ACCESSORY APPARATUS. 



113 



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 Polarized 

 light, is so greatly increased by the interposition of a rotating film of 

 Mica, that two Selenites red and blue with a Mica-film, are found to 

 give the entire series of colors obtainable from any number of Selenite- 

 films, either separately or in combination with each other. The Revolv- 



Selenite Object-carrier. 



Barker's Selenites, as fitted by 

 Messrs. Beck. 



ing Mica- Selenite Stage (Fig. 84) devised by Mr. Blankly, and made by 

 Mr. Swift, furnishes a very simple and effective means of obtaining these 

 beautiful effects; the Mica film being set in a diaphragm which can be 

 made to rotate by applying the finger at the front edge of the stage; 

 whilst the two Selenites are so placed in a slide, that either of them can 

 be brought under the aperture as desired. 



Em. 84. 



Blankley's Revolving Mica-Selenite Stage. 



112. Swift's Combination Sub-stage. In this ingenious piece of ap- 

 paratus (Fig. 85) are combined the advantages of (1) an Achromatic 

 Condenser, A, centred by two milled-headed screws, c c, and having an 

 angle of 140, which fits it for use with Objectives of very wide angular 

 aperture, whilst, by removing the upper combination, it is made to suit 

 lower powers; (2) a contracting Diaphragm worked by the lever B; (3) a 

 revolving Diaphragm, E, with four apertures, into which can be fitted 

 either (a) a series of three central stops, giving a Black-ground illumina- 

 tion scarcely inferior to that of the paraboloid, and capable of being used 

 with the small angled l-5th, (b) tinted or ground-glass Moderators, or 

 (c) two Selenite-films for the Polarizing apparatus; (4) a Polarizing prism, 

 F, mounted on an eccentric arm, so as to be brought under the axis of 

 the condenser when not in use, and thrown out when not wanted; and 

 8 



