408 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
sharp in the photograph. This will give the measure, for that objective, 
of the difference between the two foci, and whenever the same objective 
is used, you can always, by moving the fine-adjustment to that extent, 
but in the opposite direction, convert the visual into an actinic focus. 
In 1859 I left England for India, and for nearly thirty years, having 
“ other fish to fry,” being, in fact, engaged in the “ struggle for existence,” 
I had no leisure for microscopical studies. On recently resuming, about 
two years ago, one of the first things I did was to read up what had in 
the meanwhile been done as regards photomicrography. What a change 
had taken place, whether regarded from the photographic or the micro- 
scopic point of view ! I found that there were now available gelatino- 
bromide plates, infinitely more sensitive than the old collodion, and dry 
instead of wet, so that there need be no limit to the time of exposure. 
On the other hand, “ immersion ” objectives, followed by apochromatics, 
had greatly increased the delineating power of the Microscope. I 
promptly provided myself with a set of apochromatics, and proceeded to 
mount my old hobby, intending to apply it chiefly to the investigation 
of the minute structure of the diatom valve. I commenced with daylight 
(white cloud) illumination, which in the old days had been considered 
the best ; next proceeded to artificial light (oxy-hydrogen) ; and finally 
adopted, for high magnifications, sunlight, with which Colonel Woodward 
had achieved his best results. My wish is to place before you to-night 
some of the results that I have so far obtained ; to describe the apparatus 
I use in its present state of development, and explain, so far as I can 
without a “ practical demonstration,” the method of working with it. 
A general idea of the apparatus you will gather from the woodcuts 
and description, which originally appeared in the Koyal Microscopical 
Society’s Journal, 1890, pp. 429-34. 
[We omit the description and figures of the Microscope and heliostat 
as they were dealt with in the Journal, 1890.] 
Turning now to the camera. This is fixed to a base-board, which 
pivots on a tripod, so that it can be slewed round out of the way when 
not in use. There is then room for the operator to sit at the Microscope, 
find and arrange his object, and adjust his illumination, also to effect the 
necessary corrections of the object-glass for variations in the thickness 
of the cover-glass, if an eye-piece is to be used ; but if the photograph is 
to be taken without an eye-piece, this correction should be effected after 
the camera is attached, and when the image is on the ground glass. It 
is well for the table upon which the Microscope stands to be of such a 
height as to bring the tube of the instrument comfortably to the level 
of the observer’s eye, and the height of the tripod must correspond, being 
such that the axis of the camera coincides with that of the Microscope. 
The light-tight connection of the camera to the Microscope can be 
effected in a variety of ways. The one 1 employ is a collar, covered 
with velvet, which fixes on to the upper end of the draw-tube of the 
Microscope, and has a deep groove, into which fits a wide brass tube 
attached to the camera front by a small conical bellows. 
The image of the object may be projected on to the sensitive plate 
either (1) by means of the object-glass alone, or (2) by the use of what 
is termed a “ projection ” eye-piece. Much good work has been done by 
the former method, but not, so far as I can judge, the very best. I 
