44 
SIR DAVID BREWSTER ON THE PHENOMENA OF THIN PLATES 
double system of rings had the same colours and the same diameters, the rings being 
completely polarized at the polarizing angle of the glass. 
When the lens, however, was pressed against a metallic mirror, and examined with 
a doubly refracting rhomboid, two images perfectly similar appeared between a per- 
pendicular incidence, and that of 55° or the polarizing angle of glass. One of the 
images disappeared entirely at this angle of 55°, when the principal section of the 
rhomboid was perpendicular or parallel to the plane of reflexion ; but reappeared at 
greater incidences, with this remarkable peculiarity, that the colour of each of the 
rings which composed it was complementary to that of the corresponding rings in 
the image which had disappeared. 
M. Arago likewise remarks that we may easily perceive with the eye, naked and 
without the assistance of any crystal, that at a certain angle near 55° the rings are 
composed of two distinct sets having unequal diameters, the rhomboid separating in 
a great measure the two sets of rings, because they are very unequally polarized. 
He likewise found that these phenomena were not produced when the rings were 
formed upon native sulphur and diamond. 
“If the presence of a metallic mirror,” says M. Arago, “is necessary for the pro- 
duction of the phenomenon in question when the rings are formed upon a plate of air, 
the case is otherwise when the thin body has much more density, and is in contact by 
one of its faces with another medium of sufficient refractive power. Thus coal pre- 
sents often in its cleavages very bright colours, produced by an extremely thin sub- 
stance, and which are decomposed into two complementary images when they are 
examined with a rhomboid under sufficiently oblique incidences. The colours which 
are formed artificially by the progress of evaporation, on thin films of alcohol or oil 
of sassafras, deposited upon coal or any other analogous substance, give rise also to 
two images, dissimilar, and of opposite tints*.” 
In order to investigate the phenomena of the rings of vapour in the iriscope, I 
illuminated them with light polarized in an azimuth of 90°, or perpendicularly to the 
plane of incidence, and examined them 'by a magnifying glass, when the centre of 
the rings was seen by light reflected at about 53° 11', the polarizing angle of water. 
The effect, which was very striking, is shown in Plate I. fig. 2. The central part, 
A B, of the system of rings, CDE F, was without rings and colours of any kind : the 
upper half, C D, was part of a system of rings with white circumferences, and was 
formed by polarized light incident on the film at an angle greater than the polarizing 
angle of water; while the under half, E F, was part of a system of rings with black 
circumferences like those seen by common light, and was formed by polarized light 
incident on the film at an angle less than the polarizing angle of water. 
The absence of rings in the middle portion, A B, was of course owing to there 
being no light reflected from the first surface of the film with which that reflected 
from the second surface could interfere; and the reason of there being light reflected 
* Memoires de Physique et de Chimie de la Societd d’Arcueil, tom. iii. p. 363. Paris, 1817. 
