Intelligence and Miscellaneous Articles. 239 



magnetic rotation of the plane of polarization, and the rotation due 

 to the special action of the substance. 



In my experiments I have used the general method of M. Fizeau 

 and M. Foucault, which consists in intercepting on a prism the 

 light originally polarized and transmitted by the transparent body, 

 and studying the state of polarization of the various parts of the 

 spectrum. To the rays whose plane of polarization is parallel to 

 the principal section of the analysing Nicol, a black band corre- 

 sponds, the middle of which is successively brought to coincide with 

 rays whose wave-lengths are known by Fraunhofer's experiments ; 

 the displacement which the analyser must undergo in order to re- 

 establish coincidence with a given ray, when the direction of the 

 current is changed, is exactly double the rotation due to the action 

 of magnetic forces. 



The following Table contains the relative values of the rotations 

 corresponding to the five rays C, D, E, F, G*, for substances which 

 I have studied, the rotation corresponding to the ray E being taken 

 as unity : — 



C. D. E. F. G. 



Distilled water 0'63 079 l'OO 1-20 1*55 



Solution of chloride of calcium . . 061 0*80 l'OO 1*19 1'54 



chloride of zinc 0*61 078 l'OO 1-19 161 



protochloride of tin . . .. 078 l'OO 1*20 1*59 



Oil of bitter almonds 0'61 078 l'OO 1-21 



Oil of anise 0'58 075 l'OO 1*25 



Sulphide of carbon 0'60 077 l'OO 1'22 1*65 



Commercial creosote 0'60 076 l'OO 1*23 1-69 



Essence of Laurus cassia 0'59 074 1*00 1*23 



The exact law of the inverse ratio of the square of the wave- 

 lengths would have required the series of rotations 



C. D. E. F. G. 



0-64 0'80 l'OO 1-18 1-50, 



which does not much differ from any of the series in the preceding 

 Table. Considering the nature of the liquids which deviate most 

 from the law (sulphide of carbon, essences, creosote), the results of 

 my experiments may be summed up in the following three propo- 

 sitions : — 



1. The magnetic rotations of the plane of polarization of rays of 

 different colours follow approximately the law of the inverse ratio of 

 the square of the wave-lengths. 



2. The exact law of the phenomena, is that the product of the 

 rotation by the square of the wave-length increases from the least to 

 the most refrangible end of the spectrum* 



3. The substances for which this increase is most perceptible are 

 also those which have the greatest dispersive power. 



* Any observation of the rays B and H is impossible ; and I have only 

 obtained even slightly satisfactory results for the rays C and G by placing 

 in front of the eye coloured glasses which extinguished the middle and 

 brilliant region of the spectrum without sensibly weakening the lustre of 

 the adjacent portion of these rays. 



