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 ¢ 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 e W 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 18}4, p. 223. 
} 
‘ 
