AND OTHER METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
61 
the spectrum than any of the other argentine salts, and is on that account preferable 
for the influence of artificial light. The sensibility of paper prepared with this salt 
has been found to be greatly augmented by the addition of a small quantity of iodide 
of silver (figs. 1 and 2, Plate V.). In fig. 1, the development of the impression has been 
arrested when distinctly visible on the iodized portions, but invisible on the remainder 
of the paper : in fig. 2, the difference in the clearness and darkness of the several 
portions of the line is very well marked. A little isinglass is also added, partly be- 
cause the presence of a small quantity of organic matter appears to assist the catalytic 
action that ensues on the development of the impression, and partly for the purpose 
of retaining the salts dissolved with it on the surface of the paper. 
The mode of preparation is thus : to a filtered solution of four grains of isinglass in 
one fluid ounce of boiling distilled water, add ten grains of bromide of potassium, and 
two grains of iodide of potassium*: when cold, the solution is evenly laid with a camel’s 
hair brush on highly glazed paper in sufficient quantity to thoroughly wet the surface, 
but not to run off ; the paper is then quickly dried by the fire, to prevent the solution 
being absorbed by the paper. Paper thus prepared may be kept for a considerable 
time in a dry place ; but it is recommended not to prepare at one time more than 
enough for a week’s consumption. 
When about to be used, a piece of the above paper is evenly washed over by a camel’s 
hair brush with a solution of fifty grains of crystallized nitrate of silver in one fluid- 
ounce of distilled water, by the aid of red or yellow light only, and placed damp in 
the apparatus, as the sensibility of the paper is much diminished by permitting it to 
become dry. When removed from the apparatus, the latent impression is developed 
by washing the paper with a saturated solution of gallic acid in distilled water, to 
which a very small quantity of strong acetic acid is added, when used ; the addition 
of the acetic acid is found to diminish the darkening of the paper. The piece of 
paper used for each of these registers is half a sheet of folio post paper, torn length- 
wise into two strips : to the quantity of gallic acid necessary to wet the surface of 
one of these pieces, which is about a teaspoonful, the addition of three drops of acetic 
acid is found to be sufficient. As soon as the impression is sufficiently developed, all 
soluble matter is removed by washing the paper two or three times in water ; and 
lastly, the image is fixed by washing with a solution of twelve grains of hyposulphite 
of soda in one fluid-ounce of distilled water. A little practice in the manipulation is 
necessary for the success of this or any other photographic process. It may here be 
usefully remarked, that in all photographic processes, the strictest attention to clean- 
liness, as regards all instruments employed, is indispensably necessary : separate 
cloths, brushes and glasses should be used, and each retained for its respective 
purpose, as the smallest undue admixture of the materials will entirely frustrate the 
object to be attained. 
The paper prepared as above described, is placed round the outside of a cylindrical 
* See note, p. 67. 
