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 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, 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 lays 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, alfords 
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. 
