304 Professor Jagadis Chunder Bose [Jan. 29, 



I now readjust the gratings in a crossed position. I have in my 

 hand a large block of the crystal beryl ; it is perfectly opaque to 

 light. I now hold the crystal with its principal plane inclined at 

 45° between the crossed gratings, and the galvanometer-spot, hitherto 

 quiescent, sweeps across the scale. It is very curious to observe the 

 restoration of the extinguished field of electric radiation, itself in- 

 visible, by the interposition of what appears to the eye to be a per- 

 fectly opaque block of crystal. If the crystal is slowly rotated, there 

 is no action on the receiver when the principal plane of the crystal 

 is parallel to either the polariser or the analyser. Thus, during one 

 complete rotation there are four positions of the crystal when no 

 depolarisation effect is produced. 



Eotation of the crystal, when held with its optic axis parallel to 

 the incident ray, produces no action. The field remains dark. 



Here is another large crystal, idocrase, belonging to the ortho- 

 rhombic system, which shows the same action. It is not at all 

 necessary to have large crystals ; a piece of calc-spar, taken out of an 

 optical instrument, will polarise the electric ray. But the effect pro- 

 duced by the crystal epidote seems extraordinary. I have here a 

 piece with a thickness of only • 7 cm. — a fraction of the wave-length 

 of the electric radiation — and yet observe how strong is its depolaris- 

 ing effect. 



I subjoin a representative list of crystals belonging to the different 

 systems, which would be found to produce double refraction of the 

 electric ray. 



Tetragonal System. — Idocrase, scapolite. 



OrtTiorliomhic System. — Barytes, celestine, cryolite, andalusite, 

 hypersthene. 



Hexagonal System. — Calcite, apatite, quartz, beryl, tourmaline. 



Monoclinic System. — Selenite, orthoclase, epidote. 



Triclinic System. — Labradorite, microcline, amblygonite. 



Douhle Refraction produced by a Strained Dielectric. 



Effect due to Pressure. — A piece of glass, when strongly com- 

 pressed, becomes double refracting for light. An analogous experi- 

 ment may be shown with electric radiation. Instead of producing 

 pressure artificially, it seemed to me that stratified rocks, which, from 

 the nature of their formation, were subjected to great pressure, would 

 serve well for my experiment. Here is a piece of slate about an inch 

 in thickness. I interpose this piece with the plane of stratification 

 inclined at 45°, and the spot of light flies off the scale. I now care- 

 fully rotate the piece of slate ; there is no depolarisation effect when 

 the plane of stratification is parallel to either the polariser or the 

 analyser. Thus the existence of strain inside an opaque mass can 

 easily be detected, and what is more, the directions of maximum and 

 minimum pressures can be determined with great exactitude. 



Effect due to Strains in Cooling. — An effect similar to that pro- 



