Illuminating Opaque Objects. 
59 
the purpose of stopping out direct rays. This paraboloid is 
set in a ring, which is screwed underneath a flat brass plate, 
Fig. 5. 
so as to bring the upper plane surface of the glass exactly 
level with that of the plate in the manner shown by the figure. 
The parabola must be sufficiently short to prevent any rays 
from passing within the angle of total reflection relative to the 
flat top — or the paraboloid may be cut off at the point in the 
curve intersected by an angle of 45° drawn from the focus. 
If a powerful series of parallel rays be sent into the base of 
this paraboloid, not any of the light will find its way through 
the upper flat surface. The whole will be reflected down 
again into the body of the glass. If now a piece of thin glass 
is placed on the top, with a drop of water, the greater portion 
of the illuminating pencil will be transmitted to the upper 
surface of the cover, and from thence totally reflected, illumi- 
nating any small objects contained in the fluid. Glass slides 
containing balsam objects may be placed on the apex of the 
paraboloid, using an intermedium of turpentine, camphine, or 
oil of cloves, in preference to water. This same reasoning also 
applies when small objects are viewed directly in fluid, by 
being laid on the flat top of the paraboloid, and covered with 
thin glass. When the nature of the substances will admit of 
it, for the purpose of obtaining greater intensity of illumina- 
tion, they should be placed in turpentine or oil of cloves ; in 
this case the whole of the light will be reflected from the top 
surface of the cover— no separate reflection taking place from 
the upper plane of the paraboloid, as with water. In using 
this instrument, all that is required is to throw direct light 
into the parabola, by means of the concave mirror. 
Having now described some modifications of this principle 
of illuminating opaque objects, as most especially adapted for 
the highest powers, numerous experiments will justify me in 
saying a few words as to the effect. The light may be ob- 
tained of any required degree of intensity, and the field per- 
