. The physiology of domestic animals ... Physiology, Comparative; Veterinary physiology. SENSE OP SIGHT. 855 the density of the lens at its central part, in order that it may act more strongly on rays of light, by which the refracting power is increased at that point; this is accomplished in the crystalline lens of the eye in this manner since it is less dense at the circumference than in the centre : or, spherical aberration may be diminished by placing a diaphragm between the object of which the image is to be formed and the lens, so as to cut off those rays which pass through the circumfere
. The physiology of domestic animals ... Physiology, Comparative; Veterinary physiology. SENSE OP SIGHT. 855 the density of the lens at its central part, in order that it may act more strongly on rays of light, by which the refracting power is increased at that point; this is accomplished in the crystalline lens of the eye in this manner since it is less dense at the circumference than in the centre : or, spherical aberration may be diminished by placing a diaphragm between the object of which the image is to be formed and the lens, so as to cut off those rays which pass through the circumference and allow the image to be formed only by the central rays. This method, also, is adopted in the construction of the eye, where the movable diaphragm is represented by the iris. Again, another point is to be mentioned : white light, as is well known, is composed of seven colors, which vary in their different decrees of refrangibility—violet, indigo, blue, green, yellow, orange, and 1^ If a beam of white light is passed through a triangular prism of glass it is. Fig. 376.—Diarram illustrating Spherical Aberration. (Oanot.) The rays passing through the edges of the lens have a shorter focal distance than those passing nearer to the centre. decomposed into its constituent rays, the violet rays being refracted most strongly and the red the least (Fig. 311). A white point on a black ground does not form a simple image on the retina, but many colored points appear after each other. If the eye is accommodated so as to focus to a sharp image the violet rays are refracted most strongly; the other colors will form concentric diffusion circles, being most marked in the case of the red rays. In the centre of all the colors a white point is produced by their mixture, while around it are placed colored circles. Such an action, of course, produces dimness of the object, and is known as chromatic aberration. This, too, may be corrected in two ways: either hy making use of the diaphragm
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