Page:Optics.djvu/128

 thrown outside of the eye. The great Author of nature has, however, not left his work so imperfect. By means of a certain muscular mechanism, which increases or diminishes at pleasure the convexity of the chrystalline, the form of the eye is modified, so as to throw on the retina distinct images of all objects, between very wide limits. Good eyes can see distinctly objects at a distance so great, that the rays proceeding from them, admitted through the pupil, must be to all sense parallel, and with equal facility, others placed at eight or nine inches from them.

139. Persons who have been accustomed for some time to look very much at near objects, as in reading, or engraving, are apt to become short-sighted.

Others again, such as savages, who are in the constant habit of looking out for game at a distance, cannot see a thing distinctly within arms-length; they are what is called long-sighted.

Both these imperfections proceed from the same cause. The chrystalline being constantly used in one state becomes fixed in it, and the muscles which serve to modify its form, lose their power from disease.

Old age generally brings on long-sightedness, which is commonly corrected by the use of spectacles with convex lenses, to assist the eye in giving the necessary convergence to rays proceeding from a near object.

In like manner, short-sighted persons use concave glasses, to enable them to discern distant objects, as the effect of such glasses is to give additional divergency to incident rays.

140. It is of consequence to a person of an imperfect sight to know the form or focal length of the lens best suited to assist his eye; this is easily found: suppose a short-sighted man can see distinctly only to the distance of 20 inches: he will be enabled to discern a very distant object by the help of a lens, which will produce an image of such an object at the distance of 20 inches, or which will make parallel rays diverge from a focus at 20 inches from him, (supposing the glass placed close to the eye;) that is, a concave lens of 20 inches focal length.