

THE MICROSCOPE. 25 



dimensions of the latter can now be ascertained. 

 Care must, however, be taken that the magnifying 

 power is the same in both cases. 



Amongst the accessory apparatus are various ar- 

 rangements for concentrating the light on the objects 

 which are placed for examination under the Micro- 

 scope. One of these combinations is called the 

 achromatic condenser. This consists of a series of 

 lenses, which are placed between the mirror and the 

 stage, and which may consist of an ordinary object- 

 glass. The stages of the larger kinds of Microscopes 

 are fitted up with a screw or slide, by which the 

 condenser can be fastened beneath and adjusted to 

 the proper focus for throwing light on the object 

 examined. 



The illumination of opaque objects by means of 

 the bull's-eye condenser is sufficient when only the 

 lowest powers are used; but when any objective of 

 less than inch-and-half focus is used this method 

 of illumination is not satisfactory, and a form of 

 reflector called a Lieberkuhn will be found to be a 

 welcome addition to the Microscope. This instru- 

 ment consists of a concave silvered speculum with 

 $ central aperture of the diameter of the front lens 

 of the objective : a short tube is attached to the 

 convex surface of the reflector, which slides over the 

 object-glass. The action of the Lielerkuhn will be 

 easily understood from the following diagram : a 

 represents the objective with the Lieberkuhn in situ ; 

 b, the concave reflector; c, a stop for the purpose of 

 preventing any direct light entering the objective (a 

 small disk of black paper attached to the slide is 

 generally sufficient) ; d, d, rays of light from the 

 mirror ; e, e, reflected rays converging to a focus aty 

 (the object). To obtain the full effect of this mode 

 of illumination the mirror should be placed a little 

 out of the axis of the tube of the Microscope. By 



