ACCESSORY APPARATUS. 



107 



directly through it, whilst it receives adequate illumination from the 

 light around, its form is clearly discerned, and the two surfaces are dis- 

 tinguished without the least difficulty. 



104. A simple method of obtaining c black-ground ' illumination, 

 which works well with objectives of low power and small angular aper- 

 ture, consists in fixing into the top of a short tube that slides into the 

 ( cylindrical fitting ' usually carried beneath the stage in Educational and 

 Students' Microscopes, a small ' bull's eye' lens, the plane surface of 

 which (placed uppermost) has its central portion covered by a black spot. 

 When light reflected by the mirror falls on the lower surface of this Spot- 

 Lens, only the rays that fall on its marginal ring are allowed to pass; 

 and these, owing to its high curvature, are so strongly refracted inwards, 

 as to cross each other in the object (when the lens is focussed for it), and 

 then diverge again at an angle sufficiently wide to pass beyond the mar- 

 gin of the objective, like those transmitted by the Paraboloid to be pres- 



Parabolic Illuminator. 



ently described (Fig. 79, F o, F H). Thus the field is left dark; whilst 

 the light stopped by the object gives it a luminosity of its own. The 

 same effect is gained by the use of the Webster Condenser ( 100) with a 

 central stop placed immediately behind the lower lens or upon the flat 

 surface of the upper. Neither of the foregoing plans, however, will 

 answer well for objectives of high power, having such large angles of 

 aperture that the light must fall very obliquely to pass beyond them alto- 

 gether. Thus if the pencil formed by the t spot-lens' have an angle of 

 50, its rays will enter a 4-10ths objective of 60, and the field will not 

 be darkened. 



105. A greater degree of obliquity, suited to afford * black-ground ' il- 

 lumination with Objectives of larger angular aperture, may be obtained 

 by the use of the Parabolic Illuminator^ (Fig. 78); which consists of a 



1 A Parabolic Illuminator was first devised by Mr. Wenham, who, however, 

 employed a Silver speculum for the purpose. About the same time, Mr. Shad- 

 bolt devised an Annular Condenser of Glass for the same purpose (see " Transact, 

 of Microse. Soc.," Ser. I , Vol. iii., 1852, pp. 85, 132). The two principles are com- 

 bined in the Glass Paraboloid. 



