196 Sir D. Brewster on the Phenomena of Thin Plates 



succeed the white-centred ones, which vanish at about 210°; 

 then the black centred, which continue to 360°. 



At 45°, 2'2i°. The very same phaenomena appear at these 

 and other azimuths, the azimuths of tiie rhomb at which the 

 rings disappear out of the phme of incidence being a litde less 

 than tiie azimuths of the polarized hght. 



At 0^. Tlie evanescence of tlie rings takes place when the 

 azimuths of the rhomb are 0°, 90°, ISO'', and 270°, ihewJnte- 

 centred rings appearing in \\\e first and thinl, and the blaclc- 

 centrcd ones in the second ivui\ fourth cjuadrant. 

 3. Azimuth of polarized light 90°. 



Incidence of polarized light 68° 3', the polarizing angle of 

 realgar. 



At this angle all the light reflected from the realgar has 

 disappeared, excepting a dark bluish purple, in the middle of 

 which is seen, without using the rhomb, a splendid system of 

 richly-coloured rings, with a white centre. When the rhomb 

 is applied as before, and performs a complete revolution, the 

 white-centred rings are seen all round, disappearing at 90° 

 and 270°. 



4-. When the azimuth of the polarized light varies from 90° 

 to 0°, the incidence being 68° 3', as before. 



At 90° azimuth, the phaenomena are as above described. 

 At 80°, and all other azimuths, the xvhite-centred rings are 

 seen when the rhomb is at 0°; but they disappear at azimuths 

 of the rhomb a little less than the azimuths of polarization, 

 and are then succeeded by the black-centred rings. 



At 0° azimuth, the rings disappear when the rhomb is at 

 0° and 180°, and are black-centred all round. 



Without using the rhomb, the rings always disappear at 

 the azimuth ,r, at which the planes of polarization of the inter- 

 fering pencils are rectangular. 



At incidences above 68° 3', the phaenomena are of the same 

 character. The rings are 'white-ccntred in 90^ of azin)uth, and 

 when the rhomb is at 0°. They become very brilliant about 

 45°. Near 90' of rotation the rings vanish, and immediately 

 the black-centred system appears, which quickly vanishes, 

 and is succeeded by the white-centred system. 

 5. Angles of incidence less than 56° 5'. 

 In 90° of azimuth of the polarized ray, and the rhomb being 

 at 0°, the black-centred rings are seen, and continue to be 

 seen during a complete revolution of the rhomb. In all azi- 

 muths, from 90° to 0°, the rings disapj)ear by turning the 

 rhomb to the lelt, the arch diminishing from 90° to 0°; but in 

 azimuths of an intermediate magnitude, the disappearance of 

 the rings is followed by the appearance of the tsohite-centred 



