136 TKANSACTIONS OF THE 



sides on tlie object, thus allowing of no shadows ; but this holds 

 only good so long as the mirror throws up rays parallel to the axis 

 of the microscope. Altogether, I consider the Lieberkuhn the 

 easiest, and an excellent piece of apparatus for the illumination of 

 opaque objects. Lister s dark laells are little blackened cups 

 supported at the end of wires, and if one is placed by means of an 

 adapter in the sub-stage, it intercepts the central rays from the 

 mirror, and the transparent object having now a dark ground, may 

 be viewed by means of the Lieberkuhn. 



The apparatus for viewing opaque objects, especially covered 

 ones under high powers, are not very satisfactory, i?. & J. Beck's 

 patent illuminator is one of the best, and consists in an adapter 

 which has to be screwed between the nose-piece and the object- 

 glass. It has a circular aperture, and a disc of very thin glass placed 

 under an angle of 45 degs. By its means the object-glass is made 

 its own illuminator, the light being reflected through the aperture 

 on the glass disc and from thence on the uncovered object, which 

 in its turn reflects the light back again through the object-glass in 

 the eye-piece. 



Transparent objects may be viewed either by transmitted or by 

 reflected light; the latter is called dark ground illumination. The 

 simplest way of viewing transparent objects is by means of the 

 plane mirror, on which parallel I'ays are thrown by the bull's-eye 

 condenser. For thick objects direct rays are necessary, but for 

 thin and lined objects oblique rays are essential ; they are easily 

 obtained by throwing the mirror out of the axis of the microscope. 

 When the mirror is so much out of the centre that the parallel 

 rays reflected from it pass so obliquely over the object that they do 

 not enter the object-glass, we gain dark ground illumination, the 

 object then appearing self-luminous and brilliantly lit up on a 

 black ground. As the mirror gives a double reflection, namely, 

 one from the outer surface, and one from the inner or silvered 

 side, and as these two sets of rays are not parallel, those having 

 passed through the glass being refracted, a rectangular prism is often 

 substituted for the mirror. Such a prism reflects the light totally, 

 and gives therefore more, and only parallel, rays ; it is generally 

 fitted to the tail-piece of the microscope, but it may also be 

 mounted on a stand similar to that of the bull's-eye condenser. 

 Two such prisms enable the microscopist, who has good object- 

 glasses but only a small and simple stand, to resolve the most 



