58 
Wen II AM, on a Method of 
cover in this instance acts precisely the part of a Leiberkuhn, 
with the advantage of more perfect reflection. 
A lens of this description may be let into a thin plate of 
brass as in fig. 2, and used in the same^ way as an aquatic 
holder, the parabolic condenser always being used for concen- 
trating the light. When a slide containing balsam-mounted 
objects is placed above the lens, instead of using water, it is 
preferable to employ turpentine, or oil of cloves ; the refractive 
index of the latter being nearly the same as crown glass. The 
reason for introducing this agent is because light impinging 
upon the polished plane between a greater and a less refractive 
medium, will always suffer total reflection at the surface of 
the former, at a given angle dependent upon the relative 
refrangibilities. If water is used, the angle of the illuminating 
pencil will be limited to about 160° ; above this, all rays will 
be reflected down again by the flat surface of the lens, and 
lost, as shown by fig. 4 ; a a represents the glass slide, with 
Fig. 4. 
objects in balsam ; J is a hemispherical lens placed underneath 
the slide, with water interposed ; c c, rays which pass onwards 
to the top plane of the thin glass cover, to be reflected down 
again upon the object : the dotted lines, d d, are the portions 
of the illuminating pencil, that will be lost by being reflected 
from the flat surface of the lens — of course if a medium of 
nearly the same refractive power as the glass is used, such as 
oil of cloves, all this light will be transmitted and rendered 
available. 
Another variation in this principle of illuminating opaque 
objects, is that illustrated by fig. 5 : a is a small paraboloid of 
solid glass with a flat top. A black stop, b, of the same 
diameter as the apex, is fixed at the base of the parabola, for 
