ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
253 
of which the focusing is much facilitated. The method of illumination 
is either by a petroleum or albo-carbon gas lamp. The amount of 
illumination is determined by Abbe’s method, which consists in looking 
into the body-tube of the Microscope after removal of the eye-piece, and 
making the ratio of the bright and darker luminous field thus observed 
as 1 : 3. 
With respect to the choice of preparations, sections should be as thin 
as possible. The most suitable are those which have been embedded in 
paraffin and stained with safranin, eosin, haematoxylin, &c., and then 
mounted in balsam. For coloured sections the author prefers balsam to 
glycerin for mounting. 
Two methods of focusing may be employed. The first consists in 
repeating the operation before each photograph, the second in calculating 
once for all what displacement the optic system ought to undergo in 
changing from the eye-piece to the ground glass of the camera. The 
method of procedure in the first case is as follows : — After removal of 
the eye-piece and the insertion into the body-tube of the Microscope of a 
cardboard cylinder coated with black velvet for the prevention of internal 
reflections, the camera is adjusted on the Microscope. The image is 
then approximately focused on the ground glass by the micrometer 
screw. For more exact focusing a lens is employed. The lines traced 
upon the glass are brought into focus with the lens, and then, looking 
at the transparent portion of the plate, the micrometer-screw is turned 
until the image of the preparation is seen with the same clearness as 
the lines. The second method of focusing consists in determining 
once for all the lengthening of the body-tube necessary in order to 
readjust the focus when the eye-piece is substituted for the camera; but 
for high powers it is scarcely to be relied upon. 
As regards the choice of sensitive surfaces, the albumen plates fur- 
nish layers absolutely without granulation, but unfortunately they are 
too slow in action. The author uses Lumiere gelatin- bromide plates, 
of which the fineness is sufficient to allow of enlargements of three 
diameters without appreciable loss of clearness. § 
Fig. 29. 
A 
(2) Enlargement of the small negative . — The principle of all enlarging 
apparatus is represented in fig. 29, in which A B denotes the surface to 
be magnified, 0 the objective, and A' B 1 the sensitive surface intended to 
