250 RECENT IMPROVEMENTS IN PHOTOGRAPHY. 
This paper being possessed of so high a degree of sensi- 
tiveness is, therefore, well suited to receive images in the ca- 
mera obscura. If the aperture of the object lens is one inch, 
and the focal length fifteen inches, the author finds that one 
minute is amply sufficient, in summer, to impress a strong 
image upon the paper of any building upon which the sun 
is shining. When the aperture amounts to one-third of the 
focal length, and the object is very white, — as a plaster bust, 
&c. — it appears to him that one second is sufficient to obtain 
a pretty good image of it. 
The images thus received upon the calotype paper are, for 
the most part, invisible impressions. They may be made vi- 
sible by the process already related, namely, by washing 
them with the gallo-nitrate of silver and then warming the 
paper. When the paper is quite blank, as is generally the 
case, it is a curious and beautiful phenomenon to see the 
spontaneous commencement of the picture, first tracing out 
the stronger outlines, and then gradually filling up all the 
numerous and complicated details. The artist should watch 
the picture as it developes itself, and when, in his judgment, 
it has attained the greatest degree of strength and clearness, 
he should stop further progress by washing it with the fixing 
liquid. 
The fixing process. — To fix the picture, it should be first 
washed with water, then lightly dried with blotting paper, 
and then washed with a solution of bromide of potassium, 
containing one hundred grains of that salt dissolved in eight 
or ten ounces of water. After a minute or two it should be 
again dipped in water, and finally dried. The picture is in 
this manner very strongly fixed, and with this great advan- 
tage, that it remains transparent, and that therefore there is 
no difficulty in obtaining a copy from it. The calotype pic- 
ture is a negative one, in which the lights of nature are re- 
presented by shades ; but the copies are positive, having the 
lights conformable to nature. They also represent the objects 
in their natural position with respect to right and left. The 
copies may be made upon calotype paper in a very short 
