THE SENSES 



775 



object (Fig. 276). Then the image of A is the point in which all 

 rays proceeding from A and falling on the mirror, including rays 

 parallel to the principal axis, are focussed. But the ray AE, parallel 

 to the principal axis, passes after reflection through the principal 

 focus F, therefore the image of A must lie on the straight line EF. 

 If any secondary axis ACD be drawn, the image of A must lie on 

 ACD. It must therefore be the point of ' intersection, a, of EF and 

 ACD. Similarly, the image of B must be the point of intersection, 

 b, of GF and BCH. The image ab of an object AB farther 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. 292) is an example. 



After reflection from a corn-ex mirror^ rays of light always diverge, 

 and only erect, virtual 

 images are formed, *.*., 

 images which do not 

 really exist in space, but 

 which, from the direc- 

 tion of the rays of 

 light, we judge to exist. 

 The position of the 

 image of an object AB 

 (Fig. 277) may be 

 found by a construc- 

 tion similar to that for 

 reflection from a con- 

 cave mirror. The 

 image of a flame FlG ' 277 '- FoRMATI R * R lMAGE BY A CONVEX 

 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. 292). 



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 

 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 nf 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 cf 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 perpendicular. 



