894 



A MANUAL OF PHYSIOLOGY 



from the mirror than the principal focus is real and inverted. The 

 Purkinje-Sanson image reflected from the concave anterior surface 

 of the vitreous humour (Fig. 387) is an example. 



After reflection from a convex mirror, rays of light always diverge, 

 and only erect, virtual images are formed i.e., images which do not 

 really exist in space, but which, from the direction of the rays of 

 light, we judge to exist. The position of the image of an object AB 

 (Fig. 372) may be found by a construction similar to that for reflec- 

 tion from a concave mirror. The image of a flame reflected from 

 the anterior surface of the cornea or lens is erect and virtual. It 

 diminishes in size with increase in the curvature or convexity of the 

 reflecting surface (Fig. 387). 



Refraction. A ray of light passing from one medium into another 

 has its velocity, and consequently its direction, altered. It is said to 

 be refracted. The first law of refraction is that the refracted ray is 



FIG. 373. REFRACTION AT A PLANE 

 SURFACE. 



AB is the incident ; BD, the refracted 

 ray ; CB, the normal to the surface. 

 When the ray passes from air into another 

 medium, the refractive index of the latter 



sin a 

 is the fraction 



FIG. 374- REFRACTION BY A MEDIUM 

 BOUNDED BY PARALLEL PLANES, 



P AND P'. 



The ray ABDE issues parallel to its 

 original di ection : CB, FD, normals 

 to P and P' ; a, angle of incidence ; 

 (3, y, angles of refraction. 



in the same plane as the incident ray and the normal to the surface. 

 The second law is that the sine of the angle of incidence has a constant 

 ratio (for any given pair of media) to the sine of the angle of refraction. 

 The angle of incidence is the angle which the ray makes with the 

 normal to the surface, separating the two media ; the angle of refrac- 

 tion is the angle made with the normal in the second medium. This 

 ratio is called the index of refraction between the two media. For 

 purposes of comparison, the refractive index of a substance is usually 

 taken as the ratio of the sine of the angle of incidence to the sine of 

 the angle of refraction of a ray passing from air into the substance. 



When a ray strikes a surface at right angles, it passes through 

 without suffering refraction. When a ray passes from a less dense 

 to a denser medium (e.g., from air to water), it is bent towards the 

 perpendicular. When it passes from a more dense to a less dense 

 medium (as from water to air), it is bent away from the perpen- 

 dicular. 



When a ray passes across a medium bounded by parallel planes, it 



