THE MIRROR, AND EYE-PIECE. 47 



the object or objective will be prevented. When 

 about to be used it is simply necessary to place it on 

 the fixed stage of the microscope. The object about to 

 be examined is supported and kept in place by a couple 

 of clips or projecting springs. Should a tyro in the 

 use of the instrument hastily rack down the body, all 

 undue force is broken by the elasticity of the springs. 

 Messrs. Watson and Son, of Holborn, manufacture a 

 light and elegant form in ebonite of this accessory 

 safety-stage. 



The Mirror. The mode in which an object is illu- 

 minated is, in the words of Andrew Ross, " second only 

 in importance to the excellence of the glass through 

 which it is seen." To ensure good illumination the 

 mirror should be in direct co-ordination with the 

 objective and eye-piece; it must be regarded as a 

 part of the same instrument, and tending by a com- 

 bined series of acts to a common result. Illumination 

 is spoken of as of three kinds or qualities reflected, 

 transmitted, and refracted light. For the illumination 

 of transparent objects, transmitted light is brought 

 into use ; for opaque objects, reflected. The trans- 

 mitted illuminating pencil should be as large as can 

 well be received by the lens, and no larger. Any 

 light beyond this is liable to produce confusion of 

 image. In using the mirror the reflected light can be 

 made brighter, more concentrated, by employing a 

 bull's-eye condensing lens. 



The Eye-piece. The eye-piece of the compound 

 microscope consists of two plano-convex lenses; that 

 furthest from the eye, as I have already explained, is 

 the field-glass, and that nearest the eye is the eye- 

 glass. The former increases the field of vision, 

 the latter magnifies, the enlarged inverted image. 

 Combined together, the two materially assist in cor- 

 recting residual imperfections of the objective. The 

 magnifying power of the microscope depends in a 

 measure, then, upon the eye-piece, but the limit of 

 usefulness in this direction is soon reached, for, 

 although the size of the image is thereby increased, 

 this increase is achieved at the expense of per- 



