ROTATION OF LIGHT IN BISULPHIDE OF CARBON. 
363 
more favourable results than hitherto would be obtained with an undivided field of 
view. 
In the application of the polarimeter, with which the present paper is mainly con¬ 
cerned, the free play of the Nicol is advantageously replaced by an equivalent 
rocking of the plane of polarisation itself through a small angle on either side of its 
normal position, produced by the action of an auxiliary electric current, embracing the 
experimental tube a moderate number of times, and reversed at pleasure by a suitable 
key under the hand of the observer. 
In these discussions it has been convenient to take as a basis the fractional difference 
of brightnesses which can be recognised on simple presentation to the eye,* but it must 
be remembered that if suitable precautions are taken to avoid asymmetry, there is no 
theoretical limit of final accuracy. Thus in ordinary photometry with a divided field 
(e.g., Bunsen’s grease-spot photometer), the match must not be approached from one 
side only. By combining a large number of observations in which the match is 
approached as much from one side as from the other, a degree of accuracy may be 
practically attained far beyond that corresponding to the difference of brightness 
which can be directly recognised by the eye. It is not necessary actually to take 
readings on the two'sides, though it is sometimes desirable to do so; the essential 
point is to secure symmetry. Time may be saved by the plan of providing means for 
instantaneous displacements of given amount on either side, as was done in the 
experiments of the present paper by the auxiliary reversible current. 
In practical applications of the polarimeter we have almost always to determine, 
not so much a particular plane of polarisation as the rotation of this plane, due to 
electromagnetic action, to the substitution of syrop for water, etc., and it appears 
that the measurement of this angle must be affected with a possible error, double 
of the error possible in the determination of a single plane. M. Becquerel, indeed, 
in his interesting memoir upon the rotation in bisulphide of carbon under the terres¬ 
trial magnetic force,! describes a procedure by which, as he considers, the error may 
be reduced. By the introduction of a half-wave plate, adjusted so that its principal 
section coincides nearly with the plane of first polarisation, the angle of rotation is, 
as it were, reflected by the former plane, and the difference of readings taken with 
and without the plate is the double of the real angle of rotation. If e be the greatest 
angular error possible in determining a single plane, M. Becquerel shows that the 
error in setting the plate cannot exceed e, from which he argues that the whole error 
possible in determining the double angle of rotation is only 3e, or fe upon the single 
angle. It appears, however, that the error of adjustment of the half-wave plate enters 
* [August, 1885.—I find that the sensitiveness of the eye to small differences of brightness is subject 
to very rapid fatigue. Even a few seconds’ gazing is often enough to obliterate a distinction quite 
apparent at first, and appreciable again after a little repose. This defect is a great obstacle to the 
further improvement of photometric methods.] 
t Ann. d. Chimie, t. 201, p. 323, 1882. 
