-iiclicn exposed to Polarized Light. 1 95 



ized lif^lit, and subsequently analyse the light transmitted 

 through the thin plates, we shal! obtain a series of very inter- 

 esting and instructive phasnomena, analogous to those pro- 

 duced 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 analysing rhomb. If the 

 film is too thick to produce colours, it will depolarize the po- 

 larized 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 produce uniform tints, a 

 coloured band or system of rings, with black or while centres. 

 Their action is analogous to that of thin crystallized plates, ' 

 which either produce uniform tints like the lamina of sulphate 

 of lime, or uniaxal or biaxal systems of rings. 



It would be unprofitable to describe minutely the great 

 variety of phasnomena which thin plates thus exhibit, as they 

 vary with the refractive power of the fluid or solid upon which 

 they are laid, so that 1 shall confine myself to the case in w hich 

 a thin plate of oil of laurel rests on the surface oi' a .specimen 

 oi artijicial realgar. In common light, the colours of this film 

 are very beautiful, but when examined in polarized light by 

 an analysing rhomb, they are brilliant beyond description. 



1. Wlte)i the azimuth of the polarized ligJd is 90°, aiid the 

 incidence of the polarized ray S%^ 3', (he polarizing angle of 

 oil of laurel. 



When the film is viewed without the polarizing rhomb, no 

 rings a;e seen, as there is no light reflected from the first 

 surface of the film, and consequently no interference. 



When the film is viewed with the polarizing rhomb, having 

 its pnnci))al 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 

 quaiitity, then aftec the ihomb has been turned round nearly 

 90° towards the right, a system of hlack-centrcd rings is seen 

 for an instant, and these, after disappearing, are followed by 

 a system o\ 'johite-cciitred ones, the white- centred rings appear- 

 ing first if the rhomb is turned to the left. The same phae- 

 non)ena are repeated in every quadrant of the circular motion 

 of llie rhomb. 



2. When the azimuth of the polarized light varies from 90° 

 to 0°, the incidence, being .56° 5', as before. 



At 90° azimuth the phasnomena are as above described. 



At 67^°. Rhomb O", no rings. 



Rhomb turning to the right, the white centred rings appear, 

 then vanish, when the azimuth of the rhomb is less than 671°; 

 then black-centred rings appear, which vanish at 180; then 



02 



