VEKDET’S CONSTANT IN ABSOLUTE UNITS. 
33 
between two points whose magnetic potentials differ by unity, 
MR.l 2(A) 
tan 8 H ' ' 
rS { 4,rra (lA . LB + MA . Mb) } 
_ 3-14159 
where Jc is the number of units of circular measure m a minute of arc = 180x 60 
(•00029088) (77453) 
2878499< 4tt(1028-15) - 77453 
(18-95) (45-29) (50'06)(23 
1(23-72))} 
2R 
’ tan 8 . H 
(35) 
^(log- 1 10-7866630)(49808-0) 
=log" 1 5-4839621 
= 3-04763(10- 5 ). 
As the constant is the ratio of a number to a current, its dimensions are 
This is Verdet’s constant in absolute measure. For light of a given wave-length 
passing through a given substance, it is a fixed and definite physical quantity, depending 
only on the units of length, mass, and time*. 
It is the number in optical measure which is equivalent to unity in electro-magnetic 
measure. In future investigations quantities expressed in electro-magnetic measure can 
be expressed in optical measure by multiplying them by this number. 
u, then, is defined to be Verdet’s constant for the thallium ray in bisulphide of 
carbon, expressed in C.G.S. measure. 
I here insert, as I have been requested to do, the result of my former paper on the 
same subject. It is that for distilled water with white light, 
*=4-496(10- 6 )f. 
I do not, however, attach much value to the result, as the different determinations 
are in the ratios of 
7-563, 7-406, 8-295, 8-401, 6-916, 
showing variations from the mean of ±7 per cent., or giving only about yg- of the 
accuracy of the present paper. 
The methods used for determining the constants were also susceptible of less accuracy. 
Before concluding this paper, I must express my thanks, first and chiefly, to Professor 
Maxwell, who has superintended every detail of the work for the year and eight months 
* The magnetic rotative power of bisulphide of carbon here comes in the same way as the specific heat of 
water comes into Joule’s equivalent. 
f By an error in arithmetic this was printed 10~7 in the abstract of my paper published Proc. Boy. Soc. 
June 1875. 
MDCCCLXXVII. 
F 
