62 



THE MICROSCOPE. 



fig. 41. 



" The apparatus is shown in section in fig. 41 : a a is a, parabolic 

 ...^ reflector, of a tenth of an 



inch focus, with a po- 

 lished silver surface, hav- 

 ing the apex so far cut 

 away as to bring the focal 

 point at such a distance 

 above the top of the ap- 

 paratus (which is closed 

 with a screw -cap when 

 not in use) as may allow 

 the rays to pass through 

 the thickest glass com- 

 monly used for mounting 

 objects upon before com- 

 ing 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 dia- 

 meter 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 adjustments- 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 aper- 

 tures e e, placed diametrically, for the purpose of obtaining two pencils 

 of oblique light in opposite directions. The effects of the chromatic 

 and spherical aberrations, in the shape of fog and colour about the ob- 

 jects, caused by the glass slides upon which they are mounted, fre- 

 quently 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 illumination is procured ; and as 

 the silver reflector is now closed with glass, it is hermetically sealed, 

 and permanently protected from dust and damp, and will therefore re- 

 tain its polish. The light most suitable for this method of illumination 

 is lamp or candle 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 ob- 



