PROCEEDINGS OF THE AMERICAN ACADEMY 



reflected, as we merely make n' ■=-, and i becomes r, and r, ^, tlie 

 values of A and B being unchanofed. 



When i = 0°, or is infinitesimal, A = ^!" ," z~- ■■ — *- 



' sin- (n + l)r 



and ^ = (- t\ ; hence the reflected ray R = (- -j and is 



unpolarized. Table I. gives the values of H for various values of n. 



TABLE I. 

 Light reflected when i = 0°, or incident light normal to surface. 



The light therefore increases with n, being zero when n=l, and 100 

 per cent, or all reflected when r= go. 



This law also holds for other angles of incidence, and serves to 

 explain many fiimiliar phenomena. The brilliancy of the diamond is 

 mainly due to its high index, causing it to reflect about 20 per cent of 

 the light foiling normally on it, while glass returns but 4 per cent. 



The white color of fine powders is generally due to the light reflected 

 from the faces of the minute crystals of which they are composed. 

 Hence, immersing them in a liquid diminishes their brightness, since 

 the relative index being reduced, less light is reflected. This is seen 

 when snow is immersed in water, or when we write with wet chalk on 



a blackboard. In the last case, the relative index = ^-j '- — = -^ 



nidex water 1.33 



= 1.167, and 0.6 per cent only is reflected ; while, when dry, the index 



is 1.5, and 4 per cent, or six times as much, is sent back. For the same 



reason, white lead is injured as an oil paint wlien adulterated with 



sulphate of baryta, which, although as white, has an index of refraction 



of 1.68 instead of 1.9. The effect is to diminish the covering power, 



the light being transmitted instead of reflected, so that when applied as 



a paint more coats are needed. In the four cases, we have white lead, 



