278 



PHYSIOLOGY 



CHAP. 



tissue adherent to the sclerotic (which in albinos is fairly trans- 

 parent), examined the posterior surface by turning the cornea 

 towards a source of light, as a window, or in a dark room, towards 

 a candle flame. But the retinal image is much plainer in the eye 

 of large mammals like the ox, on cutting out posteriorly a piece 

 of sclerotic and choroid of about the same size as the cornea. In 

 the image thus formed on the retina external objects are greatly 

 reduced, but all the outlines are clear and the colours are faithfully 

 represented. The image is inverted, and every movement of the 

 object is reproduced on the retinal image in the reverse direction. 

 The size of the image decreases proportionately to the distance of 

 the object. 



To form a sharp image of the object, either on the sensitive 



plate of the photographic camera, 

 or on the retinal surface of the 

 eye, the light-rays starting from 

 any point of the object must in 

 passing through the correspond- 

 ing dioptric system unite at 

 given points of the receptor sur- 

 face so as to form an inverted 

 real image there. To under- 

 stand this clearly, we must con- 

 sider certain laws of optics : 



(a) Index of Refraction. 

 When a ray of light passes from 

 one medium to another separated 

 by a plane surface, it is re- 

 fracted when it falls obliquely 

 on the plane of separation of the 

 two media. Let MM' be two 

 media (Fig. 117), AB the sur- 

 face of separation, CD a ver- 

 Then, as the diagram shows, 



CD goes from M to M' without deflection, while ED takes 

 the direction DE'. The incident ray and the refracted ray 

 are in the same plane. The angle i is called the angle of 

 incidence, the angle r the angle of refraction. The relation 

 between the sine of the angle of incidence and the sine of the 

 angle of refraction or between ab and cd is called the re- 

 fractive index. This relation is a constant, indicated by n, for 

 two given media. In measuring n it is always assumed that the 

 ray of light is passing from the air to a denser medium. On 

 passing, for instance, from air to water the ray is so deflected that 

 db :cd is as 4 : 3. The refractive index of water is thus |-, more 

 exactly 1-336. That of glass is f = !. The sines of the angle 

 of incidence and of refraction vary with the velocity of the pro- 



Fio. 117. Refraction of a ray that 



through two media separated by a plane 

 surface. 



tical, and ED an oblique ray. 



