28 THE MICROSCOPE. 
objectives, they are in no way superior to the ordinary Woodward 
prism or hemispherical lens, and for this reason, and on account of 
their cheapness, the prism and lens may be found in the accessory 
box of almost any one who is in possession of an objective whose 
angular aperture exceeds the limit of dry working lenses. It may 
be urged as an objection to the use of the prism or lens, that beams 
of light from different directions at the same time, are not obtaina- 
ble; but this objection is overthrown when the light is taken di- 
rectly from two or more lamps, hence it results that the chief objec- 
tion is the necessity of having the mirror bar so constructed as to 
permit of considerable lateral movement. This objection applies 
only to such stands as are constructed in the old way—that is, to 
those in which the mirror bar is a fixture and oblique light can not 
be had from the mirror. 
Some time ago the idea occurred to me that the parabola used 
in connection with a spot lens, might be made to serve a useful pur- 
pose in regard to illuminating objects seen with the widest angled 
objectives; and a few experiments so completely confirmed the 
idea that I should like to describe the method, so that it may be 
subjected to trial by others. 
The plan is to make two comparatively inexpensive pieces of 
apparatus take the place and do the work of any first-class 
wide-angled immersion condenser. These accessories are the ordi- 
nary parabola and the hemispherical lens. Ordinarily the former is 
a dark ground illuminator, but when the aperture of the objective 
exceeds that of the parabola the effect is simply that of a dry con- 
denser, in which the central rays are stopped out. But even at its 
best the light cannot traverse the slide at greater angle than 41° 
from the axis; and it is rarely, if ever, even so great as this. Now, 
if the light reflected by the parabola could be converted into a glass 
angle, (commonly called balsam angle) without altering its angular 
direction, it would be amply sufficient to give light into the objec- 
tive at the widest balsam angle now used in the best homogeneous 
immersion objectives. This may be done by using, under the slide, — 
a hemispherical lens, whose radius is less than that of the concavity 
of the parabola—of course making optical contact by means of the 
immersion fluid. This is to be accurately centered and the para- 
bola brought up so close that the hemispherical lens will 
occupy the concavity. When properly adjusted it 
