Page:Proceedings of the Royal Society of London Vol 4.djvu/232

Rh on record a number of insulated facts and observations respecting the relations both of white light, and of the differently refrangible rays, to various chemical agents which have offered themselves to his notice in the course of his photographic experiments, suggested by the announcement of M. Daguerre's discovery. After recapitulating the heads of his paper on this subject, w^hich was read to the Society on the 14th of Mai'ch, 1839, he remarks, that one of the most im- portant branches of the inquiry, in point of practical utihty, is into the best means of obtaining the exact reproduction of iu definitely multiplied facsimiles of an original photograph, by which alone the publication of originals may be accomplished ; and for which purpose the use of paper, or other similar materials, appears to be essentially requisite. In order to avoid circumlocution, the author employs the terms positive and negative to express, respectively, pictures in which the lights and shades are the same as in nature, or as in the original model, and in which they are the opposite ; that is, light represent- ing shade ; and shade, light. The terms direct and reverse are also used to express pictures in which objects appear, as regards right and left, the same as in the original, and the contrary. In respect to photographic publication, the employment of a camera picture avoids the difficulty of a double transfer, which has been found to be a great obstacle to success in the photographic copying of en- gravings or drawings.

The principal objects of inquiry to which the author has directed his attention in the present paper, are the following. First, the means of fixing photographs ; the comparative merits of different chemical agents for effecting which, such as hyposulphite of soda, hydiiodite of potash, ferrocyanate of potash, &c., he discusses at some length ; and he notices some remarkable properties, in this respect, of a pe- culiar agent which he has discovered.

2. The means of taking photographic copies and transfers. The author la3'S great stress on the necessity, for this purpose, of pre- serving, during the operation, the closest contact of the photogra- phic paper used with the original to be copied.

3. The preparation of photographic paper. Various experiments are detailed, made with the view of discovering modes of increasing the sensitiveness of the paper to the action of light ; and particularly of those combinations of chemical substances which, applied either in succession or in combination, prepare it for that action. The ope- ration of the oxide of lead in its saline combinations as a mordent is studied ; and the influence which the particular kind of paper used has on the result, is also examined, and various practical rules are deduced from these experiments. The author describes a method of precipitating on glass a coating possessing photographic proper- ties, and thereby of accomplishing a new and curious extension of the art of photography. He observes, that this method of coating glass with films of precipitated argentine, or other compounds, affords the only effectual means of studying their habitudes on exposure to light, and of estimating their degree of sensibility, and other parti- culars of their deportment under the influence of reagents. After