284 



PHYSIOLOGY 



CHAP. 



passing through them to converge at the focal point, or vice versa 

 cause the divergent rays Irom the focal point to become parallel. 

 The rays from an object on the optical axis beyond the focal 

 point converge on the other side of the lens in an image (Fig. 

 124). The farther or nearer the object from the focal point, the 

 nearer or farther will be the image on the other side of the lens. 

 If the light-rays proceed from any point on the optical axis 

 between the focal point and the optic centre of the lens, then 

 after passing through the lens they become less divergent, but are 



l-'i". 124. Convergent action of a bi-cnnvcx lens. ". origin of clivrr_rnt rays, nr, nd, which hr\ ond 

 thi- lens become convn . :<n<l rms- at h, which is tin- him;," 1 of point a. 



still unable to form an image. Finally, if a ray of light passes 

 through a secondary axis, oblique to the lens, the same laws 

 obtain, so long as it only forms a moderate angle with the optical 

 axis. 



Concave lenses have a )ie;/nfiue focal distance (Fig. 125). They 

 make parallel rays divergent, divergent rays still more divergent, 

 and convergent rays less convergent or divergent. 



Lenses with convex-concave or concave-convex surfaces are 



Fi<;. 125. Divergent action of a bi-concave lens, a, origin of divergent rays, ac, ad, which beyond 

 the lens become more divergent, cf, de, as if they came from point b, not from point a. 



convergent or divergent according as the convex or concave surface 

 has the shorter radius of curvature. 



III. The dioptric apparatus of the eye consists in a highly 

 complex system of refractive media, separated by spherical 

 centred surfaces. From the front backward these are the cornea, 

 moistened by a very thin layer of lachrymal secretion, the aqueous 

 humour, the many variously refracting layers of the lens, and the 

 vitreous body (humour). 



To determine the path of the light-rays which traverse the 

 eye to the retinal surface it is necessary to ascertain (a) the 

 refractive indices of the different transparent media ; (b) the radii 



