ROTATION OF LIGHT IN BISULPHIDE OF CARBON. 
347 
was then abolished, and the available space filled with paper wrapped pretty tightly 
round the tube. This effected a great improvement, enhanced still further in the 
later experiments of Series III., in which, by reduction of the diameter of the tube, a 
wider space became available for heat insulation. The disturbance by conduction of 
heat from the wire to the CS 2 remained, however, the worst feature of the experi¬ 
ments, and could not be obviated without a fundamental alteration in the apparatus. 
Probably the best arrangement would be a water-jacket next the wire, and a good 
thickness of paper or other insulator between the water and the CS 3 . 
9. The bisulphide of carbon was purified by treatment with corrosive sublimate and 
grease with subsequent distillation (according to the procedure advocated by 
Becqcjerel), until most of the unpleasant odour had disappeared. The transparency 
is much greater than is readily (if at all) obtainable with water, provided proper pre¬ 
cautions are taken to avoid exposure to light. After being acted upon by light, the 
CS 3 attacks brass and becomes rapidly opaque. In this respect it would be an 
advantage to replace the metal tube by one of glass. 
10. The analyser consisted, in some experiments, of a Nicol (J), and in others of a 
double image prism, and was mounted in a circle made by the Cambridge Scientific 
Instrument Company. In order that a rotation of the plane of polarisation may be 
correctly indicated by the difference of the two circle readings, it is necessary that 
the axis of rotation should coincide with the direction of the light. This requirement 
is, however, not very easily satisfied. At the commencement of a series of experiments 
the adjustment was made with the aid of a telescope and cross wires temporarily 
substituted for the Nicol, but during the course of a set of readings the passage of 
heat into the liquid tended to make the upper strata warmer than the lower, and thus 
to bend the rays into a different direction. It is known* that the error arising from 
maladjustment in this respect is in great part eliminated by reading the Nicol always 
in both the positions (differing by about 180°) which give extinction or (in the half¬ 
shade arrangement) equality of illumination. This plan was constantly followed, but 
it is not clear that the whole error can be thus got rid of. It occurred to me that 
another term in the harmonic expansion of the error would be destroyed by use of a 
double image prism read in four positions distant about 90°. Experiment showed 
that in spite of the glare of the unextinguished image, good readings could be 
obtained after a little practice, and the comparison of the results arrived at in this 
way tends to show that the error is not wholly eliminated in the mean of two 
readings taken in positions differing by 180°. But the matter could be much better 
investigated with a simplified apparatus and the use of a strong white light. 
In Series II. and III., when the light traversed the tube but once, no magnification 
was necessary, and the eye was applied immediately behind the analyser. In 
* “ Zur Theorie des Polaristrobometer und des drehenden Nicols.” V. D. Sande Bakhuyzen. Pogg. 
Ann., cxlv., 259, 1872. 
“Notes on Nicol’s Prism,” Glazebrook, Phil. Mag., October, 1880. 
