302 



Professor Jagadis Chunder Bose 



[Jan. 29, 



suitable parallel air-space, are placed on the spectrometer-circle, the 

 receiver being placed opposite the radiator. The trouble of following 

 the deviated ray is thus obviated. The index of refraction of glass I 

 found to be 2 • 04 ; that of commercial sulphur is 1 • 73. 



Double Befraction and Polarisation. 



I now proceed to demonstrate some' of the principal phenomena 

 of polarisation, especially in reference to the polarisation produced 

 by crystals and other substances, and by dielectrics when subjected 

 to molecular stress due to pressure or unequal heating. 



As the wave-length of electric radiation is many thousand times 

 the wave-length of light,^there is a misgiving as to whether it would 



Fig. 4. — Polarisation Apparatus. 



K, crystal-holder; S, a piece of stratified rock; C, a crystal; J, jute 

 polariser ; W, wire-grating polariser ; D, vertical graduated disc, by 

 which the rotation is measured. 



be possible to exhibit polarisation effects with crystals of ordinary 

 size. I hope to be able to demonstrate that such a misgiving is 

 groundless. 



A beam of ordinary light incident on a crystal of Iceland spar is 

 generally bifurcated after transmission, and the two emergent beams 

 are found polarised in planes at right angles to each other. The 

 usual optical method of detecting the bi-refringent action of crystal, 

 is to interpose it between the crossed polariser and analyser. The 

 interposition of the crystal generally brightens the dark field. This 

 is the so-called depolarisation effect, and is a delicate test for double- 

 refracting substances. There is, however, no depolarisation when the 



