294 J. C. Bose — Polarisation of Electric Hays. [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 he increased to any extent 

 desirable. 



When the Analyser and Polariser are propei*ly 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. 



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 number 

 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. 



Ehombohedral 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 11x5 cm. 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. 



