The path of the light forming an image on the retina is shown in Fig. 33, where A B is the object, and _a b_ its image, formed in exactly the same way as the image of the candle-flame which we have just considered; in fact, the eye is exactly represented by a photographic camera, the iris acting in the same manner as the stops in the lens, limiting its available area, and by contracting, decreasing the amount of light from bright objects, and at the same time increasing the sharpness of definition, for in the case of the eye, the luminous rays obey the known laws of propagation of light in media of variable form and density, and we have only simple refraction to deal with. The next matter to be considered is that the nearer the object A B is to the eye, the larger is the angle A, _o_, B, and also _a_, _o_, _b_, and therefore the image on the retina is larger; but there is a limit to the nearness to which the object can be brought, for, as we found with the candle, the distance between the lens and the image must be increased as the object approaches, or the curvature of the lens itself must be altered, for if not the ray forming the rays from each point of the object will be too divergent for the lens to be able to bring them to a focus. Now in the eye there is an adjustment of this sort, but it is limited so that objects begin to get indistinct when brought nearer the eye than perhaps six inches, because the rays become too divergent for the lens to bring them to a focus on the retina, and they tend to come to a focus behind the retina, as in Fig. 34; but we may assist the eye lens by using a glass convex lens in front of it, between it and the object. It is for this reason that spectacle glasses are used to enable long-sighted persons to see clearly. [Illustration: FIG. 34.—Action of a Long-sighted Eye. ] We may also use a much stronger lens, and so get the object very near the lens and eye, as in Fig. 35, where _a b_ is the object so near the eye that, if it were not for the lens L, its image would not come to a focus on the retina at all. The effect of the lens is to make the rays proceeding in a cone from _a_ and _b_ less divergent, so that after passing through it, they proceed to the eye-lens as if they were coming from the points A and B, a foot or so away from the eye, and so the object _a b_ appears to be a much larger object at a greater distance from the eye. [Illustration: FIG. 35.—1. Diagram showing path of rays when viewing an object at an easy distance. 2. Object brought close to eye when the lens L is required to assist the eye-lens to observe the image when it is magnified. ]