An American text-book of physiology . d how this is effected it will be necessary to consider the refractingmedia of the eye by means of which an optical image of the luminous objectis thrown upon the expanded termination of the optic nerve—viz. the retina. Dioptric Apparatus of the Eye.—For the better comprehension of thisportion of the subject a few definitions in elementary optics may be given. A The axes of rotation of the internal and external recti, however, deviate but sli<,htly fromthe equatorial plane. - P. Tergast: Ueber das Verhiiltniss von Nerven und Muskehi, Archie fur tuikr. A
An American text-book of physiology . d how this is effected it will be necessary to consider the refractingmedia of the eye by means of which an optical image of the luminous objectis thrown upon the expanded termination of the optic nerve—viz. the retina. Dioptric Apparatus of the Eye.—For the better comprehension of thisportion of the subject a few definitions in elementary optics may be given. A The axes of rotation of the internal and external recti, however, deviate but sli<,htly fromthe equatorial plane. - P. Tergast: Ueber das Verhiiltniss von Nerven und Muskehi, Archie fur tuikr. 36-46. THE SENSE OF VISION. •-47 dioptric system in its simplest form consists of two adjacent media which havedifferent indices of refraction and whose surface of separation is the segmentof a sphere. A line joining the middle of the segment with the centre of thesphere and prolonged in either direction is called the axis of the svstem. Letthe line .1 P B in Figure 213 represent in section such a spherical surface the. M^ B Fig. 213.—Diagram of simple optical system (after Foster). centre of which is at iV, the rarer medium being to the left and the denser me-dium to the right of the line. Any ray of light which, in passing from therarer to the denser medium, is normal to the spherical surface will be unchangedin its direction—i. e. will undergo no refraction. Such rays are represented bvthe lines 0 F, 31D, and 31 E. If a pencil of rays having its origin in the rarermedium at any point in the axis falls upon the spherical surface, there will beone ray—viz. the one. which coincides with the axis of the system, which willpass into the second medium unchanged in its direction. This ray is calledthe principal ray {OP), and its point of intersection (P) with the sphericalsurface is called the principal point. The centre of the sphere (jV) throughwhich the principal ray necessarily passes is called the nodal jjoint. All theother rays in the pencil are refracted
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