CH. xxxili. MALUS ON POLARIZATION. 3*3 



Bartholinus discovered that a ray of light is split into two 

 rays in passing through Iceland spar in any direction except 

 along the axis of the crystal ; and that Huyghens explained 

 this by saying that the crystal was more elastic in one direc- 

 tion than in another, so that the waves moved at different 

 rates through it (see p. 179). To understand Malus's dis- 

 covery 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. 



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 

 discovered that, whenever light is reflected from glass at this 

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

 a divided ray which has passed through Iceland spar. Light 

 reflected from other substances is also divided up in this 

 vay, only the angle at which this change takes place is dif- 



1 The polarizing angle of glass varies between 45 35' and 58, accord- 

 ing to the nature of the glass. 

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