344 
LORD RAYLEIGH ON THE CONSTANT OF MAGNETIC 
3. The laws of the phenomenon were investigated in detail by Verdet, who proved 
experimentally that in a given medium the rotation between any two points on a ray 
of light of given kind is proportional to the difference of magnetic potential at those 
points. When the path of the ray is singly or doubly curved, the rotation is to be 
estimated upon principles similar to those applicable to twist in curved rods.t 
4. Absolute determinations of magnetic rotation in bisulphide of carbon have 
been made by Gordon, l and by H. Becquerel, § whose results differ, however, 
by about 9 per cent. The former obtained his magnetic force by means of an 
electric current circulating a great many times round the column of CS 3 . This 
column being a good deal longer than the coil, the electromagnetic effect is 
approximately determined by the strength of the current and the number of 
turns. Of these data the first was found by a comparison with H (the horizontal 
component of terrestrial magnetism). The number of windings in the coil was 
determined, not by a simple counting, but cl posteriori by an electrical process. 
In M. Becquerel’s experiments the magnetic force was that of the earth acting 
on a column of CS 3 more than 3 metres in length. The very small effect (obtained 
by reversal of the apparatus in azimuth) was augmented by causing the light to 
pass the tube 3 or 5 times, but even with 5 passages the double rotation amounted 
to only about 30 minutes. M. Becquerel regards his determination for sodium light 
as accurate to within 1 per cent., which would be indeed a wonderful result con¬ 
sidering the smallness of the rotation. 
5. It is important to observe that great care is required in order to define with 
sufficient accuracy the kind of light employed. Since the rotation is approximately 
proportional to \ -i , a change from one sodium line to the other would make a 
difference of two parts per thousand. Both of the above-mentioned experimenters 
started with white light. Gordon threw a spectrum upon a screen, perforated with 
a slit, the position of which was adjusted to correspond with the thallium line; while 
Becquerel corrected his results indirectly by a subsequent comparison between the 
effects of the more mixed light used by him and that emitted by sodium. 
Considering that the employment of white light involved very elaborate arrange¬ 
ments for analysis (according to wave length), in order to avoid errors exceeding- 
in magnitude those likely to be encountered in the polarimetric or electric determina¬ 
tions, I decided to use light actually emitted from sodium vapour. The sodium 
chloride was held by a spoon of platinum gauze in the flame of a small ordinary 
* Thomson and Tait’s ‘Natural Philosophy,’ §§ 119-123. 
f When polarised light passes from one medium to another, e.g., from air to glass, the plane of 
polarisation is in general twisted without the operation of any magnetic force. This effect, however, 
depends upon a part of the light being diverted by reflection, and would disappear if the transition 
from one medium to the other were gradual, i.e., occupied a stratum a few wave-lengths thick. (See 
Proc. Math. Soc., vol. xi., No. 159). 
+ Phil. Trans. 1877, p. 1. 
§ Ann. d. Chimie, 1882. 
