TRANSACTIONS OF THE SECTIONS. 2h 
object, over and into the darker portions of the’picture. To illustrate this argument 
the following fact in connexion with dry-plate photography should be borne in 
mind :—If Bp aniscape photograph consisting of trees and sky be taken in a bright 
and actinic light, and when a short exposure is therefore only necessary, there will 
be on a rapid dry plate but little radiation ; but if the same view be taken in a dull 
light, and when the actinism is infinitely less, the longer exposure required will 
certainly produce a great amount of radiation from the sky over the trees: now the 
sky, even in a dull light, would be impressed on the film tolerably rapidly, and the 
necessity for the long exposure is in order to get the detail in the darker parts of the 
picture; and the radiation is thus produced by the creeping of the superabundance 
of light that has already finished its work on the sky over the dark parts of the 
apn which require the prolonged exposure. That. this ‘‘ creeping of the light” 
oes take place, the following experiment will, I think, show with certainty. 
In order to find out which of the salts of silver were least liable to give this 
radiation I have experimented with a very large number of them, to which I will 
presently allude ; and the following singular fact was discovered by me in connexion 
with the chloride of silver, which salt has much more tendency to give radiation 
than any other that I have experimented with. I had found that a sensitive film 
which contained a chloride was always possessed of less satisfactory keeping qualities 
than one from which chloride was absent ; and in course of some experiments on 
the various keeping qualities of plates between exposure and development the 
following observations were made :—A dry film containing a considerable quantity 
of chloride was after exposure cut in half; one half developed immediately, and the 
other half put away in the dark for forty-eight hours. It was found that on the 
half of the plate that had been kept in the dark for forty-eight hours the “ blurring,” 
as it is called by photographers, was considerably greater than on the half plate 
that was ipecloed. at once; and on repeating the experiment several times ta 
enabled to convince myself that the action of radiation, or what I have called the 
“ creeping of light” on to the dark parts of the picture, continues after the film has 
been removed from the action of the light, and after it has been put away in the 
dark. I have noticed this with chloride of silver only, but I was thus led to inves- 
pee the behaviour of other salts of silver in connexion with this radiation of 
ight. 
No photographic process gives such good and rapid dry plates as the one in which 
an emulsion is made of bromide of silver formed in the presence of a large excess of 
the nitrate of the metal; but when we use the bromide alone it is unfortunately 
strongly addicted to the blurring before spoken of. We can, however, entirely 
counteract this tendency by the use of other salts of silver. I will not take up your 
time by going into very minute details on this point, but will merely say that I 
have found that the addition either of the malate, succinate, fluoride, or iodide of 
silver to the bromide will, if used in the proper proportion, give a sensitive film from 
which radiation shall be entirely absent, and that their suitability to the purpose is 
found in the order in which I have written their names ; and with a film containing 
a large proportion of malate of silver I have been enabled to take subjects which it 
would be impossible to take by any other method, owing to their strong contrasts 
of black and white. It should also be noticed that these salts have a peculiar effect 
on the colour of the finished negative—the malate giving a golden brown, the suc- 
‘cinate a red-brown, the fluoride a pink, and the iodide a delicate green. It is also 
a great preventive of radiation to add nitrate of uranium to the emulsion, and what- 
ever other salts are used this should never be omitted. I may also mention here 
that I have found these various salts to act in a remarkable manner in connexion 
with the colours of the spectrum, and to give peculiar results in connexion with 
some of the more difficult lines thereof. 
There is one final point as to the obtaining of good astronomical photographs. It 
appears to me to be essential that they should be developed by the strong alkaline 
method of development which was introduced by myself in the course of last year. 
It is impossible with the old method of development to obtain results in any way 
as satisfactory as those obtained by my new method. Not only is there a very great 
increase of sensitiveness obtained, but the development is unusually certain and 
rapid, and where the sun has to be photographed it has 4 peculiar effect in giving 
