i8o SEVENTEENTH CENTURY. FT. IIL 



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 the ordinary 

 law of refraction (see p. 105), and the other in an extra- 

 ordinary way. Thus .these two separate rays entering 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 of unequal brightness, but the 

 light of all four was never greater than the light of the one 

 ray, out of which they had all come. These four rays con- 

 tinued apart while he turned the second crystal more and 

 more round; till, when he had turned it 90, or a quarter 

 of a circle, the rays became two again, with this remarkable 

 peculiarity, that they had changed characters ! The ray 

 which before had been refracted in the ordinary way now 

 took the extraordinary direction, while the other chose the 

 ordinary one. 



This curious effect observed by Huyghens is now known 

 as the ' polarisation of light ' by crystals. There is a beauti- 

 ful explanation of it, but we must wait for that till we consider 

 the science of the nineteenth century, for it is now much 

 better understood. Huyghens' ' Theory of Light ' was pub- 



