ixiJien exposed to Polarized Light. 1 87 



faces of the film. At other angles of incklence the rings dis- 

 appeared at different azimuths, varying from 90^ to about 45°, 

 as the angle of incidence varied from 53° 11' to 90°. I found 

 it difficult, however, to measure these azimuths with any ac- 

 curacy, as the rings were not permanent ; and I was there- 

 fore obliged to form the colours of thin plates upon highly 

 refracting substances, such as diamond, chromate qf lead, arti- 

 Jicial realgar, and grcenockite (the most refractive of all bodies), 

 which had high polarizing angles. A solution of fine soap 

 gave brilliant colours when dried, and in this way I obtained 

 the following results with the surface of a very fine diamond. 

 The index of refraction of the soap was 1*475, and that of the 

 diamond 1'^\, and their respective polarizing angles 5SP 52', 

 and 67° 43'. 



Azimuth of the plane of polarization at which 

 Angle of incidence of the the rings disappear, 



polarized light. Observed. Calculated. 



55 52 

 60 

 65 



67 43 

 70 

 75 

 90 



As the disappearance of the rings was not owing to the extinc- 

 tion of one of the interfering pencils^ as at hb° 52', for a suffi- 

 cient quantity of polarized light was reflected from both sur- 

 faces of the film, there was reason to believe that it might 

 arise from the two pencils being polarized at right angles to 

 each other, in conformity with the law relating to the action 

 of the second surfaces of plates which I have given in a 

 former paper*. 



Calling X the azimuth of primitive polarization, i the angle 

 of incidence on the^rs^ surface of the film, i' the correspond- 

 ing angle of refraction, and consequently the angle of inci- 

 dence on the second surface, i" the angle of refraction at the 

 second surface, and 



f = the inclination of the plane of polarization of the re- 

 flected pencil CA, fig. 3, 

 f' = that of the refracted pencil CD, 

 <p" = that of the reflected pencil DE, and 

 (p"'= that of the refracted pencil EB, with which CA inter- 

 feres ; then by Fresnel's formula we have for the ray CA, 



cos(i + i') 

 tan a = tan x . — ; . — .,. ; 

 cos{t — r) 



* Philosophical Transactions, 1830, pp. 148, 149. 



