192 Sir D. Brewster on the Phanomenn of Thin Plates 



of incidence, whereas on transparent surfaces of low refractive 

 power, they can scarcely be seen at such angles. When the 

 azimuth of the polarized ray varies from 90° to 0°, the rings 

 disappear at different angles of incidence ; or when the angles 

 of incidence vary, the rings disappear in different azimuths. 

 I measured these azimuths when the polarized ray was inci- 

 dent upon speculum metal, and obtained the following num- 

 bers : — 



Azimuth in which the rings disappear. 

 Angles of incidence. 



90 6 

 71 50 

 60 

 56 5 



In computing column third from the formula in p. ISS, I 

 used r49 as the index of refraction of o;7 of laurel, and 4"011 

 as the index of refraction for specnlum metal, as deduced from 

 my experiments on its elliptic polarization*. 



I have made similar experiments when the rings were 

 transferred to silver, whose elliptical polarization approaches 

 nearest to circular polarization ; and to grain tin, which ap- 

 pears to have the highest refractive power of any of the metals ; 

 but I found it very difficult to ascertain with any accuracy the 

 azimuths in which the rings disappear. 



If we use common in place of polarized light in the prece- 

 ding experiments, and analyse the reflected light by a rhomb 

 of calcareous spar, the very same phaenomena will be exhibited. 



When the films or thin plates are not laid upon the surfaces 

 of fluid or solid bodies, the phaenomena are of an entirely 

 different kind. At all angles of incidence, and in all azimuths, 

 the colours and character of the rings are the same, whether 

 we use common or polarized light. In obtaining this result 

 I stretched thin films of various oils, such as oil of laurel, oil 

 of cassia, oil of turpentine, and many others, across circular 

 apertures, and examined them in light polarized in different 

 azimuths. The rings of course vanished at the polarizing 

 angle of the oil, and the brilliancy of the colours varied with 

 the angles of azimuth and incidence, but the complementary 

 rings never appeared, the rings being always those with the 

 black centre f. 



* Philosophical Transactions, 1830, p. 324. 



■\ The physical jjhainoniena exhibited in these attenuated films are very 

 remarkable. A current of fluid is projected from the margin and centre of 

 the ring of fluid across the fluid surface, resembling the top of a pine apple. 

 This movement makes the film thinner at some places than others, and 



