RECENT IMPROVEMENTS IN PHOTOGRAPHY. 251 
time, the invisible impressions being brought out in the way 
already described. But the author prefers to make the co- 
pies upon photographic paper prepared in the way which he 
originally described in a memoir read to the Royal Society, 
in February, 1839, and which is made by washing the best 
writing paper, first, with a weak solution of common salt, 
and next, with a solution of nitrate of silver. Although it 
takes a much longer time to obtain a copy upon this paper, 
, yet, when obtained, the tints appear more harmonious and 
pleasing to the eye ; it requires in general from three to thirty 
minutes of sunshine, according to circumstances, to obtain a 
good copy on this sort of photographic paper. The copy 
should be washed and dried, and the fixing process (which 
may be deferred to a subsequent day) is the same as that al- 
ready mentioned. 
The copies made by placing the picture upon the photo- 
graphic paper, with a board below and a sheet of glass above, 
and pressing the papers into close contact by means of screws, 
or otherwise. 
After a calotype picture has furnished several copies, it 
sometimes grows faint, and no more good copies can then be 
made from it. But these pictures possess the beautiful and 
extraordinary property of being susceptible of revival. In 
order to revive them and restore their original appearance, it 
is only necessary to wash them again by candle light with 
gallo-nitrate of silver, and warm them : this causes all the 
shades of the picture to darken greatly, while the white parts 
remain unaffected. The shaded parts of the paper thus ac- 
quire an opacity, which gives a renewed spirit and life to the 
copies of which a second series may now be taken, extend- 
ing often to a very considerable number. In reviving the 
picture it sometimes happens that various details make their 
appearance which had not before been seen, having been la- 
tent all the time, yet nevertheless not destroyed by their long 
exposure to sunshine. 
The author terminates these few observations by stating a 
few experiments calculated to render the mode of action of 
the sensitive paper more familiar, viz : — 
