THE MICROSCOPE. 25 



be traced on the paper in the same way as the 

 outline of an object: the 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 

 arrangements for concentrating the light on the 

 objects which are placed for examination under 

 the Microscope. 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 

 a 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 Lieberkuhn will 

 be easily understood from the following diagram : 

 a represents the objective with the Lieberkuhn in 

 situ ; 6, 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 at / (the object). To obtain the full 

 effect of this mode of illumination the mirror should 



