56 SIR DAVID BREWSTER ON THE PHENOMENA OF THIN PLATES 



At 67 J°. Rhomb 0°, no rings. 



Rhomb turning to the right, the white-centred rings appear, then vanish, when the 

 azimuth of the rhomb is less than 67i° ; then black -centred rings appear, which 

 vanish at 180°; then succeed the white-centred ones, which vanish at about 210°; 

 then the black-centred, which continue to 360°. 



At 45°, 22J°. The very same phenomena appear at these and other azimuths, the 

 azimuths of the rhomb at which the rings disappear out of the plane of incidence 

 being a little less than the azimuths of the polarized light. 



At 0°. The evanescence of the rings takes place when the azimuths of the rhomb 

 are 0", 90°, 180°, and 270°, the white-centred rings appearing in the Jlrst and third, 

 and the black-centred ones in the second and fourth quadrant. 



3. Azimuth of polarnzed 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 splen- 

 did system of richly-coloured rings, with a ivhite 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. IVhen the azimuth of the polarized light varies from 90° to 0°, the incidence 

 being 68° 3', as before. 



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



At 80°, and all other azimuths, the white-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 x, at which 

 the planes of polarization of the interfering pencils are rectangular. 



At incidences above 68° 3', the phenomena are of the same character. The rings 

 are white-centred in 90° of azimuth, 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 i?icidence 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 azimuths, from 90° to 0°, the rings disappear by turning the rhomb to the left, 

 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 white-centred 

 system, which quickly disappears, and is succeeded by the black-centred si/stem. This 

 phenomenon is seen best near 45° of azimuth. 



