140 ACCESSORY APPARATUS. 



the microscope, and capable of being adjusted to the different 

 distances required by the several objectives, a separate speculum 

 is required for every object-glass. The disadvantages of this 

 mode of illumination are chiefly these : first, that by sending 

 the light down upon the object almost perpendicularly, there is 

 scarcely any shadow, so that the inequalities of its surface, and 

 any minute markings which it may present, are but faintly or 

 not at all seen; second, that the size of the object must be so 

 limited by that of the speculum, 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 Lieberkiihn, 

 which will then send it down obliquely upon the object ; the 

 illumination, however, will not even then be so good as that 

 which is afforded by the side-reflector. The mounting of opaque 

 objects in wooden slides (Chapter Y), which affords in many 

 cases the most convenient means of preserving them, completely 

 prevents the employment of the Lieberkiihn in the examination 

 of them; and they must either be set, for this purpose, upon 

 disks which afford them no protection, or in glass cells with a 

 blackened background. The cases wherein the Lieberkiihn is 

 most useful, are those in which it is desired to examine small 

 opaque objects, such as can be held in the stage-forceps ( 66), 

 or laid upon a slip of glass, with lenses 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 ob- 

 struction to the passage of the rays from the mirror to the specu- 

 lum. 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 the " dark well" serves to throw 

 out a few objects with peculiar force, yet, for all ordinary pur- 

 poses, a spot made with black paper or black sealing-wax- varnish 

 upon a slip of glass will answer the required purpose very 

 effectually, the slip being simply laid upon the stage beneath 

 the object. 



SECTION 2. APPARATUS FOR THE PRESENTATION OF OBJECTS* 



66. Stage-Forceps. Every Microscope should be furnished with 

 a pair of Stage-forceps (Fig. 47) for holding minute objects 

 beneath the object-glass. They are mounted by means of a joint 

 upon a pin, which fits into a hole either in the corner of the stage 

 itself, or in the object-platform ; the object is inserted by pressing 

 the pin that projects from one of the blades, whereby it is sepa- 



