124 APPENDAGES TO THE MICROSCOPE. 



fitted the ring b, which turns easily round it, and carries the hori- 

 zontal arm c c, jointed at each end ; from this hangs vertically the 

 stem d, which can be lengthened or shortened at pleasure ; and to the 

 lower end of this the Speculum f is attached by the ball-and-socket 

 joint e. This arrangement may be used not only with the Objec- 

 tives already named, but alsoAvith those of one-half or4-10ths inch 

 focus, if these do not approach the object so nearly as to interfere 

 with the reflection of the illuminating rays from the Speculum. 



92. Lieberkuhn. — A mode of illuminating Opaque objects by a 

 small concave Speculum reflecting directly down them upon the 

 light reflected up to it from the Mirror, was formerly much in use, 

 but is now comparatively seldom employed. This concave Speculum, 

 termed a 'Lieberkuhn,' from the celebrated Microscopist who 

 invented it, is made to fit upon the end of the Objective, having a 

 perforation in the centre for the passage of the rays from the object 

 to the lens ; and in order that it may receive its light from the 

 Mirror beneath (Fig. 73, 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 

 Object-glass, 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 con- 

 sequently 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 limited by that 

 of the Speculum, so as to allow the rays to pass to its marginal por- 

 tion ; 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 

 clown obliquely upon the object (Fig. 73, 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 azimuth : the illumination, however, will in neither 

 case be so good as that which is afforded, with powers up to 2-3rds 

 inch, by the Parabolic Speculum just described. The mounting of 

 Opaque objects in wooden slides (Fig. 98), 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 (Fig. 106) with a blackened 

 back-ground. The cases wherein the Lieberkuhn is most useful are 

 those in which it is desired to examine small Opaque objects, such 

 as can be held in the Stage-Forceps (§ 94), or mounted on small disks 

 (§ 95), or laid upon a slip of glass, with Objectives of half-inch 



