ROTATION OF LIGHT IN BISULPHIDE OF CARBON. 
365 
independence of other circumstances. M. Becquerel speaks of this method as new, 
but it is in reality that employed by Gordon in 1877.’“ Most of the complication in 
Gordon’s memoir relates to the determination of the current, and especially to the 
circumstance that the number of turns in the spiral was not ascertained (as it should 
have been) during construction, but subsequently by electrical processes. When the 
number of turns and the current are known, there is no difference between the pro¬ 
cedure of Gordon and Becquerel and that of the present memoir. 
There is a pretty close resemblance between M. Becquerel’s recent work and mine. 
In both a soda flame is used as the source of light, and in both the number of windings 
on the helices is ascertained during construction. In the current determinations, 
M. Becquerel used a galvanometer as an intermediate standard, while I employed 
for the same purpose a Clark’s cell, the ultimate standard being a silver voltameter 
(and in my case a current-weighing apparatus). Inasmuch as M. Becquerel uses 
the same number as that which I obtained for the electro-chemical equivalent of silver, 
there should be no difference between us in the estimation of currents. 
In M. Becquerel’s experiments the temperature of the CS 3 was usually about 0° C., 
and he reduces his results to that standard temperature. He regards Bichat’s formula 
as confirmed by his observations. According to this my result for 18° would become 
•04302'; 
whereas M. Becquerel obtains 
•04341', 
nearly 1 per cent, higher. I am at a loss to understand the cause of this discrepancy. 
M. Becquerel estimates that his result should be correct to about the same 
degree of accuracy which I also had hoped to have attained. So far as I can judge, I 
should consider that in respect of current measurement the advantage lay with me, 
but that on the optical side M. Becquerel’s arrangements were probably superior. 
M. Becquerel repeats his proposal! to found upon his value of the constant a 
method for current measurement. I had considered this question at (I believe) an 
earlier date; and the less sanguine view expressed in the following paragraph seems 
to be justified by the discrepancies between the results of various observers at various 
times as to the value of the constant in bisulphide of carbon :— 
“ Another method, available with the strong currents which are now common, 
depends upon Faraday’s discovery of the rotation of the plane of polarisation by 
magnetic force. Gordon found 15°J as the rotation due to the reversal of a current 
* See his equation. (24), p. 15. 
t C. R., t. xcviii., p. 1253; 1884. 
1 Jan., 1884. In a note recently communicated to the Royal Society (Proceedings, Nov. 15, 1883), 
Mr. Gordon points out that, owing to an error in reduction, the number given by him for the value of 
Verdet’s constant is twice as great as it should be. The rotations above mentioned must therefore be 
halved, a correction which diminishes materially the prospect of constructing a useful instrument upon 
this principle. 
MDCCCLXXXV. 3 B 
