3IO NINETEENTH CENTURY, PT. HI. 



thirty-seven years of age. He was a most accomplished 

 mathematician, and if he had lived longer would probably 

 have been one of the most celebrated men of our century. 



You will remember that in 1669 a Danish physician 

 named Bartholinus discovered that a ray of light is split 

 into two rays in passing through Iceland spar in any direc- 

 tion except along the axis of the crystal ; and that Huyghens 

 explained this by saying that the crystal was more elastic in 

 one direction than in another, so that the waves moved at 

 different rates through it (see p. 180). To understand 

 Malus's discovery you must also remember that one of 

 these divided rays, if it falls upon a second crystal in the 

 same manner as the first, goes on its way as a single ray, 

 but if the second crystal is turned round a little the ray splits 

 up again into two rays, one much baighter than the other. 



In the year 1808, M. Malus was standing at his study 

 window in the Rue d'Enfer, in Paris, looking through a 

 prism of Iceland spar at the sunlight reflected from the 

 windows of the Luxembourg Palace, which stood opposite. 

 All at once he observed to his surprise that he saw only one 

 image through the prism instead of two. Turning his prism 

 a little, he got the two images again, but one was much 

 brighter than the other, and when he turned the crystal a 

 little farther the other image disappeared, and he had only 

 one again. In fact, the light which was reflected from the 

 window at one particular angle (56° 45') behaved just like 

 one of the divided rays which has come out of a crystal, and 

 not like an ordinary ray which comes from the sun. 



This remarkable peculiarity puzzled Malus greatly, and 

 led him to make a great many experiments, by which he dis- 

 covered that, whenever light is reflected from glass at this 

 particular angle of 56° 45', it has the peculiar characters of a 



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