1889.] 



of the Plane of Polarisation of Light 



69 



to see that the value of O/O is greater or less than unity, according 

 as a is greater or less than unity. For if /3 is small, 



Q = ft(l + a 2 ) 



/3(sec/3 + a 2 cos/3) 



1 + 



a fraction which is greater or less than 1 according as 1 — a 2 is 

 negative or positive. The curve tracings also show that the apparent 

 rotation changes sign whenever (3 is any multiple of a right angle. 



[If ft is greater than 90°, equal say to n 90° + 7, where 7 is less than 

 a right angle, then if n is even, 



"_/(«Vy). 

 o-~-ft ' 



and if n is odd, 



Q _ /(V, 90°- 7 ) 



quantities which become very small when (3 is large. — May 14.] 



In the case of quartz one centimetre thick, where the direction of the 

 ray of light makes an angle of 90° with the optic axis, /3 = 627 tt/2 

 very nearly. Hence Q/Q in this case is about g-^y at best, which 

 gives a value of O quite inappreciable by any known methods. To 

 find the true magnetic rotation in quartz we must use thin sections 

 about O'Ol mm., and ci 2 should be equal to 1. 



