ARTICLES 

 OPTICAL ACTIVITY 



By F. D. CHATTAWAY, F.R.S., D.Sc. 



Towards the close of the seventeenth century Erasmus Bar- 

 tholin, 1 a Danish physician and philosopher, noted that a ray 

 of light, in passing through a crystal, which had been sent to 

 him from Iceland, was divided into two equal parts. Some 

 few years later his observations were confirmed and extended 

 by Christian Huygens, who found that the two new rays acquired 

 peculiar properties which became evident when they were 

 passed through a second crystal. 



Little further progress was made for over a century until 

 fitienne Louis Malus, a French Colonel of Engineers, discovered 

 in 1808 that light reflected at a certain angle from the surface 

 of any transparent body acquired properties corresponding to 

 those of the rays transmitted through Iceland spar. He was 

 led to the discovery by casually examining, through a doubly 

 refracting crystal of calcite, the sunlight reflected from the 

 windows of the Luxembourg Palace, when he was surprised 

 to find that the two rays alternately disappeared as the crystal 

 was rotated through successive right angles. 



To the alteration of the properties of light thus produced, 

 Malus gave the name "polarisation," as on the emission theory 

 which he held at the time he attributed it to a kind of polarity 

 of the light corpuscles, and the term has been retained. The 

 reflection plane itself — that is, the plane passing through the 

 incident ray and the normal to the reflecting surface — was 

 designated the "plane of polarisation." 



1 " Erasmus Bartholin : An accompt of Sundry Experiments made and com- 

 municated by that Learned Mathematician, Dr. Erasmus Bartholin, upon a 

 Chrystal-like Body, sent to him out of Island," Phil. Trans. January 16, 1670/71, 

 No. 67, p. 2039. 



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