610 SIR D. BREWSTER ON THE ACTION OF UNCRYSTALLISED FILMS, ETC. 
crystal. The polarised tints have the same value in every azimuth at the same 
angle of incidence, and therefore form rings which, when crossed with plates of 
sulphate of lime, descend in Newton’s Scale like the tints of negative uniaxal 
crystals. 
Out of hundreds of specimens now on the table, I have found a few so colour- 
less and so perfect as to produce, at different incidences, all the polarised tints or 
rings up to the dlue of the second order of Newton’s Scale. These colours are so 
pure, and so regularly developed by the inclination of the plate, that the most 
skilful observer could not fail to pronounce it to be a portion of a doubly refract- 
ing crystal. 
The production of the leading phenomena of doubly refracting crystals,— 
namely, two oppositely polarised pencils,—and the system of coloured rings by 
the interference of these pencils, is certainly one of the most remarkable facts in . 
physical optics; and, in a theoretical point of view, no less remarkable is the fact 
that one of the interfering portions is a fasciculus of pencils returned into the 
refracted beam by different routes, and having different origins. 
Owing to the extreme thinness of the combined films, we cannot, as with thick 
plates of uniaxal crystals, see at once the black cross and its attendant rings; 
but in numerous specimens of decomposed glass to which I have already referred, 
the films are spherical shells of different diameters and thicknesses, and exhibit 
the black cross with the greatest sharpness and beauty. In many specimens 
these circular combinations are perfectly colourless; and the colours of the four 
luminous sectors which embrace the black cross rise only to the white of the first 
order. 
When the films are so thin as to give the colours of thin plates, the colour -of 
the luminous sectors is generally the same as that of the film in which the cir- 
cular portions occur, and the rings or bands which surround them have a very 
peculiar character, owing to the manner in which the spherical shells are joined 
to the films which compose the plate. 
How far these results may lead to new views of the structure which paaees 
double refraction, it would be unprofitable to inquire in the present state of our 
knowledge of the atomical constitution of transparent bodies. 
