POLARISING APPARATUS RINGS AND BRUSHES 



319 



scope does not readily admit of the connection of the selenite plate 

 with the polarising prism, it is convenient to make use of a plate of 

 brass (fig. 263) somewhat larger than the glass slides in which 

 objects are ordinarily mounted, with a ledge near one edge for 

 the slide to rest against and a large circular aperture into which 

 a glass is fitted, having a film of selenite cemented to it ; this 

 ' selenite stage ' or object-carrier being laid upon the stage of the 

 microscope, the slide containing the object is placed upon it, and, by 

 an ingenious modification contrived by Dr. Leeson, the ring into 

 which the selenite plate is fitted being made movable, one plate may 

 be substituted for another, whilst rotation may be given to the ring 

 by means of a tangent-screw fitted into the brass plate. The 

 variety of tints given by a selenite film under polarised light is so 

 greatly increased by the interposition of a rotating film of mica that 

 two selenites red and blue wilh a mica film, are found to give the 

 entire series of colours obtainable from any number of selenite 

 films, either separately or in combination with each other. 



The compact apparatus made by Swift as a general sub-stage 

 illuminator is useful and commendable, 

 and is capable of adaptation to most 

 English microscopes. It is shown in fig. 

 264. The special a, A vantage of this con- 

 denser lies in its having the polarising 



FIG. 263. 



prism, the selenite and mica films, the black ground and oblique- 

 light stops, and the moderator all brought close under the back lens 

 of the achromatic ; whilst it combines in itself all the most important 

 appliances which the sub-stage of a good moderate microscope can 

 require. 



Rings and Brushes.- Mr. Nelson has pointed out (' Journ. 

 II. M.S..' 1892) that it is remarkable the microscopical text-books 

 give no account of the method of viewing the rings and brushes 

 which certain minerals show under polarised light. If the instru- 

 ment be set up as if for viewing ordinary polariscope objects, not a 

 ring or a brush will be seen. 



The whole point lies in the fact that it is a wide-angled telescope 

 that is required, and not a microscope. Once this is recognised the 

 whole matter is simple. As the microscope has to be turned into a 

 wide-angled polarising telescope, all that is necessary is to screw a low 

 power on the end of the draw-tube, as in fig. 265 . As^the light requires 

 to be passed through the crystal at a considerable angle, a wide- 

 aiigled condenser should be employed, but it need not be achromatic. 



