114 



THE MICROSCOPE AND ITS REVELATIONS. 



Ens, 



(5) an upper arm carrying two revolving cells geared together by fine 

 teeth (one of them shown at D, while the other is under the condenser), 

 so that a revolving motion may be given to either by acting on the other; 

 one of these cells carries a plate of Mica, the revolution of which over the 



selenite-films gives a great variety 

 of color-tints with Polarized light; 

 while the other serves to receive 

 oblique-light disks, to which rota- 

 tion can be given by the same 

 means. The special advantage of 

 this Condenser lies in its having 

 the polarizing 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 Secondary body 

 of First-class Microscopes is able to 

 afford. It may be specially recom- 

 mended to such as make much use 

 of Polarized light. 



113. Illuminators for Opaque 

 Objects. All objects through 

 which sufficient light cannot be 

 transmitted to enable them to be 

 viewed in the modes already de- 

 scribed, require to be illuminated 

 by rays, which, being thrown upon 

 the surface under examination, 

 shall be reflected from it into the 

 Microscope; and this mode of view- 

 ing them may often be advantage- 

 ously adopted in regard to semi- 

 transparent or even transparent 



Swift's combination Sub-stage. objects,^ or the sake of the diverse 



aspects it affords. Among the va- 

 rious methods devised for this purpose, the one most generally adopted con- 

 sists in the use of a Condensing Lens (Fig. 86), either attached to the 

 Microscope, or mounted upon a separate stand, by which the rays pro- 

 ceeding from a lamp or from a bright sky are made to converge upon the 

 object. For the efficient illumination of large opaque objects, however, 

 it is desirable to employ a nil's eye Condenser (which is a plano-convex 

 lens of short focus, two or three inches in diameter), mounted upon a 

 separate stand, in such a manner as to allow of being placed in a great 

 variety of positions. The mounting shown in Fig. 87, is one of the best 

 that can be adopted: the frame which carries the lens is borne at the bot- 

 tom upon a swivel joint, which allows it to be turned in any azimuth; 

 whilst it may be inclined at any angle to the horizon, by the revolution of 

 the horizontal tube to which it is attached, around the other horizontal 

 tube which projects from the stem; by the sliding of one of these tubes 

 within the other, again, the horizontal arm may be lengthened or short- 

 ened; the lens may be secured in any position (as its weight is apt to drag 



