ACCESSORY INSTRUMENTS. 



117 



Fig. 62. 



%ht reflected from a metallic surface is preferred for this 

 method of illumination/on account of its purity. Fig. 62 re- 

 presents a section of 

 the apparatus, drawn 

 half the size of the 

 original, a a is a trun- 

 cated parabolic reflec- 

 tor, with a polished 

 silver surface; at the 

 apex of the reflector is 

 placed a meniscus, b, 

 of a focus and curva- 

 ture suitable for cor- 

 recting the aberrations 

 caused by the plate of 

 glass under the object. 

 At the base of the 

 parabola is a disc of 

 glass, "c c, in the centre of which is cemented a dark well, d, 

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

 the reflector. The dark well is less in diameter than the 

 flange, and has a eliding adjustment, by which it is raised till 

 the fleld appears dark under the highest powers; therefore 

 the aperture of the illmninator must exceed that of any of the 

 object-glasses. The reflector is moved to or from the object 

 by means of the rack and pinion, e, and has similar adjust- 

 ments for centring, and is fixed under the stage of the micro- 

 scope in the same way as the ordinary achromatic condenser. 

 In addition there is a revolving diaphragm, _^ made to shde on 

 the bottom tube of the apparatus ; it has two apertures, ff g, 

 placed diametrically opposite, for the purpose of obtaining two 

 pencils of oblique light in opposite directions, which is useful 

 for viewing some test objects. Before use, the axis of the 

 illuminator must be made to coincide with that of the object- 

 glass ; to effect which, fix the apparatus under the stage, and 

 move the lateral or longitudinal adjusting screws, till the hole 

 in the centre of the cap, which screws on the top of the re- 

 flector, is in the centre of the field of view, using the inch 



