DISCOVERY BY EXAMINING PECULIAR TRUTHS. 491 



those rays which pass by the two edges and meet in the 

 middle of the shadow, having to travel the same distance, 

 the crests of their waves coincide with each other, and 

 therefore strengthen the light ; whilst, of the rays which 

 meet at a little distance from the middle of the shadow, 

 those which have passed by the one edge .have travelled a 

 different distance from those which have passed by the 

 others, and thus the crests of their waves do not coincide, 

 and therefore interfere with and weaken each other. As 

 also white light is composed of rays of all colours, and 

 the waves of rays of different colours have different 

 lengths, bands of different colour are also produced. It 

 was by investigating the peculiar and unexplained (or 

 wrongly explained) phenomenon of a bluish colour pro- 

 duced by white light in an aqueous solution of sulphate of 

 quinine, and of blood-red colour in a green solution of 

 chlorophyl, that Professor Stokes, in the years 186162, 

 made the important discovery of the change of refrangi- 

 bility of light, and was subsequently led to discover the 

 great length of the invisible portion of the spectrum of 

 the electric light. 



It was for a long time an unexplained fact that the velo- 

 city of sound, as deduced from theory and that found by 

 experiment, differed about one-sixth ; but Laplace, by study- 

 ing this circumstance, and allowing for the residual effect 

 of heat produced by the transmission of sound, was enabled 

 to reconcile the difference and discover its true cause. 



It was investigation of the peculiar circumstance that 

 a piece of amber which had been rubbed with silk pos- 

 sessed the power of attracting a feather, which led to 

 the discovery of the source of static electricity ; and that 

 of the attraction of warm ashes by a heated tourma- 

 line was in a similar manner the origin of the discovery 

 of pyro- or crystal-electricity. By investigating the pecu- 



