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 eftect, which was very striking, is shown in Plate I. fig. 2. The central part, 

 A B, of the system of rings, C D E 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 

 * M^moires de Physique et de Chimie de la Soci^te d'Arcueil, torn. iii. p. 363. Paris, 1817. 



