ROTATION OF LIGHT IN BISULPHIDE OF CARBON. 
345 
Bunsen burner (fig. 1, A). As in Mr. Glazebrook’s optical investigations, the 
evaporation of the salt and the temperature of the flame were stimulated by a jet 
of oxygen gas, brought in laterally and caused to play round the gauze. 
© 
A 
C B 
A. Bunsen burner. 
B. Mirror with slit. 
C. Back mirror. 
D. Direct vision prism. 
E. Collimating lens. 
Fig. 1. 
i 
F. Polarising Nicou. 
G. Sugar cell. 
H. Tube of bisulphide of carbon. 
I. Screen (blackened inside). 
J. Analysing Nicol. 
At the close of the experiments I examined the light thus obtained with a powerful 
spectroscope, and found that under the influence of the oxygen the originally narrow 
bright lines dilate almost to the point of contact, thus forming a bright field upon 
which the dark D-lines are seen with beautiful definition. Although the distribution 
of light appeared to be tolerably symmetrical, it is a question to what degree of 
accuracy the mean quality of this light can be identified with that coming from 
midway between the D-lines. Probably we shall be safe in estimating that the error 
from this cause is well below y^Yo- 
The bright part of the flame being much larger than is required, a screen (B), 
perforated with a slit, may conveniently be interposed. In this course there are two 
advantages. It allows us to purify the light from rays of other refrangibilities (of 
which there is always a sensible accompaniment, both red and blue), by use of a 
direct-vision prism (D). Again, by making this screen of looking-glass, from which 
a narrow strip of silvering is removed, and by backing the flame with a parallel 
mirror (C), we gain by repeated reflections to and fro, an important increase of 
illumination. The success of the polarimetry is very dependent upon the intensity of 
the light, but there must be also a reasonable steadiness. Several arrangements of 
flame which at first promised well failed in the latter requirement. 
6. The rays from the slit, after purification by the direct vision prism, are rendered 
parallel by a collimating lens (E) and pass into the polarising Nicol (F). The polari- 
meter employed is on the principle of Laurent, but according to a suggestion of 
Poynting* the half-wave plate of quartz is replaced by a cell (G) containing syrop, so 
arranged that the two halves of the field of view are subjected to small rotations 
differing by about 2°. The difference of thicknesses necessary is best obtained by 
introducing into the cell a piece of thick glass, the upper edge of which divides the 
* Phil. Mag., July, 1880. 
