64 
Illumination of Objects by Polarized Light, 
to the ordinary developments of the microscope; and the 
brilliancy of colour resulting from its action has fixed the 
appearance of many an object in the mind of the observer, 
which, by ordinary light, would have made scarcely any mental 
impression at all. 
But with the advantage of attractive appearance two great 
defects were constantly perceptible : one, the impossibility of 
transmitting the light through structures of more than a cer- 
tain density ; the other, that although the differing densities 
of structure were separated by it, the definition of these were 
disfigured in some cases, and lost in others. 
It therefore became obvious that if objects could be illu- 
minated by polarized light, in such a manner as to give the 
structure a stereoscopic effect, by a due contrast of light and 
shadow, causing the object to appear as if in relief, great 
benefit would result from the application, as giving a clearer 
development of the form and constitution than had before been 
obtained. Tiiis expression at once suggests the dark-ground 
illumination, which some of our Members have so largely im- 
proved by various useful adaptations. I have found it pre- 
ferable to use for this purpose a plano-convex lens, three-fourths 
of an inch in diameter. This fitting is of so small a size that it 
can be adapted to almost any instrument. An illuminating lens 
thus constructed will show intended objects on a black ground 
through the low powers, and should be arranged with a 
system of both central and external stop, each revolving on a 
separate axis, as generally supplied. If we now put an ad- 
justable cap on to the top of the lens, containing a crystal of 
Herapathite mounted between thin glass, and a plate of Selenite 
(also mounted on thin glass) above it, we shall find that ob- 
jects of too great density for transmitted light, will appear, 
under this illumination, as if in relief; and not only will the 
varying density of the structure be most beautifully displayed, 
but the definition of the different parts will be so accurately 
given as to constitute a perfect method of viewing the object. 
I do not profess to write a long article to engage your 
attention here ; but I would fain occupy some portion of your 
time at home in perfecting this arrangement, which, in my 
opinion, is most conducive to truth in microscopic analysis, 
and which will amply reward (as well in the highest as in 
the lowest sense) the careful labours of those who sympathise 
with me in the delights arising from those researches which it 
is the peculiar province of this Society to encourage and to 
stimulate. 
