WHEN EXPOSED TO POLARIZED LIGHT. ft5 



second surface we can make it fall upon it at a greater angle than the polarizing 

 angle. The phenomena may be still more varied by inclining the surface of emer- 

 gence to the surface of incidence*; but as it is not easy to obtain films with faces 

 suitably inclined to each otlier, it is unnecessary to pursue this branch of the subject 

 any further. 



Such are the phenomena of thin and thick plates when viewed by polarized light, 

 or by common light subsequently analysed by a doubly refracting rhomb. But if we 

 use polarized light, and subsequently analyse the light transmitted through the thin 

 plates, we shall obtain a series of very interesting and instructive phenomena, analo- 

 gous to those produced by plates of doubly refracting crystals which exhibit the 

 polarized tints. In both these cases, the film is interposed between a polarizing plate 

 and an analyzing rhomb. If the film is too thick to produce colours, it will depolar- 

 ize the polarized ray, in a manner analogous to that of a crystallized plate, which is 

 not thin enough to give the polarized tints ; and if the film is sufficiently thin to pro- 

 duce uniform tints, a coloured band or system of rings, with black or white centres. 

 Their action is analogous to that of thin crystallized plates, which either produce uni- 

 form tints like the laminse of sulphate of lime, or uniaxal or biaxal systems of rings. 



It would be unprofitable to describe minutely the great variety of phenomena 

 which thin plates thus exhibit, as they vary with the refractive power of the fluid or 

 solid upon which they are laid, so that I shall confine myself to the case in which a 

 thin plate of oil of laurel rests on the surface of a specimen of artificial realgar. In 

 common light, the colours of this film are very beautiful, but when examined in po- 

 larized light by an analyzing rhomb, they are brilliant beyond description. 



1 . When the azimuth of the polarized light is 90°, and the incidence of the polar- 

 ized ray 56° 5', the polarizing angle of oil of laurel. 



When the film is viewed without the polarizing rhomb, no rings are seen, as there 

 is no light reflected from the first surface of the film, and consequently no inter- 

 ference. 



When the film is viewed with the polarizing rhomb, having its principal section in 

 the plane of incidence, no rings appear, either in its ordinary or extraordinary image. 

 But if the plane of polarization is less or more than 90°, by even a small quantity, 

 then after the rhomb has been turned round nearly 90° towards the right, a system 

 of black-centred rings is seen for an instant, and these, after disappearing, are followed 

 by a system of white-centred ones, the white-centred rings appearing first if the 

 rhomb is turned to the left. The same phenomena are repeated in every quadrant 

 of the circular motion of the rhomb. 



2. IVhen the azimuth of the polarized light varies from 90° to 0°, the incidence, 

 being 56° 5', as before. 



At 90° azimuth the phenomena are as above described. 



* Philosophical Transactions, 1830, p. 147, fig. 3. 



