OPTICS

(Yunanca köken — orijinale bakınız.) That branch of physical science which treats of light and vision. The theory of light, and the different hypotheses respecting its propagation, having been explained under the term Light, and its most remarkable properties being described under their respective heads (Chromatics, Diffraction, Interference, Polarization, Reflexion, Refraction, .), we shall here confine ourselves to the explanation of the phenomena and laws of vision, and the formation of images; and this, in fact, comprehends all that is meant by Optics, in the strict sense of the word. Description of the Eye. — The human eye is of a spherical form, having a slight projection in front. The annexed figure represents a horizontal section of it through its axis. It consists of three principal chambers, filled with different humours, or transparent media of different refractive powers. The first of these media, occupying the anterior chamber A, is called the aqueous humour, and consists almost entirely of pure water. The cell in which the aqueous humour is contained is bounded on its anterior side by a strong horny transparent substance, called the cornea, the figure of which is an ellipsis of revolution about its major axis. The posterior side of the cell is formed by the iris, a kind of circular opaque screen, consisting of muscular fibres, by the contraction or expansion of which an ai)erture in its centre, called the pupil, is increased or diminished according to the illumination, in order to protect the eye and preserve its sensibility by equalizing the quantity of light admitted into it. The second humour, called the crystalline lens, B, enclosed in its capsule, lies immediately behind the pupil. Its figure is a solid of revolution, having its anterior surface much le curved than the posterior; and both surfaces are ellipsoids of revolution about their le er axes. The crystalline is somewhat denser towards the centre than at the outside; the increase of density serving to correct the aberration, by shortening the focus of the rays near the centre. The third or vitreous humour, C, fills the posterior chamber of the eye. This fluid scarcely differs from the aqueous humour, either in specific gravity, or chemical composition, or refractive power. The following are the refractive powers of the different humours of the eye, according to Sir David Brewster, the ray of light being incident on them from air: — Aqueous CrysUlIine Lens. Vitreous Humour. Surface. Centre. Mean. Humour. 1-336 1-3767 1-3990 1-3879 1-3394 But as the rays refracted by the aqueous humour pa into the crystalline, and from the crystalline into the vitreous humour, the indices of refraction of the separating surfaces of these humours will be — From aqueous humour to outer coat of crystalline 1-0466 From ditto to crystalline, using the mean index - 1-0353 [s. 860]