Mr. W. Spottiswoode on a new Revolving Polariscope, 473 



with a quartz plate. The order of the tints will be that due to 

 a right-handed quartz for one position of the quarter-undulation 

 plate, and will be that due to a left-handed quartz for a position 

 at right angles to the first. 



In order that this effect may be successfully produced, the 

 thicknesses of the seleniteand quarter-undulation plate must be 

 adapted to one another. The latter is usually constructed for 

 the yellow rays ; and with such plates we should consequently 

 use a selenite giving yellow and blue images. 



If the quarter-undulation plate be used with a quartz per- 

 pendicular to the axis, the tints may be reduced to a pair of com- 

 plementaries, as given by a selenite. 



To these many other and varied experiments may be added, 

 but on the present occasion I will confine my remarks to two of 

 them. 



In the cases hitherto described the central image appears 

 colourless and uniformly illuminated, while the excentric image 

 or ring has been the chief subject of observation. But if instead 

 of a plane plate we use quartz wedges giving Savarfs bands, 

 or a concave quartz cut parallel to the axis, the central image 

 will be the most interesting. We shall then see light and dark 

 (or coloured and colourless) bands taking alternate places at 

 each half revolution of the analyzer. The ring shows no very 

 striking feature, if a quarter- undulation plate be used, the 

 bands will be seen to travel across the plate, in one direction 

 when it is placed so as to produce right-handed, or in the other 

 when placed so as to produce left-handed circular polarization. 



Analogous effects are seen with a concave quartz plate cut 

 parallel to the axis. The central figure then shows rings, which 

 with a quarter-undulation plate expand or contract according to 

 the position of that plate. When, however, the analyzer revolves 

 rapidly, it will be noticed that the rings assume the form of spi- 

 rals. This is due to the fact that the central image, when pro- 

 duced by a circularly concave plate, is not accurately circular, 

 but elliptical, owing to the unequal refraction of the double- 

 image prism in two rectangular directions. To the same cause 

 is attributable the apparent wabbling of the central image, even 

 when the instrument is in perfect adjustment. 



The principle of the revolving analyzer is applicable alike to 

 a table polariscope for eye-observations and to one constructed 

 for projection. In the table polariscope it is used above the 

 analyzer, a diaphragm being placed immediately over the object 

 on the stage of the instrument ; in that for projection it may 

 be placed in the focus of the focusing-lens of the system. 



