LIEBERKUHN. VERTICAL ILLUMINATOR. 



125 



focus or less ; since a stronger light can he thus concentrated upon 

 them, than can be easily obtained by side-illumination. In every 



Fig. 73. 



such case a black back-ground must be provided of such a size as 

 to fill the field, so that no light shall come to the eye direct from 

 the Mirror, and yet not large enough to create any unnecessary 

 obstruction to the passage of the rays from the mirror to the 

 speculum. With each Lieberkiihn is commonly provided a blackened 

 stop of appropriate size, having a well-like cavity, and mounted 

 upon a pin which fits into a support connected with the under side 

 of the stage ; but though this ' dark well' serves to throw out a 

 few objects with peculiar force, yet, for all ordinary purposes, a 

 spot of black paper or black varnish will answer the required pur- 

 pose very effectually, this spot being either made on the under side 

 of the cell which contains the object, or upon a separate slip of 

 glass laid upon the stage beneath this. 



93. Beck's Vertical Illuminator— Various attempts have been 

 made by Mr. Wenham and other Opticians to view Opaque objects 

 under powers too high for the advantageous use of the Lieberkiihn, 

 by employing the Objective itself as the illuminator, light being 

 transmitted into it downwards from above. By Professor Smith, of 

 Kenyon College, U.S., a pencil of light admitted from a lateral 

 aperture above the Objective, is reflected downwards upon the object 

 through its lenses, by means of a small silvered speculum placed 

 on one side of its axis and cutting off a portion of its aperture. By 



