258 DISEASES OF THE IIOKSE. 



found smaller also after it has crossed the other nerve, and to 

 the point where it enters the sclerotic coat of the eye. ^ The 

 thalami nervorum opticorum I consider as the fountain of 

 vision, for it is here tlie impressions of objects are ultimately 

 received. 



On Vision. 



[Before we enter on the diseases of the eye, it would be well 

 to notice briefly the phenomena of vision. 



A ray of light, when it impinges on a body, is subject either 

 to reflection, absorption, or transmission. If reflected, it recoils 

 from the surface of the object Avhich it cannot permeate, in the 

 same manner as a ball would rebound if thrown against a wall. 

 Each ray of light is composed of seven colours, which may, 

 indeed, be separated by a triangular piece of glass, called a 

 prism, and different objects have the power of absorbing some 

 of these colours and reflecting others. The colours absorbed 

 are, of course, rendei'ed invisible, and those reflected are seen ; 

 and thus it is according to the colour or colours reflected that 

 an object appears green, or blue, or yellow. When it appears 

 Avhite all the colours are reflected, and when black they are all 

 absorbed. 



It is a law of optics that the angle of reflection and that of 

 incidence are equaL The former signifies the direction in 

 which the object or ray strikes a mirror, and the latter the angle 

 at which it may be seen. They may be represented by two per- 

 sons standing at an equal inclination from a mirror in which 

 they may be able to see each other's image, though not suf- 

 ficiently opposite as to discern their own. It is found that the 

 deo-ree of reflection depends on the degree of obliquity with 

 which the ray impinges on the surface of the reflecting object, 

 and thus the reflected light of the sun and moon is greatest 

 when rising or setting. 



All bodies have the power of absorbing rays of light, but in 

 different degrees. Opaque bodies absorb more light than trans- 

 parent ones ; and of these, black substances absorb most, and 

 wliite the least. 



When a ray of light enters a transparent medium of greater 

 density than what it has just left, it becomes refracted or turned 

 from the perpendicular ; and when the density is less than before, 

 it appears refracted towards the perpendicular. Thus, when a 

 stick is plunged into water, it appears broken at the surface of 

 the liquid, which is owing to the rays of light being broken as 

 they pass from the object into the air. So, likewise, when a 

 ray of light impinges on a curved surface it is refracted ; and 

 thus a dense transparent body, of a convex figure, will collect 

 the rays into one common point or focus, whicli may be shown 



