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XXII. Is Rotatory Polanzation influenced by the EartTCs 

 Motion ? By Lord Rayleigh, F.R.S* 



THE question whether the rotation o£ the plane of polari- 

 zation of light propagated along the axis of a quartz 

 crystal is affected by the direction of this axis relatively to 

 that of the earth's orbital motion, is of considerable theoretical 

 importance. According to an investigation of Lorentz, an 

 effect of the first order might be looked for. Such an effect 

 would be rendered apparent by comparing the rotations when 

 the direction of propagation of the light is parallel to that 

 of the earth's motion and in the reverse direction, and it 

 might amount to _ n \ n ■ of the whole rotation f . According- 



& 10.000 ' & 



to Larmor's theory J there should be no effect of the first 

 order. 



The question was examined experimentally many years ago 

 by Mascart § , who came to the conclusion that the reversal of 

 the ray left the rotation unchanged to x Q * part. In most 

 of the experiments, however, the accuracy was insufficient to 

 lend support to the above conclusion. 



Dr. Larinor (I. c. p. 220) having expressed the opinion that 

 it might be desirable to re-examine the question, I have made 

 some observations which carry the test as far as can readily 

 be done. It appears that the rotation is certainly not altered 

 by ] part, and probably not by the half of this, when 



the direction of propagation of the light is altered from that 

 of the earth's motion to the opposite direction. 



I should scarcely have been able to carry the test to so 

 satisfactory a point, had it not been for the kindness of Prof. 

 MacGregor, who allowed me the use of certain valuable quartz 

 crystals belonging to the Edinburgh collection of apparatus. 

 These crystals, five in number, are all right-handed, and 

 measure about 50 mm. each in the direction of the optical 

 axis, to which the polished faces are approximately perpen- 

 dicular. They were prepared for Prof. Tait, and were em- 

 ployed by him for his " rotatory polarization spectroscope of 

 great dispersion " |J. For the most part they are nearly free 

 from blemish, and well adapted to the purpose in view. 



In principle the experiment is very simple, scarcely differing 



* Communicated by the Author. 



t This fraction representing approximately the ratio of the velocity of 

 the earth in its orbit to the velocity of light. 

 T 'JEtheY and Matter,' Cambridge, 1900. 

 § Annates de VEcole Normate, vol. i. p. 157 (1872). 

 || Nature, vol. xxii. 1880 ; Tait's < Scientific Papers/ vol. i. p. 423. 



