292 J. C. Bose — Polarisation of Electric Kays. [No. 2, 



short wave lengths, I have succeeded in obtaining very satisfac- 

 toi*y results with crystals of moderate size. These experiments show 

 that certain crystals are double refracting as regards electric rays, and 

 that they polarise the transmitted beam. With the help of a rudely 

 constructed apparatus, I was able last year to detect traces of these 

 effects. The apparatus has since been improved in detail ; it is now 

 possible to detect the polarisation effects with certainty. 



The usual optical method of detecting the bi-refringent action of 

 crystals, is to interpose the double refracting structure between two 

 crossed Nicols. The interposition of the crystal generally brightens 

 the dark field. This is known as the depolarising effect, and is regarded 

 as a delicate test for double refracting substances. There is however, 

 no depolarising effect, when the principal plane of the crystal 

 coincides with the polarisation planes of either the polariser or analysei\ 

 The field also remains dark, when the optical axis of the crystal is 

 parallel to the incident ray. 



A similar method was adopted for experimenting with polarised 

 electric radiation. The electric ray is first polarised by a wire grating. 

 A similar grating acts as an analyser. The two gratings are crossed, 

 and the crystal to be examined is interposed. The Receiver is a 

 modified form of ' Coherer' with its associated Voltaic cell and Galvano- 

 meter. Brightening of the field is indicated by a throw of the Galvano- 

 meter needle. 



Apparatus used. 



Radiator. — A small Ruhmkorff's coil is used for the production of 

 oscillatory discharges between two small metallic spheres, the diameter 

 of each sphere being 1*5 c. m. The choice of a coil to produce electric 

 oscillation has been a matter of necessity. I obtained oscillatory effects 

 with ease and certainty by using a small influence machine of the Reple- 

 nisher type. But in the damp atmosphere of Calcutta, the satisfactory 

 working of such a machine is a matter of great difficulty, at least for 

 the greater portion of the year. I had therefore to abandon the influ- 

 ence machine with regret, and to use a Ruhmkorff's coil instead. This 

 coil caused me the greatest trouble. The discharge would of a sudden 

 cease to be oscillatory ; after a great deal of coaxing it would work 

 satisfactorily just for a short time. The only coil I could get, was a 

 badly constructed one, with defective insulation. I made it serviceable 

 by changing the condenser and improving the vibrator. By looking 

 to many points of detail I succeeded in making the apparatus work 

 with fair uniformity for several hours. It must be borne in mind that 

 the Receiving apparatus also requires careful adjustment. 



