142 ACCESSOEY APPARATUS. 



the object; and it is advantageous that this should be made 

 to revolve. A very convenient mode of eiFecting this, is to 

 mount the selenite plate in a revolving collar, which fits into the 

 upper end (a) of the tube (Fig. 41, b) that receives the polarizing 

 prism. In order to obtain the greatest variety of coloration 

 with different objects, films of selenite of diflerent thickness 

 should be employed; and this may be accomplished by sub- 

 stituting one for another in the revolving collar. A still greater 

 variety may be obtained by mounting three films, which sepa- 

 rately give three different colors, in a frame resembling that in 

 which hand-magnifiers are usually mounted, so that they may 

 be used singly or in double or triple combinations ; as many as 

 thirteen different tints may thus be obtained ; but the advantage 

 of revolution is sacrificed. When the construction of the 

 microscope does not readily admit of the connection of the 

 selenite plate with the polarizing prism, it is convenient to make 

 use of a plate of brass (Fig. 43) somewhat larger than the glass 



slides in which objects are 

 Fw- 43. 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" 



Selenite Object-Carrier. or obj ect-carricr being laid 



upon the stage of the micro- 

 scope, and the slide containing the object being placed upon it, 

 the effect of the selenite is obtained, as in the previous arrange- 

 ment ; 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. Such a " selenite stage" answers 

 every purpose that can be required ; but as there is no provision 

 for using two or three plates in combination, it is necessary to 

 have a distinct selenite plate for every modification of colors 

 that may be desired. 



A very beautiful effect may be obtained with certain kinds of 

 semi-opaque objects, by illuminating them by means of a " spotted 

 lens" (§ 61), with a polarizer of Herapathite placed at such a dis- 

 tance above it as to receive the converging hollow pencil near its 

 termination in the object, and an analyzer of the usual descrip- 

 tion, — a combination devised by Mr. Furze ;' for the solidity 

 which this mode of oblique illumination imparts to certain ob- 

 jects, is remarkably heightened by the play of colors afforded by 

 the polarization of the light. When the polarizing apparatus is 

 being employed with any but the lowest powers, it is very advan- 



' " Transactions of the Microscopical Society" (2d series), vol. iii, p. 63. 



