608 SIR DAVID BREWSTER ON THE ACTION OF UNCRYSTALLISED FILMS 
mining it, I found it to have the same relation to the refracted pencil “as the 
nebulous tmage has to the bright image of the agate, or as the first has to the second 
pencil of doubly refracting crystals.” * 
In making the experiment with a small bright image of a candle, and using 
plates of parallel glass, I found that the reflected images a, a, a, a, were dis- 
tinctly separated from the bright or refracted image A, and were all polarised by 
reflection in a plane opposite to that of A.f 
Although these two facts, which have much theoretical importance, were not 
only minutely described, but represented in diagrams, in my paper of 1814, yet 
they escaped the notice of the three distinguished philosophers I have named, 
and of all subsequent writers; and the consequence of this has been, that the 
true action of a pile of plates, or films, has never been the subject 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 portions of light polarised in opposite 
planes. 
In submitting the films of decomposed glass to the polarising microscope, I 
observed a number of polygonal portions, approaching more or less to circles, 
but often perfectly circular, and exhibiting the black cross with coloured sectors 
and rings, analogous to those produced by uniaxal crystals. This observation, 
which was made with decomposed glass given me forty years ago by the late 
Marouis of NorTHAMPTON, was communicated to the British Association at Glas- 
gow 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 
polarisation in different azimuths of the light transmitted through the spherical 
shells, like a group of watch-glasses, of which the circular portions were com- 
posed. The light surrounding 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, and which enabled me to arrive at re- 
sults that could not have been obtained from the finest and the thinnest plates of 
glass artificially produced. 
* Phil. Trans., 1814, p. 226, and Plate VIIL,, figs. 2, 3. 
Tels, ib, 
