292 
J. 0. Bose — Polarisation of Electric Pays. 
[No. 2, 
short wave lengths, I have succeeded in obtaining very satisfac¬ 
tory 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 analyser. 
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. 
