exhibited by Polarized Light. 467 



2. If the polarizer and analyser be rotated with equal velo- 

 city in opposite directions, that system of rings is seen which 

 is obtained when both are in repose, and inclined to each 

 other at an azimuth of 45°. This is particularly distinct when 

 the light is rendered monochromatic by using a flame of spirit, 

 coloured yellow by common salt. The cause of this phaeno- 

 menon, which appears so remarkable at first, is at once seen 

 when we recollect that, when one apparatus only rotates, the 

 analysing or the polarizing, during an entire revolution, the 

 arms of the white cross fall twice as often on the same spot as 

 those of the black, and the dark lines of interference alternate 

 with the bright lines of coincidence exactly in the same spots. 

 When however both apparatus rotate in opposite directions, 

 the system of rings with the white cross appears at the points 

 0°, 90°, 180°, 270°, whilst the system of rings with the black 

 cross corresponds to the points 45°, 135°, 225° and 315°. As 

 the bright arms of the white coincide with the coloured spaces 

 of the dark cross, the impressions of the two are added, and 

 we obtain the appearance above stated. If the rapidity of 

 rotation of both rollers is exactly the same, the figure appears 

 motionless ; but if the rapidity of rotation of the two is slightly 

 different, it slowly revolves, because the points of coincidence 

 gradually alter. 



3. If tiie polarizing and analysing apparatus revolve rapidly 

 with the same velocity in the same directions, the linear system 

 of rings becomes converted into that which is obtained when 

 circularly polarized light is circularly analysed. If, before 

 rotation, the Nichol's prisms are so placed that in calcareous 

 spar the black cross is visible, on rotation we obtain the New- 

 Ionian reflected rings with a black centre, but without the cross ; 

 but if, when at rest, the white cross is visible, we then see, on 

 rotation, the Newtonian transmitted rings with a white centre. 

 If the polarizing and analysing apparatus do not rotate with the 

 same rapidity, we see the remarkable phaenomenon, that the re- 

 flected Newtonian rings with the black centre, and the trans- 

 mitted ones with the white centre, constantly alternate in 

 definite periods in the same spot of the field. 



4. If a circularly polarizing apparatus be rotated in an op- 

 posite direction to the linear analyser, the same phaenomenon 

 is seen as when a linear polarizer and a linear analyser re- 

 volve in opposite directions, but it is less distinct (2). 



5. If the circularly polarizing apparatus be rotated, and 

 the linear analyser be kept at rest, we obtain the system of 

 rings appearing at the quadrants exactly on rotation as on 

 repose. 



6. If the circularly polarizing apparatus be kept at rest, 



2 I 2 



