28 Eemarhs on the Structure [^^SlumS^^ 
Highest Powers of the Microscope," on the principle of " causing 
rays of Hght to pass through the under-side of the glass slip upon 
which the object is mounted, at the proper angle for causing total 
internal reflexion from the u]0]oer surface of the cover, which is 
thus made to act the part of a speculum for throwing the light 
down upon the underlying objects immersed in the balsam or fluid." 
The effect of this is that the objects are shown beneath an intense 
sheet of light, not any of which can enter the object-glass except 
that from the object itself ; consequently the field is perfectly dark. 
Though this is very easily managed, and in the form here repeated 
costs but a few pence, yet it has attracted but little notice, and, as 
far as I know, has been used by no one else but myself. Various 
modes of effecting this are described in my published paper, but I 
transcribe the one employed in the present question. Besides the 
apparatus possessed by most microscopists, it only requires the little 
truncated lens, h (shown full size), which is stuck on the slide by a 
film of any highly refractive oil, such as that of cassia, or cloves : — 
" Fig. 3 is another method ; a a is a glass slide — under this is 
cemented with Canada bal- 
sam a lens, h, nearly hemi- 
spherical, with a segment 
removed so as to leave the 
thickness equal to about 
one-third the diameter of 
the sphere. The flat facet 
of the lens is blackened. The 
radius of curvature should 
be about two-tenths of an 
inch : the use of the black- 
ened facet is to exclude all 
rays below the incident 
angle of total reflexion. 
This lens is intended to be used in conjunction with the parabolic 
condenser, in the manner represented by the figure. The rays 
from the parabola pass through the surface of the lens in a radial 
direction without refraction, and proceed till they reach the upper 
surface of the thin glass cover, where they are totally reflected and 
converge upon the object ; the cover in this instance acts precisely 
the part of a Leiberkuhn, with the advantage of more perfect 
reflexion." 
As Podura scales are generally mounted on the thin glass cover, 
it must not be expected that these will be illuminated as dry objects 
by such means, for the total reflexion takes place entirely from the 
top surface of the slide, and not a vestige of the upper objects will 
be seen. But in the intense black field a few soKtary scales will be 
found illuminated with singular beauty and brilliancy. These have 
become detached from the cover and lie upon the under slide, and 
