6
THE ART OP PHOTOGRAPHY.
iodide of silver, and gave moreover the peculiar properties of the "iodisedpaper." He says: "The papers so prepared (with iodide of potassium and nitrate of ailver) vary considerably in sensitiveness, aceo:rding to the strength of the solutions used. A concentrated solution of iodide of potassium gives a paper nearly or altogether insensible; with a weak solution the result is the reverse." At the meeting of the British Association at Plymouth, in July 1841, Mr. Robert Hunt read a paper "On the influence of the yellow ferrocyanide of potassium (a compound of equivalents of cyanide of potaaaium with one of protocyan ide of iron, and three of water) upon iodide of silver, and on the high sensitiveness of the same as a photographic preparation.'' He gaye also instructions how to prepare the " iodised paper," viz., as follows: "Select good smooth letter paper, waah it on one side with a solution of one drachm of nitrate of silver in one ounce of distilled water ; let it dry quickly, and then wash it once more with the same solution. As soon as it is dry again, saturate it with a aolution of two drachms of iodide of potassium in six ounces of distilled water; waah it afterwards cautioualy in distilled water, and then let it dry in the dark, at the common temperature... ..._ Iodised paper was used, likewise by Ryan, Lassaigne, and others ; and it seems moreover pretty certain that this paper, as prepared according to the instructions given by Herschel, Hunt, and others, was an article of commerce, before the patent for the Oalotype had been obtained. In Sir John Herschel'• paper above alluded to, we find also mentioned already the use of" gallic acid" as an accelerative agent. In that paper, Sir John Herschel states, that he " failed to obtain satisfactory results (in the way of accelerating the process) by any of the agents tried, with the somewhat problematical exception of gallic acid and its compounds... The direct production in the oamera-obscura of positive pictures on paper, was the next point which attracted the attention of the friends of the photographic art. Prepared paper blackened by the action of light was saturated with a solution of iodide of potassium, and placed in the camera, where it was bleached by the luminous rays, and an 'I{ image thereby produced, in which the distribution of light and The most successful shade Willi found to be the same as in nature.