200 USE OF THE MICROSCOPE. 



light is effected by the Lieberkuhn, before described at 

 page 127 : this consists, as there stated, of a concave silvered 

 cup, adapted to a short tube that slides over the outer part of 

 the setting of the object-glass with which it is intended to be 

 used, and is so contrived that, when converging rays of light 

 are thrown on it by the concave mirror, they wiU be brought 

 to a focus on an object placed in the focus of the object-glass. 

 All the object-glasses, from the two inch to the one-fourth, are 

 provided with Lieberkuhns, but the one-eighth and one-six- 

 teenth approach so close to the object, that it would be useless 

 to apply one to them. The older microscopes were sometimes 

 fitted with only one Lieberkuhn for aU the magnifying powers, 

 and this was made to slide up and down upon the end of the 

 compound body, as described at page 22, so as to condense 

 the light upon any object placed in the focus of either of 

 the magnifiers, but it will be found far preferable to have a 

 Lieberkuhn adapted to each object-glass: the old way was 

 economical, but, as microscopes are now constructed, the plan 

 cannot well be adopted. 



"When it is required to use a Lieberkuhn, the diaphragm 

 plate should be removed from the under surface of the stage, 

 and an object-glass, with its accompanying Lieberkuhn, 

 having been screwed to the compound body, the object in- 

 tended to be viewed must either be placed on the stage-plate 

 or held in the forceps ; the light must then be thrown upon 

 the Lieberkuhn by the concave mu-ror, as shown in figs. 128 

 and 129, where A represents the end of the compound body, 

 B the object-glass, C the Lieberkuhn in section, D the 

 concave mirror, and E an object in the focus of the object- 

 glass. The converging rays, r r, &c., reflected from the mirror, 

 will be condensed upon the object E, which, if perfectly flat, 

 wiU be well exhibited, but if the surface be uneven, no part of 

 it wiU be correctly defined, in consequence of there being no 

 shadows. The method of illuminating then to be employed is 

 shown by fig. 129, where it wiU be seen that the rays, rr, &c., 

 from the mirror, are only thrown on one side of the Lieber- 

 kuhn, C ; these are reflected obliquely upon the object, E, and 

 no part of it being illuminated perpendicularly, a shadow 



