RECENT IMPROVEMENTS IN PHOTOGRAPHY. 249 
case, and the author therefore recommends that it should be 
used in a few hours after it has been prepared. If it is used 
immediately, the last drying may be dispensed with, and the 
paper may be used moist. Instead of employing a solution 
of crystallized gallic acid for the liquid B, the tincture of 
galls, diluted with water, may be used, but he does not think 
the results are altogether so satisfactory. 
Use of the paper. — The calotype paper is sensitive to 
light in an extraordinary degree, which transcends a hundred 
times, or more, that of any kind of photographic paper 
hitherto described. This may be made manifest by the fol- 
lowing experiment : — Take a piece of this paper, and having 
covered half of it, expose the other half to day light for the 
space of one second, in dark cloudy weather in winter. This 
brief moment suffices to produce a strong impression upon the 
paper. But the impression is latent and invisible, and its 
existence would not be suspected by one who was not fore- 
warned of it by previous experiments. 
The method of causing the impression to become visible is 
extremely simple. It consists in washing the paper once 
more with the gallo-nitrate of silver, prepared in the way 
before described, and then warming it gently before the fire. 
In a few seconds the part of the paper upon which the light 
has acted begins to darken, and finally grows entirely black, 
while the other part of the paper retains its whiteness. Even 
a weaker impression than this may be brought out by re- 
peating the wash of gallo-nitrate of silver, and again warming 
the paper. On the other hand, a strong impression does not 
require the warming of the paper, for a wash of the gallo- 
nitrate suffices to make it visible, without heat, in the course 
of a minute or two. 
A very remarkable proof of the sensitiveness of the calotype 
paper is afforded by the fact stated by the author, that it will 
take an impression from simple moonlight not concentrated 
by a lens. If a leaf is laid upon a sheet of the paper, an image 
may be obtained in this way in from a quarter to half an 
hour. 
VOL. VII. — NO. III. 32 
