Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/853

Rh EYE 817 FIG. 1. Refraction of Light. produced in photography. In general terms, therefore, the lower end of the spectrum may be called thermal, the middle luminous, and the upper actinic or chemical; but the three merge into and overlap one another. It may be observed that the number of vibrations in the extreme violet is not double that of the low red, so that the sensibility of the eye to vibrations of light does not range through an octave. The ultra-violet rays may act on the retina in certain condi tions, as when they are reflected by a solution of sulphate of quinine, constituting the phenomenon of fluorescence. 2. OPTICAL ARRANGEMENTS OF THE EYE. (1.) General. &quot;When light traverses any homogeneous transparent medium, such as the air, it passes on in a straight course with a certain velocity ; but if it meet with any other transparent body of a different density, part of it is reflected or returned to the first medium, whilst the remainder is propagated through the second medium in a different direction and with a different velocity. Thus we may account for the phenomena of reflection and of refraction, for which see the article LIGHT. Let a b, in fig. 1, be a plane surface of some transparent sub stance, say a sheet of glass ; a ray, c d, per pendicular to the sur face, will pass through without refraction ; but an oblique ray, e f, will be sent in the direction e h. If the ray e h had passed from a dense into a rarer medium, then the direction would have been e g. It might also be shown that the sine of the angle of incidence always bears a certain ratio to the sine of the angle of refraction ; this ratio is termed the index of refraction. Thus, if a ray pass from air into water, the sine of the angle of incidence will have to the sine of the angle of the refraction the ratio of 4 : 3, or |. Before a ray of light can reach the retina, it must pass through a number of transparent and refractive surfaces. The eye is a nearly spherical organ, formed of transparent parts situated behind each other, and surrounded by various membranous structures, the anterior part of which is also transparent. The transparent parts are (1) the cornea ; (2) the aqueous humour, found in the anterior chamber of the eye ; (3) the crystalline lens, formed by a transparent convex body, the anterior surface of which is less convex than the posterior; and (4) the vitreous humour, filling the posterior chamber of the eye, The ray must therefore traverse the cornea, aqueous humour, lens, and vitreous humour. As the two surfaces of the cornea are parallel, the rays practically suffer no deviation in passing through that structure, but they are bent or re fracted during their transmission through the other media. From the optical point of view, the eye may be regarded as a dioptric system consisting of various refractive media. In such a system, as shown by Gauss, there are six cardinal points, which have a certain relation to each other. These are Two focal points: every ray passing through the first focal point becomes, after its refraction, parallel to the axis, and every ray which before refraction is parallel to the axis passes after its refraction to the second focal point ; (2) Two principal points: every ray which passes through the first point before refraction passes after refrac tion through the second, and every ray which passes through any point of a plane elevated on a perpendicular axis from the first principal point (the first principal plane) passes through the corre sponding point of an analogous plane raised upon the axis at the second principal point (the second principal plane) and (3) Two nodal points, which correspond to the optical centres of the two principal planes just alluded to. The distance of the first principal point from the first focal point is called the anterior focal length, and the term posterior focal length is applied to the distance of the posterior focal point from the second principal point. Listing lias given the following measurements in millimetres from the centre of the cornea for the cardinal points in an ideal eye : Anterior focal point 12-8326. Posterior focal point 22-6470. First principal point 2 1746. Second principal point... 2 5724. A view of such an ideal eye is shown in fig. 2. First nodal point 7-2420. Second nodal point 7-6398. Anterior focal length 15-0072. Posterior focal length.... 20-0746. FIG. 2. Transverse section of an Ideal or Schematique Eye. A, Summit of cornea; SC, Sclerotic; S, Schlemm s canal; CH, Choroid; I, Iris; M, Ciliary muscle; K, Retina; N, Optic nerve; HA, Aqueous humour; L, Crystalline lens, the anterior of the double lines on its face showing its form during accommodation; HV, Vitreous humour; I)X, Internal rectus muscle; DE, External rectus; YY, Principal optical axis; &amp;lt;1&amp;gt;&amp;lt;I&amp;gt;, Visual axis, making an angle of 5 with the optical axis; C, Centre of the ocular globe. The cardinal points of Listing- H,H 2 . principal points; K^, nodal points; F^, principal focal points. The dioptric constants according t&amp;lt;&amp;gt; Giraud-Tett!on: H, Principal points united; &amp;lt;/&amp;gt;i&amp;lt;/) 2. principal foci during the repose of accommodation; (f&amp;gt; ]&amp;lt;p * principal foci during the maximum of accommodation; 0, fused nodal points. The remaining measurements of such an eye are as follows : Radii of curvature. Of anterior face of cornea = 8 millimetres. Of anterior face of lens =10 ,, Of posterior face of lens =6 ,, Indices of Refraction. Aqueous humour W 3 = 1 3379 Crystalline lens if = 1 4545 Vitreous humour W = 1 3379 The optical constants of the human eye may be still further simplified by assuming that the two principal points and the two nodal points respectively are identical. Thus we may construct a reduced eye, in which the princi pal point is 2 3448 mm. behind the cornea, and the nodal point is 7 49C9 mm., having an anterior focal length of 15 mm. and a posterior focal length of 20 mm. The refracting surface, or lens, has a radius of 5 mm., and is 3 mm. behind the cornea; and the index of refraction is that of the aqueous humour, or yy 3. (2.) The Formation of an Image on the Retina. This may be well illustrated with the aid of an ordinary photo graphic camera. If properly focussed, an inverted image will be seen on the glass plate at the back of the camera. It may also be observed by bringing the eye-ball of a rabbit FIG. 3. Inversion by action of a Lens. near a candle flame. The action of a lens in forming an inverted image is illustrated by fig. 3, where the pencil of rays proceeding from a is brought to a focus at a, and those from I at b, consequently the image of a b is inverted as at b a. The three characteristic features of the retinal image YIII. --103