182 Sir D. Brewster 07i the Phcjenomena of Thin Plates 



a surface of glass, M. Arago observed that they were black 

 centred, as usual ; and whether viewed with the eye or with 

 a doubly refracting rhomb of Iceland spar, that the single or 

 the double system of rings had the same colours and the same 

 diameters, the rings being completely polarized at the polari- 

 zing 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 perpendicular 

 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 peculiai'ity, that the colour 

 of each of the rings which composed it was complementary to 

 that of the corresponding rings in the image which had dis- 

 appeared. 



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 di- 

 stinct 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 phaeno- 

 mena 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 production of the phsenomenon 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 presents often in its cleavages 

 very bright colours, produced by an extremely thin substance, 

 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 sub- 

 stance, give rise also to two images, dissimilar, and of opposite 

 tints*." 



In order to investigate the phsenomena of the rings of va- 

 pour in the iriscope, I illuminated them with light polarized 

 in an azimuth of 90°, or perpendicularly to the plane of inci- 

 dence, and examined them by a magnifying glass, when the 

 centre of the rings was seen by light reflected at about 53° 1 1', 



* Mcmoires de Phydque ct dc C/iimie de la SociHe d'Arcueil, torn. iii. 

 p. 363. Paris, 1817. 



