments and 
objects which will produce a perfectly distinct image. If either 
the eye (retina), the lens, or the object be shifted, instead of a 
distinct image, a blurred one, or simply diffusion-circles, appear. 
A photographer must alter either the position of the object 
or the position of his lens when the focus is not perfect. The 
eye may be compared to a camera, and since the retina and 
lens can not change position, either the shape of the lens must 
change or the object assume a different position in space. As 
a matter of fact, any one may observe that he can not see 
objects distinctly within a certain limit of nearness to the eye, 
known as the near point (punctum proximum); while he be- 
comes conscious of no effect referable to the eye until objects 
approach within about sixty-five to seventy yards. Beyond 
the latter distance objects are seen clearly without any effort. 
We thus learn that the range of accommodation lies between 
about five inches and sixty to seventy yards, though it is cus- 
_ tomary to speak of the far point as infinity («), which simply 
means that the rays from objects beyond the distance given 
above are practically parallel, and are, therefore, focused on 
the retina without any alteration in the shape of the lens (neg- 
ative accommodation); while nearer ones require this. When 
objects are nearer to the eye than about five inches, for most 
persons, the eye can not accommodate sufficiently to bring the 
rays of light emanating from them to a focus on the retina. 
There are many ways in which we may be led to realize 
these truths: 1. When one is reading a printed page it is only 
