254 
8UMMARY OF CURRENT RESEARCHES RELATING TO 
receive the magnified image of A B. If the distance of A B from the 
objective is double the focal length, the image A' B' will be formed at 
an equal distance on the other side of 0, and there will be no magnifica- 
tion. The nearer A B approaches O, the farther from O will the image 
A' B' be formed and the greater will be the magnification. Thus 
the proper choice of the focal length of the photographic objective is an 
important point. The author recommends either a very small aplanatic 
of 8 cm. focal length, or a simple objective of short focus. This is 
fitted into the middle compartment of a universal camera, the front and 
back portions of which receive respectively the small negative and the 
ground glass and slides. Instead of a universal camera an ordinary 
camera with long draw-tube can be used, though not so conveniently. 
In this case a cone of cardboard, fitted to the holder of the objective, 
and provided at the other end with a draw-tube containing the small 
negative, replaces the front part of the universal camera. 
For the production of proofs with the same magnification, an appa- 
ratus of constant focus can be very simply constructed on the principle 
of the enlarging slide of M. Carpentier. This consists of a light-proof 
wooden box, the bottom of which receives a sensitive gelatin bromide 
paper while the top carries a draw-tube, in the base of which is inserted 
a lens which serves as objective. The proof to be enlarged is contained 
in a box attached to the other end of this draw-tube. 
In all these enlarging apparatus the focusing is effected in precisely 
the same way as in the ordinary apparatus. Daylight is the best means 
of illumination. 
The best way of arranging the sensitive paper in the slide is to 
place it between two glass plates. In this case the focusing must not 
be made on the ground glass, but upon a second plate placed against 
the small transparent window made by the layer of balsam. 
"When only low magnifications (1 to 5 diameters) of microscopic 
preparations are required, as e. g. of sections of embryos, brain, &c., 
which may measure several centimetres in diameter, direct enlargement 
of the preparation can be made by means of the photographic objective, 
without the aid of a Microscope. For this purpose the preparation takes 
the place of the small negative in the enlarging apparatus. It is fixed 
by two bands of gummed paper to a card pierced by a suitable aperture. 
Preparations which lend themselves best to this kind of reproduction 
are those which are a little thick and rather strongly coloured. The 
author obtained the best results with those which had been treated 
with haematoxylin, decolorized with acid alcohol, washed in alcohol 
and mounted in balsam. Preparations that are too thin or too feebly 
coloured should be illuminated by yellow light. 
(5) Microscopical Optics and Manipulation. 
Index of Refraction.* — Mr. A. B. Aubert describes some of the 
simpler methods for determining indices of refraction. An instrument 
for this purpose which he has found to work very satisfactorily is 
Bertrand’s Eefractometer, described in this Journal, 1887, p. 469. A 
simple method, proposed by Mr. Gordon Thompson as sufficiently accu- 
* Amer. Microsc. Journ., xiii. (1892) pp. 225-9. 
