1895.] J. C. Bose — Polarisation of Electric Bays. 295 



The crystal was now placed with its optic axis parallel to the direc- 

 tion of the incident ray. There was no action on the Galvanometer. 

 Rotation of the crystal round this axis, did not produce any effect on 

 the Galvanometer. The field continued to be dark. 



2° Apatite. — This specimen exhibited decided double refraction. 



3° Nemalite. — This is a fibrous variety of Brucite. This specimen 

 exhibited a very strong de polarisation effect. It also exhibited certain 

 interesting peculiarities which will form the subject of a future com- 

 munication. 



Rhombic system. — A large piece of Barytes was found strongly double 

 refracting. 



Triclinic system. — Microcline, a greenish blue crystal of the double 

 oblique type, exhibited polarisation effect to a remarkable degree. 



Regular system. — A large crystal of Rock-salt was taken. This as 

 was expected did not produce any effect. 



Having satisfied myself of the fact that systems of crystals other 

 than regular, produce double refraction and consequent polarisation of 

 electrical ray, I tried the action of electric radiation on crystals ordin- 

 arily used in optical experiments. 



I got a fairly large piece of black Tourmaline. On interposing this 

 with its plane vertical, there was prompt movement of the spot of light. 

 There was no action on the Galvanometer, when the principal plane coin- 

 cided with the planes of polarisation of either the Polariser or Analyser. 



With ordinary light a piece of Tourmaline of sufficient thickness 

 absorbs the ordinary, but transmits the extraordinary ray. With the 

 piece of Tourmaline used in the last experiment I found both the rays 

 transmitted, but, it seemed to me, with unequal intensities. In other 

 words, one ray suffers greater absorbtion than the other. It seems pro- 

 bable that with greater thickness of crystal one ray would be completely 

 absorbed. I found other crystals behaving more or less in the same 

 way. I reserve for another communication particulars of experiments 

 bearing on this subject. 



Lastly I tried an experiment with a crystal of Iceland spar, taken 

 out of a Polarising apparatus. With this I got distinct depolarising 

 action. 



The above results, with the exception of the last, were obtained 

 with uncut specimens. Their faces were often rough and irregular. 

 Better results, were they needed, could no doubt be obtained by judicious 

 cutting and polishing the faces. 



Summary. — It will thus be seen that crystals which do not belong 

 to the Regular system, polarise the electric ray, just in the same way as 

 they do a ray of ordinary light. Theoretically all crystals, with the 



