294 
J. 0. Bose — Polarisation of Electric Rays. 
[No. 2, 
cell. The leading wires are shielded from radiation by enclosing them 
inside two coatings of tin foil, -along the whole length. As an addi¬ 
tional precaution the Galvanometer is also enclosed in a metallic case, 
with a slit in front of the Galvanometer mirror. A spot of light reflect- 
ed from the mirror is received on a scale. By adjusting the electromo¬ 
tive force of the circuit, the sensitiveness may be increased to any extent 
desirable. 
When the Analyser and Polariser are properly constructed, and 
the two exactly crossed, no radiation will reach the sensitive surface, 
and the Galvanometer will remain unaffected. The field is then said to 
be dark. But any slight rotation of either Polariser or Analyser, will 
partially restore the field, and the spot of light will sweep across the, 
scale. / 
Method of Experiment. 
r 
The spark gap 2 m.m. in length is adjusted in a line inclined at 
45° to the horizon. The wires of the Polariser are placed at right 
angles to this line. The transmitted beam is then plane polarised, 
its plane of vibration being inclined at 45° to the horizon. The 
Analyser is now adjusted in a crossed position. On starting the 
electric vibration, by closing the Ruhmkorff’s coil circuit, the Galva¬ 
nometer remains unaffected. The crystal to be examined is now 
interposed with its principal plane vertical. 
The Geological Department of India kindly lent me a large nunjte^ 
of crystals for examination, for which I have to express my thanks.. 
Out of a large number of experiments, I give below an account of some 
typical cases. 
Rhombohedral System. —1° Beryl .-—The first piece experimented on. 
was a large crystal of Beryl. It is a Hexagonal prism with basal 
planes. The specimen examined has each face 13x5c.m. The three 
axes lying in the same plane are inclined at 60° to each other, the 
fourth axis which is also the optical axis, is at right angles to the 
plane containing the other three. This crystal was optically opaque. 
On interposing this block with its principal plane vertical, the 
Galvanometer spot flew off the scale. The crystal had thus produced 
the well known depolarising action. The crystal was now gradually 
inclined till its principal plane coincided with the polarising plane of the 
Polariser. There was now no action on the Galvanometer. On con¬ 
tinuing the rotation the Galvanometer at once responded. The spot 
became quiescent a second time, when the principal plane coincided 
with the polarisation plane of the analyser. 
