i So SEVENTEENTH CENTURY. FT. in. 



fact about light which Huyghens explained; namely, the 

 double refraction of light through a crystal called Iceland spar. 

 A physician of Copenhagen named Erasmus Bartholinus had 

 received from Iceland a crystal in the form of a rhomboid 

 (see Fig. 34), which, when broken, fell into pieces of the 



same shape. Bartholinus called this 



FIG. 34. 



crystal ' Iceland spar/ and while mak- 

 ing experiments with it he observed 

 that an inkspot or any small object 

 A spot of ink seen through a seen through it appeared to be doubled. 



crystal of Iceland spar. ^ .> 



He was not able to explain this curious 

 fact, but he published an account of it in 1669, and Huy- 

 ghens accounted for it quite correctly by suggesting that the 

 crystal was more elastic in one direction than in the other, 

 so that a wave of light passing into it was divided into two 

 waves moving at different rates through the crystals. This 

 would cause them to be bent differently one according to 

 Snell's ordinary law of refraction (see p. 107), and the other 

 in an extraordinary way. Thus these two separate rays fall- 

 ing upon the eye would cause there the impression of two 

 objects. 



This curious effect is very interesting to study, and it led 

 Huyghens to make a number of remarkable experiments. 

 He found that the two rays when they passed out at the 

 other side of the crystal remained quite separate the one from 

 the other, and if they were afterwards sent through another 

 crystal in the same direction that they had gone through the 

 first, they went on each their own way. But now came a very 

 extraordinary fact : if the second crystal was turned round 

 a little so that the rays passed in rather a different direction 

 through it, each ray was again split up into two, so that there 

 were now four rays, sometimes all equally bright, sometimes 



