490 SPECIAL METHODS OF DISCOVERY. 



refracted by the second, so as to produce four rays.' l It 

 was by investigating the phenomenon of double refraction 

 of light that Mains also, about the year 1800, found the 

 same property in a large additional number of bodies, 

 including arragonite, the sulphates of lime, lead and 

 barium, carbonate of lead, corundum, zircon, felspar, 

 euclase, emerald, cymophane, mellite, peridote, and meso- 

 type. 



By investigating the peculiar phenomenon known as 

 Grimaldi's frirfges, Young, in the year 1801, discovered 

 interference of light. He held a vertical strip of card 

 in a cone of brilliant white sun-light proceeding from a 

 minute pin-hole in the shutter of a darkened room, and 

 observed that, instead of casting a simple shadow of itself 

 upon the wall behind, a series of vertical dark and bright 

 bands appeared in the space occupied by the shadow, with 

 a faint white band in the middle. But what was very 

 remarkable was, that by entirely intercepting that portion 

 of light which was passing by one edge of the card, and 

 allowing that on the other side to pass as before, the bands 

 disappeared ; and he therefore concluded that the portion 

 of light which had passed by one edge of the card had, in 

 some way, so acted upon that which had passed by the 

 other as to produce the bands. He studied those effects, 

 made further experiments, and showed that, as the dif- 

 ferent rays of light which pass round either edge of the 

 card are bent unequally by diffraction, they have to travel 

 unequal distances before they fall upon the different parts 

 of the shadow ; and he inferred and discovered that, in 

 consequence of this, the undulations or waves of some of 

 the rays which pass by one edge of the screen strengthen 

 some, and weaken others, which pass by the other. Thus 



1 Whewell, History of the Inductive Sciences, vol. ii. 3rd edit. p. 297. 



