apex is in the object, and its base at the cornea. Those rays which enter through the pupil, proceed through the transparent media—viz., the aqueous humour—the lens and vitreous body. Thus are the rays of light made to converge in the interior of the eye. This convergence presents a cone, whose point rests on the concave surface, or posterior hemisphere, i.e. on the retina; its base being on the cornea, or anterior hemisphere. The point in the retina is the focus, and it is thus the point of the external object from which the rays diverge is represented at the bottom of the eye, by a point exactly corresponding to that of the external object. But as every point of the object sends rays to the eye in the same manner, these must represent as many foci in the retina; and as the points in the object lie close to one another, the foci at the bottom of the eye must also occupy the same relative position among themselves as these points in the object. The object then forms an image in the interior of the eye, though small, just as a picture is represented on the table of the camera.
The optician's business is to make an instrument which is analogous to the mere mechanism of the eye, and he is well able to explain the mode by which objects arrive on the retina; but who can say how they communicate with the brain, and become a part of man's knowledge? Of this hereafter.
When there is any malformation, either in quality of refraction, media, or mechanical parts, or adjustment of the eye, short sight, or other imperfections, and even blindness are the result. Even light, or rays which enter the eye, cannot be used by all eyes alike, but to some they create indistinctness and even pain.
As to the quantity of light admitted into the eye, this is regulated by the iris, which contracts when too much is
