upon the cornea, and are focused upon the retina, we have emmetropia; if focused in front of the retina, we have myopia; if behind the retina,—that is to say, if the rays after their refraction in the dioptric apparatus converge toward a point lying behind the retina,—we have hypermetropia.
Perhaps it will contribute to the elucidation of this subject if we call attention to the fact that the eye considered as an optical instrument is constructed like a camera obscura. In fact, it is just as important with the camera obscura of the photographer as with the eye that the optical image of the object be thrown with perfect distinctness upon the sensitive plate. Now, under what conditions will this requirement be satisfied?
Let us first examine the camera obscura, which in its simplest form consists merely of a convex lens and of a ground-glass plate, upon which the optical image is received. This image depends upon the fact that the rays of light which proceed from each separate point of the object are brought again to a point. We may therefore consider both the object and the image as composed of an infinite number of points, and what is true of one object-point and of its corresponding image-point is true of all.
If the position of the object-point—that is, its distance from a convex lens—be given, the position of the image-point depends upon the focal length of the lens, or in other words, upon the distance at which parallel rays are brought together. The less this distance, the greater is the refractive power of the lens. Focal length and refractive power are therefore in inverse proportion. If one lens has, for instance, a focal length of 1 inch, while another lens has one of 2 inches, and a third one of 3 inches, then the refractive power of these lenses is as . We therefore express the optical value of a lens by a fraction whose numerator is 1 and whose denominator is the focal length of the lens. The optical value of a lens of ten inches focal length is thus expressed by . This is ten times less than that of a lens of one inch focal length. In expressing the power of lenses any other unit of measure may of course be taken as well as the inch.
If in Fig. 1 be the image-point belonging to an object-point infinitely distant, the point of union for parallel rays—that is, the principal focus of the convex lens—lies at . The optical value of the lens would therefore be expressed by .
