28 DESCRIPTION OF A NEW DARKENING GLASS 



plates, cemented together by a thin layer of indurated Canada 

 balsam, or any other transparent substance, having the same 

 refractive power. The rays of light will then pass from the 

 one plate into the other, without suffering either reflexion or 

 refraction. This compound plate may even be formed of 

 glasses of different colours, if we wish to produce a great de- 

 gree of attenuation. 



The simple apparatus which has now been described, pos- 

 sesses a still more valuable property than that of attenuating 

 the incident light. The pencil c V, which has undergone two 

 reflexions, emerges completely polarised, unless when the 

 angle of incidence is very small ; and the polarisation continues 

 complete, although this angle suffers a very considerable varia- 

 tion. The other pencil eW is also polarised, and preserves 

 this character, even when the angle of incidence has a much 

 wider range. 



If a plate of fine flint-glass is used in the construction of the 

 eye-piece, we may employ it to great advantage in experiments 

 on polarisation, even when the light has a moderate degree 

 of intensity. But if the light of the sun is under examination, 

 the eye-piece will possess the peculiar advantage, of at the 

 same time attenuating and polarising the incident pencil. 



The polarisation of the emergent pencils c V, e W, enables 

 us to explain a very perplexing anomaly in the law of the po- 

 larisation of light by oblique refraction *. From numerous 

 experiments made with piles of glass plates, I found that the 

 tangents of the angles at which they polarised the transmit- 

 ted light, were inversely as the number of plates of which the 

 pile was composed. The coincidence of this law, with the ex- 

 perimental results, was extremely accurate when the number 

 of plates was between eight and forty-seven, corresponding to a 



series 



* See the Philosophical Transactions, for 1814, p. 223. 



