1876.] 



Determination of Verdefs Constant. 



147 



The Light. 



Monochromatic light was used, obtained by throwing a spectrum on a 

 card, in which was a slit to admit the colour required. A method of 

 localizing the light used is given. 



The £ollo\Yiug results for — , where y = tan 3, are given, and the wave- 

 length of the light was that of the green thallium line : — 



Grove's 2R_ Const . 

 Set. 2E,. cells. y ~ B. ' 



2.... 11° 58' 30" 5 8861-1 min. 

 3. . . . 13° 39' 30" 6 8820-6 min. 

 1.. .. 15° 26' 0" 7 8917-5 min. 

 where 2R is the difference of the circle readings corresponding to the 

 two directions of the current. 



Value of H. 



This was determined by vibrating the same magnet at Kew and at the 

 author's laboratory at Pixholme, Dorking, where all the optical part of 

 the work was done, and then deducing the force at Pixholme from the 

 Kew magnetograph curves at the times of experiment. The magnet used 

 was very kindly lent to the author by Mr. Whipple. 



The result obtained was 



H at Pixholme =(-993366) H at Kew. 



Value of the Constant. 

 The values of H at Pixholme at the times of the optical experiments 

 having been calculated, we have for the three values of the quantity 

 which should be constant : — 



211 



1 . . . . 50118-4- 



1 3 : : : : r 49808-o. 



mean 



Extreme difference 0-6 per cent. 



We have finally, if w be the rotation in bisulphide of carbon of the 

 plane of polarization of the ray whose wave-length is 



X=(5-349)10-5 



between two points whose magneti<3 potentials differ by unity, 

 w=3-04763(10-0. 



The dimensions of the constant are 



[w]=[M-*L-^T]. 



The paper concludes with a few very inadequate words of thanks to 

 Prof. Maxwell for his great kindness in superintending the whole of the 

 work for the year and eight months during which it has been in progress. 



The author also records his thanks to Mr. Whipple and several other 

 friends for assistance and suggestions. 



An Appendix contains an analysis of the bisulphide used. 



