( 607°) 
XXVII.—On the Action of Uncrystallised Films upon Common and Polarised 
Light.* By Sir Davin Brewster, K.H., F.R.S. 
(Read 20th February 1860.) 
In a paper on the Polarisation of Light by Refraction, published in the “ Philo- 
sophical Transactions” for 1814, I have shown that when a pencil of light is inci- 
dent on a number of uncrystallised plates, inclined at the same or different angles 
to the incident ray, all their surfaces being perpendicular to the plane of the first 
incidence, the transmitted pencil will be wholly polarised, when the sum of the 
tangents of the angle of incidence upon each plate is equal to a constant quan- 
tity, depending upon the refractive power of the plates and the intensity of the 
incident pencil. 
This law, though admitted by,M. Araco in his article on Polarisation in the 
* Encyclopedia Britannica,”+ was called in question by Dr Youne,} on 
the ground that no finite number of plates could polarise the whole trans- 
mitted beam, as a small portion of light must always remain unpolarised, or in 
the state of natural light. This is doubtless true; but, as Sir John HrrscHen 
has shown, it does not affect the truth of the law, which involves the intensity of . 
the incident pencil. According to the law of geometrical progression, indeed, a 
small portion of unpolarised light exists mathematically in the transmitted beam, 
but a beam of light may be said to be completely polarised when the unpolarised 
portion is invisible, ‘vanishing entirely in certain positions of the analysing 
prism. 
Neither M. AkAco nor Dr Youne has made the slightest reference to that por- 
tion of the refracted light which is reflected at the surfaces of each plate, and 
returned into the transmitted beam. Sir Joun HeERscHEL, however, has dis- 
tinctly referred to it, and remarks, that ‘“‘it mixes with the transmitted beam, 
and, being in an opposite plane, destroys a part of its polarisation.” § 
Although the law of the tangents, which I have mentioned, refers only to the 
transmitted pencil, yet, in the paper which contains it, I have shown that the 
light reflected back into that pencil is distinctly visible, not as ordinary light, as 
Sir Jonn HerscHeEt maintains, but as light polarised in an opposite plane to the 
refracted pencil. 
When the angle of incidence is considerable, this oppositely polarised light 
appears as a nebulous mass, like the nebulous image in the agate, and, after exa- 
_* This paper was read at the meeting of the British Association held at Aberdeen in Sept, 1859. 
¢ Encyc. Brit., vol. xviii. part 1, sect. v. t+ Ibid., in the passage within brackets. 
§ Treatise on Light, art, 868. 
VOL. XXII. PART III. 78 
