Dr. Brewster on the Polarisation of Light. 225 
brilliant. The indistinctness occasioned by the interference of 
the two images in the case of Iceland spar, and of the bright 
and nebulous light in the case of the agate, is here in a great 
measure avoided. 
The polarisation of light for this class of experiments may 
be effected in a very beautiful manner, by introducing into a 
glass trough about two-tenths of an inch wide, the fragments 
of a globe of glass blown to the utmost thinness, or the ele- , 
mentary films of which mica is composed. I have produced 
the same effect by coats of grease, by thick plates of mica,* 
by folds of gold beaters* skin,-f and even by gold leaf itself. 
The gold beaters' skin is particularly fitted for this experiment 
when a strong light is used, as it disperses and equalises the 
light, and thus exhibits the rings to peculiar advantage. 
Owing to the reflection which takes place at the surface of 
each plate, the image of a luminous object seen through a 
parcel of fifteen plates is encircled with a great number of 
faint images, which exhibit new phenomena. When the plates 
are placed at a little distance from each other in a wooden 
trough ABDC, PI. VIII., fig. 1, a luminous object at O will be 
seen by the eye at E in the direction EE', encircled with these 
faint images. By turning the trough round D, in the direc- 
• The following measures were taken with two thick pieces of mica. 
Thickness. Angle of Polarisation. 
Mica - - 0.127 * “ 63* o* 
Ditto, another specimen 0.093 - - 7 * 45 
f The angle at which the complete polarisation of the transmitted light is effected 
may, in some instances, be employed as a good measure of the refractive power of the 
polarising plates or films. In the case of gold beaters’ skin, and other substances 
which neither reflect nor refract light regularly, it i« the only method which can be 
put in practice. . 
MDCCCXIV, G g 
