248 RECENT IMPROVEMENTS IN PHOTOGRAPHY. 
All this is best done in the evening by candle light. The 
paper so far prepared, the author calls iodized paper, because 
it has a uniform pale yellow coating of iodide of silver. It is 
scarcely sensitive to light, but nevertheless it ought to be 
kept in a portfolio drawer until wanted for use. It may be 
kept for any length of time without spoiling or undergoing any 
change if protected from the light. This is the first part of 
the preparation of calotype paper, and may be performed at 
any time. The remaining part is best deferred until shortly 
before the paper is wanted for use. When that time is ar- 
rived, take a sheet of the iodized paper and wash it with a 
liquid prepared in the following manner : — 
Dissolve one hundred grains of crystallized nitrate of sil- 
ver in two ounces of distilled water, add to this solution one 
sixth of its volume of strong acetic acid. Let this mixture be 
> called A. 
Make a saturated solution of crystallized gallic acid in cold 
distilled water. The quantity dissolved is very small. Call 
this solution B. 
When a sheet of paper is wanted for use, mix together the 
liquids A and B in equal volumes, but only mix a small 
quantity of them at a time, because the mixture does not keep 
long without spoiling. I shall call this mixture the gallo-ni- 
trate of silver. 
Then take a sheet of iodized paper and Wash it over with 
this gallo-nitrate of silver with a soft brush, taking care to 
wash it on the side which has been previously marked. This 
operation should be performed by candle light. Let the pa- 
per rest half a minute, and then dip it into water. Then dry 
it lightly with blotting paper, and finally dry it cautiously at 
a fire, holding it a considerable distance therefrom. When 
dry, the paper is fit for use. The author has named the pa- 
per thus prepared calotype paper, on account of its great 
utility in obtaining the pictures of objects with the camera 
obscura. If this paper be kept in a press it will often retain 
its qualities in perfection for three months or more, being 
ready for use at any moment ; but this is not uniformly the 
