468 Prof. Dove on some Phenomena 



and the linear analyser be rotated, we obtain a faint system of 

 rings without a cross, with a gray centre surrounded by a 

 black circle. The direction of the rotation is here indifferent. 



7. If the circularly polarizing apparatus be rotated in the 

 same direction as the linear analyser, we obtain the appear- 

 ance, but fainter, which is seen when a linear polarizer and a 

 linear analyser are rotated in the same direction, the rotating 

 velocity not being the same; hence the alternating system of 

 rings (3). 



8. But if a circularly polarizing and a circularly analysing 

 apparatus be rotated, either in the same or in opposite direc- 

 tions, this has no influence on the appearance, which is per- 

 fectly identical with that which is seen when botli apparatus 

 are at rest. 



9. If a lamina of mica, circularly polarizing at an azimuth 

 of 45°, be introduced between the Nichol's prisms, and be kept 

 at rest whilst the polarizing Nichol's prism rotates, the systems 

 of rings neutralize each other on linear and circular analysis, 

 forming while light, in uni- and binaxial crystals and in rock- 

 crystal. 



10. If the lamina of the polarizing apparatus, which pro- 

 duces circular polarization at an azimuth of 45°, be rotated 

 between the Nichol's prisms which are at rest, we obtain as 

 the resultant of all the phaenomena of light polarized ellipti- 

 cally, circularly and rectilinearly in a single plane, on linear 

 analysis, the phaenomena of partly rectilinearly polarized light, 

 on circular analysis, those of a mixture of ordinary and cir- 

 cular light, i. e. in the first case the system of rings with a 

 dark cross, in the latter the system of rings which vanishes 

 at the quadrants ; they are however faint. 



In all the foregoing experiments the Nichol's prisms were 

 crossed before the rotation was commenced. 



11. It is so difficult to set two laminae of mica or two Ni- 

 chol's prisms either at an exactly equal or exactly opposite 

 rotating velocity by means of a band passing round a wheel, 

 that in those experiments in which the laminae of mica being 

 kept at rest, the Nichol's prisms rotate in the same or in op- 

 posite directions, or in which the Nichol's prisms being at rest, 

 the laminae of mica rotate, the relative position of the laminae 

 or of the Nichol's prisms is constantly becoming somewhat 

 changed, and hence the phsenomena of elliptical become mixed 

 with those of the circular light. Hence, for the production of 

 these phaenomena, the motion must be effected by wheel-work. 



12. Light, originally polarized rectilinearly, when trans- 

 mitted through a rotating Nichol's prism and then analysed, 

 yields light partially polarized in the original plane. 



