9. This phenomenon is called the camera obscura, or dark chamber, because it is necessary to darken the room to exhibit it. The image at the focal point within the room is in an inverted position. The reason why it is thrown in this manner will be readily understood by observing the direction of the reflected rays, as they pass from the object through the lens. In the camera obscura, it is customary to place a small mirror immediately behind the lens, so as to throw all the light which enters, downwards upon a whitened table, where the picture may be conveniently contemplated.
10. From the preceding explanation of the camera obscura, the theory of vision may be readily comprehended, since the eye itself is a perfect instrument of this kind. A careful examination of the following representation of the eye will render the similarity obvious. The eye is supposed to be cut through the middle, from above downwards.
a a, the sclerotica; b b, the choroides; c c, the retina; d d, the cornea; e, the pupil; f f, the iris; g, the aqueous humor; h, the crystalline humor; i i, the vitreous humor.
11. The sclerotica is a membranous coat, to which the muscles are attached which move the eye. The cornea is united to the sclerotica around the circular opening of the latter, and is that convex part of the eye, which projects in advance of the rest of the organ. The space between this and the crystalline lens is occupied by the aqueous humor and the iris. The iris is united to the choroides, and it possesses the power of expanding and contracting, to admit a greater or less number of rays.
12. The crystalline lens is a small body of a crystalline appearance and lenticular shape, whence its name. It is situated between the aqueous and vitreous humors, and consists of a membranous sack filled with a humor of a crystalline appearance. The vitreous humor has been thus denominated on account of its resemblance to glass in a state of fusion. The retina is a membrane which lines the whole cavity of the eye, and is formed chiefly, if not entirely, by the expansion of the optic nerve.
13. The rays of light which proceed from objects pass through the cornea, aqueous humor, crystalline lens, and vitreous humor, and fall upon the retina in a focal point, to which it is brought, chiefly by the influence of the cornea and the crystalline lens. The image, in an inverted position, is painted or thrown on the cornea, which perceives its presence, and conveys an impression of it to the brain, by means of the optic nerve.
14. Optical instruments.—The art of constructing optical instruments is founded upon the anatomical structure, and physiological action of the eye, and on the laws of light. They are designed to increase the powers of the eye, or to remedy some defect in its structure. In the cursory view which we may give of a few of the many optical instruments which have been invented, we will begin with the spectacles, since they are the best known, and withal the most simple.
15. The visual point, or the distance at which small objects can be distinctly seen, varies in different individuals. As an average, it may be assumed at eight or nine inches from the eye. In some persons, it is much nearer, and in others, considerably more distant. The extreme, in the former case, constitutes myopy, or short-sightedness, and, in the latter case, presbyopy, or long-sightedness.
16. Myopy is chiefly caused by too great a convexity of the cornea and the crystalline lens, which causes the rays to converge to a focus, before they reach the retina. Objects are, therefore, indistinctly seen by myoptic persons, unless held very near the eye to throw the focus farther back. This defect may be palliated by the use of concave glasses, which render the rays proceeding from objects more divergent.