HOW TO PHOTOGRAPH MICROSCOPIC OBJECTS. 
59 
good instruments may be employed, or an objective just lower 
than the one in use ; and we have found this (N) with Reade’s 
drum (0), the latter having its convex side to the source of illu- 
mination, a most excellent combination, giving the very best 
results. 
Illumination . — The instrument being now complete, a source 
of illumination must be decided upon. Many substances have 
been used, but our own experience has been confined to oil, oxy- 
calcium, and magnesium. The first is of course weakest in actinic 
rays, but very good results were obtained with it up to 150 dia- 
meters. Wet plates were kept moist during long exposures, some- 
times reaching to forty minutes, by means of a small cup of hot 
water inserted within the camera. A solar burner was employed ; 
and though we have long deserted it for the more brilliant mag- 
nesium, it is still found useful and economical for adjusting the 
light and the more lengthy process of focussing. The oxycal- 
cium light gave excellent results even at the higher powers, but 
the process of making oxygen was tedious, and the apparatus is 
costly. I can scarcely think that it would be used now that the 
more actinic light of magnesium is within the reach of every 
one. After some little experience with the magnesium wire, we 
have the most sanguine expectations of its ultimate usefulness. 
With a proper concentration of the light, a few flashes are suffi- 
cient to produce a picture even with a object-glass. The 
light fails only in steadiness, and if some means could be devised 
for burning the metal uniformly and at a fixed point nothing 
would be left to desire. The method we have adopted for burn- 
ing the ribbon is as follows : a small telescope upright of brass 
(S) regulated by a screw, is fixed to a block (V) adapted to slide 
on the support (U) common to microscope and light. At the 
apex of the brass upright is fixed a small tin gutter or pipe of 
sufficient capacity to admit the wfire easily, and directed down 
wards at an angle of 45 degrees. 
By this simple contrivance the point of the wire maybe raised 
or depressed at pleasure, and it will be found a great assistance 
in ascertaining the best point of illumination to provide a mov- 
able stop of tin (P) with a pinhole aperture, which can be 
attached to the boot of the condenser during this preliminary 
but very important operation. 
The apparatus may now be supposed to be complete ; improve- 
ments will suggest themselves to anyone who seriously takes up 
the art of photomicrography; but it is only after a series of trials 
and many failures that certainty in the results can be obtained. 
Some directions will be necessary as to the mode of using the 
various instruments which have been described. 
In using artificial light, many difficulties incidental to sun- 
light exposures are got rid of, but new troubles arise. In place 
