200 



DR. BREWSTER ON REFLECTED LIGHT? 



II. Those in which periodical colours are produced at the confines of parti- 

 cular kinds of glass, and various fluids and soft solids. -^ Ji^i^' 



From the first of these classes of facts the following conclusions may be drawn. 



1 . The reflective and refractive forces in media of the same refractive power 

 do not follow the same law. This result is clearly established by the experi- 

 ments with the prism B, which produced no orders of colours. Not only was 

 there a strong reflected pencil when a perfect equilibrium was effected between 

 the opposite refracting forces, but there was not even an approximation to 

 evanescence as the forces advanced to their point of compensation. The same 

 result was obtained with a prism newly ground and polished. 



2. The force which produces reflection varies according to a diffferent law in 

 diff"erent bodies. If the curve which represents the law of the reflective force 

 were exactly the same in the prism B and the fluids combined with it, then the 

 ordinates which represent the intensity of the force at any given point would 

 be exactly equal, and consequently there would be a perfect equilibrium 

 of opposite actions, and no reflection of the passing light. But as a copious re- 

 flection takes place even when the opposite forces are balanced, we are entitled 

 to infer that the law of the two forces is different. 



The reflective forces in the solid and fluid may be conceived to decrease in 

 various ways. ^'S- ^• 



1. They may extend to diff*erent distances from 

 the reflecting surface, and decrease according to 

 the same law. This relation is shown in Fig. 2, 

 where MN is the reflecting sui-face, AB the limit 

 of the sphere of reflecting activity in the solid, 

 and C D that in the fluid, — a o h the curve which 

 represents the reflecting force of the solid, and 

 en rf that of the fluid. In this case there can be 

 no compensation of opposite reflections, and an unbalanced reflecting force will 

 exist at almost every point of the sphere of reflecting activity. From a to c 

 the light will be acted upon by the undiminished force of the solid. At c the 

 force of the fluid begins to oppose that of the solid, and the unbalanced force 

 at any other line mo is equal to no, the difiference of the two forces mn,mo. 

 In this case there will be a sphere of reflecting activity extending from A B to 

 A' B', and such a combination must reflect light without refracting it. 



