114 THEORY OF THE MICROSCOPE. 



manufactured. The Wenham paraboloid, 1 and other devices, which 

 have been popularized more especially in England, agree with it in 

 providing side illumination of greater or less extent, though only 

 within rather narrow limits of inclination. This applies also to 

 the apparatus of Abbe, above mentioned, as employed for illumin- 

 ation with reflected light. In this case the peripheral rays of 

 about 60 48 inclination are totally reflected at the upper surface 

 of the thin cover-glass upon the object ; those of less inclination 

 must be partially stopped off; when immersion objectives are em- 

 ployed, total reflexion no longer takes place. On the other hand, we 

 have in the Lieberkiihn mirror a counterpart of Fig. 50, since it pro- 

 duces exactly the radiation corresponding to the angle q p n, and 

 therefore represents that part of the sky which in our case was 

 shut out. This effect is obtained by means of a small concave 

 mirror perforated in the centre, and sliding on the end of the 

 objective ; the front lens of the objective fills the opening in the 

 mirror, and thus completes the reflecting surface. 2 The ordinary 

 illuminating mirror reflects the light through a wide aperture in 

 the stage to the Lieberkiihn, the central rays being stopped off by 

 a suitable opaque disc. The curvature of the Lieberkiihn must be 

 such, that the pencils from the object-points, if produced backwards, 

 fall upon the illuminating mirror, which is only possible if the foci 

 of the Lieberkuhn and the objective lie near to each other, or 

 coincide, though the latter condition is not strictly necessary. 



adjustment. The apparatus was described and figured by the late Richard 

 Beck in the " Transactions Micr. Soc." xiii. (1865) p. 117. [ED.] 



1 Mr. Wenham' s original device consisted of a parabolic mirror turned down- 

 wards, sliding beneath the stage in the manner of a cylindrical diaphragm. 

 Mr. George Shadbolt suggested the internal reflexion from a glass paraboloid 

 in preference to the ordinary reflexion from the parabolic mirror, and this was 

 adopted by Mr. Wenham. The apex of the paraboloid is truncated suitably 

 below the focus, and the truncated end is ground deeply concave to permit the 

 adjustment of live-cells, or diaphragms, which are mounted on a movable stem 

 passing through the axis of the paraboloid. Mr. Wenham subsequently dis- 

 covered that by making the truncated surface plane, the paraboloid might be 

 utilized as an animalcule cage, and to obtain various effects of dark ground 

 illumination on suitably mounted objects when placed in immersion- contact with 

 a slide of proper thickness. " Transactions Micr. Soc." iii. (1852) pp. 83 and 

 155, and iv. (1856) p. 59, and " Monthly Micr. Journ." ii. (1869) p. 28. [Ed.] 



2 The reflexion at the front lens is so considerable, that the image of a 

 globule of mercury formed by it again produces a diminished image in the 

 latter, which is still clearly visible in the Microscope. 



