upon Common and Polarized Light. 



271 



three distinguished philosophers I have named, and of all sub- 

 sequent writers ; and the consequence of this has been that the 

 true action of a pile of plates or films has never been the sub- 

 ject of research during the last forty-six years, though such 

 piles have been used in some of the most delicate and important 

 researches in physical optics. 



The difficulty of procuring transparent plates with parallel 

 surfaces, and of sufficient thinness, would have prevented the 

 most skilful observer from making any progress in the inquiry ; 

 and had I not been fortunate enough to obtain, from the museum 

 of the Marquis Campana in Rome, a large quantity of glass in 

 different stages of decomposition, I could hardly have done more 

 than confirm the result which I obtained in 1813, that the light 

 transmitted by a pile of transparent plates consists of two por- 

 tions of light polarized in opposite planes. 



In submitting the films of decomposed glass to the polarizing 

 microscope, I observed a number of polygonal portions, approach- 

 ing more or less to circles, but often perfectly circular, and ex- 

 hibiting the black cross with coloured sectors and rings analo- 

 gous to those produced by uniaxal crystals. This observation, 

 which was made with decomposed glass given me forty years 

 ago by the late Marquis of Northampton, was communicated to 

 the British Association at Glasgow in 1855 ; but at that time I 

 regarded the black cross and its accompanying tints, as shown 

 in the drawings on the table, as produced by the refraction and 

 polarization in different azimuths of the light transmitted 

 through the spherical shells, like a group of watch-glasses, of 

 which the circular portions were composed. The light surround- 

 ing the black cross was so highly coloured with the colours of 

 the thin plates which composed the film of glass, that I failed in 

 every attempt to analyse it. After examining, however, many 

 hundreds of these films from the new specimens which I have 

 mentioned, I succeeded in finding a few in which there were no 

 such colours, an d which A 



enabled me to arrive at 

 results that could not 

 havebeen obtained from 

 the finest and the thin- 

 nest plates of glass arti- 

 ficially produced. 



These results will be 

 understood from the an- 

 nexed diagram, in which 

 M N is a thin plate and 

 A B a ray of common 

 light incident perpen- 



\ 



\b g -k 



