LIEBERKUHN ITS DRAWBACKS 335 



formerly much in use, but is now comparatively seldom employed. 

 This concave speculum, termed a ' Lieberkiilm,' from the celebrated 

 microscopist who invented it, is made to tit upon the end of the 

 objective, having a perforation in its centre for the passage of the 

 rays from the object to the lens; and in order that it may receive 

 its light from a mirror beneath (fig. 282, A), the object must be so 

 mounted as only to stop out the central portion of the rays that are 

 reflected upwards. The curvature of the speculum is so adapted to 

 the focus of the objective that, when the latter is duly adjusted, the 

 rays reflected up to it from the mirror shall be made to converge 

 strongly upon the part of the object that is in focus ; a separate 

 speculum is consequently required for every objective. 



It has two manifest drawbacks : the first one, that of requiri m/ u 

 separate Lieberkilhn for each objective, is a difficulty which in the 

 nature of things cannot be overcome. The radius of the Lieberkiihn 



Fir;. 282. 



must alter with the focus of the objective employed, and each should 

 have a certain amount of play on the objective to allow for slight- 

 alterations of focus ; for if we employ parallel rays it is obvious that 

 the Lieberkiihn will focus nearer to the object than if divergent 

 rays are used. This is met by an allowance being made to com- 

 pensate it on the tube which slides the Lieberkiihn on to the nose 

 of the objective. 



The second <lr<nrli<ick has reference to the special n-mj in n-hich 

 objects have to be mounted in order to be suitable for the Lieberkiihn. 

 This could be easily avoided if professional and other mounters 

 would attend to the following simple suggestions: 



1. Slides should never be covered with paper : it is without use, 

 and fails as an ornament ; and opaque glass slips should be entirely 

 avoided. 



2. The ring of cement should not be made of greater width than 

 is necessary for security. 



