3S8 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
them owing to the variability of the results obtained. He was also 
unable to produce a good copie de copie. The author, however, has 
succeeded in obtaining some which could scarcely be distinguished from 
the original model, and believes that it would be possible to push the 
reproduction still farther. 
These successful results were obtained by means of the gelatin 
bichromate process. This photocollographic method also allows of 
successive gratings, placed in any position, being impressed upon the 
same plate of glass, for each proof can be treated as the original plate, 
and covered with a new layer of gelatin, and so on. 
The technique of the process is simple. Solution of gelatin in the 
proportion of 1 gr. to 30 grs. of water, with addition of 0* *10 gr. to 
0*15 gr. of bichromate of ammonia, is liquefied over the water-bath and 
filtered through cotton- wool on to a plate of glass, which is then set up 
vertically and allowed to dry in the dark. The shutter in which the 
plate is exposed should be furnished with a chimney of stout black paper 
terminated by a cover which only admits the solar rays when the appa- 
ratus has been disposed in such a way that they fall perpendicularly 
upon the grating. With a good, sun, the duration of exposure is from 
six to ten seconds. In diffused 
light, one to two hours may be 
required, but in this case the 
result is less satisfactory. After 
the exposure the plate is plunged 
into tepid water, then rinsed 
with cold distilled water, and, if 
necessary, brushed very lightly. 
It is well to protect with black 
paper all the part of the grating 
which is not engraved. To ob- 
tain a grating by reflection it is 
only necessary to substitute for 
the simple glass plate, a plate 
which has been previously 
silvered. 
Camera for Microphoto- 
graphy.* — Mr. D. W. Barker 
gives the following description 
of his “ home-made ” apparatus 
(fig. 48 ) . A wooden table is made 
somewhat wider around its top 
than the front of the camera, and 
contains near the centre an ap- 
J erture through which the Micro- 
scope tube can project slightly. 
After removing the lens from the camera, place the latter on the table 
with its front downward. Place the Microscope underneath and close 
the aperture to rays of light by means of the small silk sleeve a. A 
good lamp is to be used for illuminating. Focus roughly by hand, and 
* Amer. Mon. Micr. Journ., xiii. (1892) p. 39. 
Fro. 48. 
