Illuminating Opaque Objects, 
57 
play in the cavity at the apex of the paraboloid ; if the right- 
angled faces are one quarter of an inch square, it will perform 
very well. The objection to the plan just described is the 
necessity of having a separate prism for every object, which, 
though of advantage in some remarkable and peculiar cases, is 
not necessary for all. Fig. 2 is more universal in its appli- 
cations ; a a is a thin plate of brass, Z>, a right angle prism let 
in exactly flush with the upper surface ; any small objects 
Fig. 2. 
such as animalcules, Diatomacew, pollen, &c., must be laid 
upon the prism with water, and covered with thin glass ; total 
reflection will then occrur from the uppermost surface, in the 
same way as in fig. 1, and illuminate the objects in the fluid. 
Any ordinary plane slide containing objects mounted in 
balsam may be placed upon the plate and prism, first inter- 
posing a drop of water. It is almost unnecessary to remark 
that if this, or some other fluid is not interposed, the rays will 
all be reflected from the back of the prism itself, instead of 
passing onwards into the slide. 
Fig. 3 is another method ; « is a glass slide — under this 
is cemented with Canada 
balsam a lens, Z>, nearly 
hemispherical, with a seg- 
ment removed so as to 
leave the thickness equal 
to about one-third the dia- 
meter 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 blackened facet 
is to exclude all rays below 
the incident angle of total 
reflection. 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 
