64 
POPULAR SCIENCE REVIEW. 
diaphragm (D 1 ) to a portion of the plate, the corners which are 
cut off should retain a transparency equal to the glass plate itself. 
The occurrence of a fine dust over these portions, when the 
intensifying process is complete, shows that the developer has 
been allowed to remain too long, or that the arrangement of the 
condensers and diaphragms has allowed too much interference of 
oblique rays in the picture. 
When the image is fully out, and cleared by hyposulphite of 
soda, its merits can be fully considered. The negative will be 
weak, in some cases so weak as to discourage the inexperienced 
from any attempt at intensifying. But very much can be done 
to supplement the deficiency of actinic power, and there is no 
doubt that means will eventually be found of doing so to a 
much larger extent than is now possible. 
It will be found useful to vary the mode of intensifying 
according to the character of the picture. If at a low power, 
with plenty of light and open definition, it may be got up at 
once by pyro and silver before clearing ; but if at a low light 
and high power, with lines of extreme delicacy, it is best to clear 
and dry the film before intensifying by daylight, with iodine, 
pyrogallic, and silver. In pictures of dead black and white, 
good results may be obtained by the bichloride of mercury, 
followed by hydrosulphuret of ammonia, as recommended by 
Col. Sir H. James, R.E. in the preparation of photozincographs 
from the ordnance maps. With the 64 pyro and silver plans ” 
risk is run of filling up the finer lines, and rendering the whole 
picture woolly. 
In printing, care must be taken to get the most perfect con- 
tact between the paper and the negative, or much fine definition 
will be lost in the transfer. Great pressure must be used at 
the risk of cracking a few glasses from their expansion under 
heat. The paper should be of the finest grain, and it may per- 
haps be useful to amateurs whose time is valuable to know that 
paper sensitised in a bath rendered decidedly acid by the addi- 
tion of nitric acid (twenty drops to the pint of bath) will keep 
perfectly well in a dry place for a week or more. 
A word may be added on the selection of objects, for success, 
especially in the earlier essays, will much depend on this. 
Yellow or reddish tissues will generally be found very im- 
penetrable to artificial light ; but I do not say that they cannot 
be photographed, for even opaque objects, such as Foraminifera, 
have been conquered by Professor Kingsley. Objects mounted 
specially with a view to photography should have their colour, 
if non-actinic, discharged by maceration in some fluid such as 
chlorine water or strong liquor potassse, followed by glycerine, 
varying the solution according to the chemical composition of 
the object. 
