ACCESSORY APPARATUS. 117 



but a much stronger light may be concentrated on it, when the Speculum 

 receives its rays from a lamp placed near the opposite side of the stage, a 

 1 bull's-eye ' being interposed to give parallelism to the rays. For the 

 sake of Microscopists who may desire to use this admirable instrument 

 with Objectives to which it has not been specially fitted, an adapter is 

 made by Mr. Crouch, consisting of a collar (Fig. 89, A) interposed between 

 the lower end of the body of the Microscope and the objective; on this is 

 fitted the rings, which turns easily round it, and carries the horizontal 

 arm c c, jointed at each end; whilst the stem D, which can be lengthened 

 or shortened at pleasure, hanging from this, carries at its lower end the 

 Speculum F attached to it by the ball-and-socket joint E. By this arrange- 

 ment the Parabolic Speculum may be used not only with the objectives 

 already named, but also with those of one-half or 4-10ths inch focus, if 

 these do not approach the object so nearly as to interfere with the reflec- 

 tion of the illuminating rays from the Speculum. 



115. Lieberlculm. A mode of illuminating opaque objects by a small 

 concave Speculum reflecting directly down upon them the light reflected 



Diagram of Lieberkuhn 



tip to it from the Mirror, was formerly much ;n use, but is now compara- 

 tively seldom employed. This concave Speculum, termed a ' Lieber- 

 kuhn ' from the celebrated Microscopist who invented it, is made to fit 

 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. 90, 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 

 rsys 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 conse- 

 quently required for every objective. 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 might 

 present, are but faintly or not at all seen; second, that the size of the ob- 



