i8o POPULAR LECTURES AND ADDRESSES. 



see the zinc line very clear across it near its 

 middle. Here then we have gold and silver trans- 

 parent. The silver is opaque for all except 

 that very definite light of wave-lengths from 

 about 3*07 to 3-32 io~ 5 cms. 



The different refrangibility of different colours 

 is a result of observation of vital importance in 

 the question of the size of atoms. You now see 

 on the screen before you a prismatic spectrum, 

 a well-known phenomenon produced by the 

 differences of the refractions of the different 

 colours in traversing the prism. The explana- 

 tion of it in the undulatory theory of light has 

 taxed the powers of mathematicians to the utmost. 

 Look first, however, to what is easy and is made 

 clear by that diagram (Fig. 35) before you, and 

 you will easily understand that refraction depends 

 on difference of velocity of propagation of light 

 in the two transparent mediums concerned. The 

 angles in the diagram are approximately correct 

 for refraction at an interface between air or 

 vacuum and flint glass ; and you see that in this 

 case the velocity of propagation is less in the 



