WHEN EXPOSED TO POLARIZED LIGHT. 53 



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 appears 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 preceding experiments, and 

 analyse the reflected light by a rhomb of calcareous spar, the very same phenomena 

 will be exhibited. 



When the films or thin plates are not laid upon the surfaces of fluid or solid bodies, 

 the phenomena 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 cir- 

 cular 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*. 



In order to understand the cause of this, we must inquire into the state of polar- 

 ization of the interfering pencils. The ratio of refraction being the same at both 

 surfaces of the film, we have 



. ^ , cos (? + i') J ^ Iff ^ cos^ (i — «0 

 tan (p = tan x . -—7^ — 7^, and cot ® " = cot a: . ;. . ■/ ; 



^ cos {i — t'y ^ cos {t + 1') * 



and when tan <p = cot (p'", which is the case when <p + <p"' = 90°, we have 



cos' (i — i') 



tan X = — /. , .,\ » 

 cos [t + v) 



COS V 



When i = 90°, tan 9 = A, or the azimuth of the polarized ray, and cot p'" = ~ — j. 



If we now compute the values of p and <p"' at different angles of incidence and in 

 different azimuths of the polarized light, we shall obtain the results in the following 

 Table. In azimuths 0° and 90°, (p and f = 0. 



* The physical phenomena 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 hence arises an irregular sy- 

 stem of coloured bands, with an incessant play of varying tints, as if the fluid were animated. The bands of 

 colour are serrated with salient points, from which the fluid seems to shoot across the film. In the oils of 

 cinnamon, naphtha, spearmint, wormwood, rapeseed, nutmegs, bergamot, savine, rosemary, &c., the phenomena 

 are peculiarly beautiful. With poppy oil, the red and green tints of the 4th, 5th, and 6th orders were also seen. 



