180 THE MICROSCOPE. 



polished silver surface, having the apex so far cut away aa 

 to bring the focal point at such a distance above the top 

 of the apparatus (which is closed with a screw-cap when 

 not in use) as may allow the rays to pass through the 

 thickest glass commonly used for mounting objects upon 

 before coming to a focus. 



" At the base of the parabola is placed a disc of thin 

 glass b b, in the centre of which is cemented a dark well, 

 with a flange equal in diameter to the aperture at the top 

 of the reflector, for the purpose of preventing the direct 

 rays from the source of light passing through the 

 apparatus. 



" The reflector is moved to and from the object by 

 means of the rack and pinion c, and has similar adjust- 

 ments for centering, and is fixed under the stage of the 

 microscope in the same way as the ordinary achromatic 

 condenser: in addition, there is a revolving diaphragm d, 

 made to slide on the bottom tube of the apparatus ; it has 

 two apertures e e, placed diametrically, for the purpose of 

 obtaining two pencils of oblique light in opposite direc- 

 tions. The effects of the chromatic and spherical aber- 

 rations, in the shape of fog and colour about the objects, 

 caused by the glass slides upon which they are mounted, 

 frequently require compensation ; for as the parabola has 

 the property of throwing parallel rays uncoloured to a 

 point, when used alone, it is most suitable for objects 

 without glass underneath. 



" By the addition of a meniscus, this compensation is 

 obtained, and also greater purity and intensity of illumina- 

 tion is procured ; and as the silver reflector is now closed 

 with glass, it is hermetically sealed, and permanently pro- 

 tected from dust and damp, and will therefore retain its 

 polish. The light most suitable for this method of illumi- 

 nation is lamp or caudle light, the rays of which must in 

 all cases be rendered parallel by means of a large plano- 

 convex lens, or condenser; the light may then be used 

 direct, or reflected from the plane mirror. The object 

 having been adjusted, the illuminator is moved to and fro 

 till the best effect is produced. For the purpose of viewing 

 some objects, such as naviculae, the circular diaphragm 

 should be slid on the extremity of the apparatus, and 



