B. Oxyhydrogen jet with zirconium cylinder, covered by the cowl A when working.
C. Doublet parallelising condenser, with centering screws.
D. Iris diaphragm.
E. Holder for trough and light-filtering media.
F. Plano-convex lens, 41⁄4ins. diameter, with centering screws G
H. Plano-concave lens, with iris diaphragm T.
K. Connecting pulleys between focussing rod of camera and fine adjustment of Microscope.
L. Triangular frame in which Microscope feet are placed.
M. Flap shutter.
N. Door through which image is observed on card screen, etc.
O. Solid block of mahogany on which camera body is fixed and supported.
P. Dark slide.
This efficient photo-micrographic apparatus ([Fig. 446]) is made by Messrs. W. Watson & Sons, under the instructions of Mr. E. B. Stringer. The illuminating condensing system is mounted on a square brass bar, the illuminant being oxygen-hydrogen light burning on zirconium. Immediately in front of this is a condenser, c, four and a half inches diameter, with an iris diaphragm, D, immediately in front of it. The holder, E, carries the light filtering media through which the beam passes and enters the condenser, F. It then goes through a tank of water contained in the cone, F to H, and emerges a practically parallel beam of great intensity through a plano-concave lens, h, of such a diameter as to exactly fill the back lens of the substage condenser. There is an iris diaphragm, T, for cutting off stray light.
The whole of the apparatus is fitted with centring screws and clamps, and after having been once adjusted it is ready for use at any moment without preparation. By means of this apparatus, instantaneous pictures can be taken of living rotifers, so brilliant is the illumination, while photographs of such fine objects as the flagella of bacteria cannot be secured with the same amount of certainty by any other microphotographic apparatus with which I have made myself acquainted.
Appendix C.
FORMULÆ AND METHODS:—CEMENTING, CLEARING, HARDENING AND MOUNTING.[90]
CLEARING AGENTS.
The object of employing a clearing agent is to replace the alcohol in the dehydrated section by a liquid which has a refractive index about the same as the balsam into which it is to be placed, and which will readily mix with it.
Oil of Bergamot will clear quickly from 90 per cent. of alcohol. Clove oil clears more rapidly, but it dissolves out aniline colours to a considerable extent. Xylol is without action on aniline colours. This strength of alcohol is chosen because of its being that of the methylated spirit sold in London, and which is much used in washing and dehydrating on account of its cheapness.
Oil of Cedar Wood, although an essential oil, resembles xylol, but evaporates slowly. It has very little solvent action on the aniline colours. It clears rapidly from absolute alcohol, but not well from 90 per cent. Sections can be left in it for several days. It is a convenient medium in which to examine tissues before mounting them permanently. It clears celloidin without dissolving it; and as a connecting fluid between the object and objective nothing better has been discovered.
Other clearing agents have been tried, but as they dissolve out the aniline colours, are no longer used.