118 THE MICROSCOPE AND ITS REVELATIONS. 



ject must be limited by that of the speculum, so as to allow the rays to 

 pass to its marginal portion; and third, that a special mode of mounting 

 is required, to allow the light to be reflected from the mirror around the 

 margin of the object. The first objection may be in some degree removed 

 by turning the mirror considerably out of the axis, so as to reflect its 

 light obliquely upon the Lieberkuhn, which will then send it down 

 obliquely upon the object (Fig. 90, B); or by covering one side of the 

 Lieberkuhn by a diaphragm, which should be made capable of rotation, 

 so that light may be reflected from the uncovered portion in every azi- 

 muth: the illumination, however, will in neither case be so good as that 

 which is afforded with powers up to 2-3ds inch, by the Parabolic Spec- 

 ulum just described. The mounting of Opaque objects in wooden slides 

 (Fig. 124), which affords in many cases the most convenient means of 

 preserving them, completely prevents the employment of the Lieberkuhn 

 in the examination of them; and they must be set for this purpose either 

 upon disks which afford them no protection, or in cells ( 169) with a 

 blackened background. The cases wherein the Lieberkuhn is most use- 

 ful, are those in which it is desired to examine small opaque objects, 

 such as can be held in the Stage-Forceps ( 118) or mounted on small 

 disks ( 119), or laid upon a slip of glass, with objectives of half-inch 

 focus or less; since a stronger light can be thus concentrated upon them, 

 than can be easily obtained by side-illumination. In every such case, a 

 black background 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 Lieberkuhn is com- 

 monly 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 pur- 

 poses, a spot of black paper or black varnish will answer the required 

 purpose 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. 



116. Vertical Illumination for High Powers. Various attempts have 

 been made by Mr. Wenham and others to view opaque objects under 

 powers too high for the advantageous use of the Lieberkuhn, by employ- 

 ing the Objective itself as the illuminator, light being transmitted into "it 

 downwards from above. By Prof. H. L. Smith, of Geneva College, U. 

 S., a pencil of light admitted from a lateral aperture above the objective, 

 was reflected downwards upon the object through its lenses, by means of 

 a small silver speculum placed on one side of its axis and cutting off a 

 portion of its aperture. By Messrs. Powell and Lealand, a piece of plane 

 glass was placed at an angle of 45 across a tube placed like an adapter 

 between the Objective and the body of the Microscope; and whilst a pencil 

 of light, entering at the side aperture and striking against this inclined 

 surface, is reflected by it downwards through the objective on to the 

 object, the rays proceeding upwards from the object pass upwards (with 

 some loss by reflection) through the plane glass into the body of the 

 Microscope. For this fixed plate of glass, Mr. R. Beck substituted a 

 disk of thin glass attached to a milled-head (Fig. 91, 'B), by the rotation 

 of which its angle may be exactly adjusted; and this is introduced by a 

 slot (shown at e, Fig. 91, A) into the interior of an adapter that is inter- 

 posed between the objective (c, d) and the nose (c) of the Microscope. 



