332 Royal Society : — Sir John Herschel on the Chemical 



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 pajDer, are the following. First, the means 

 of fixing photographs ; the comparative merits of different chemical 

 agents for effecting which, such as hyposulphite of soda, hydriodite 

 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 laj^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 

 stating the result of his trials with the iodide, chloride, and bromide 

 of silver, he suggests that trials should be made with the fluoride, 

 from which, if it be found to be decomposed by light, the corrosion 

 of the glass, and consequently an etching, might possibly be ob- 

 tained, by the liberation of fluorine. 



As it is known that light reduces the salts of gold and of platinum, 

 as well as those of silver, the author was induced to make many ex- 

 periments on the chlorides of these metals, in reference to the ob- 

 jects of photography ; the details of which experiments are given. 

 A remarkable property of hydriodic salts, applied, under certain cir- 

 cumstances, to exalt the deoxidating action of light, and even to 

 call into evidence that action, when it did not before exist, or else 

 was masked, is then described. 



