M. C. Lea—The Latent Photographic Image. 49 
Art. VII.—On certain new and powerful means of rendering 
visible the Latent Photographic Image; by M. Carey LEA. 
THE development of the latent or invisible photographic 
image produced by the action of light is beyond all question 
the most remarkable and the most interesting fact with which 
pboto-chemistry has made us acquainted. Our knowledge, 
however, of the substances capable of exhibiting this power, 
has increased with singular slowness. It commenced with the 
discovery, by the late Rev. Mr. J. B. Reade, in the year 1836, 
of the existence of this power in gallic acid, and not long after 
pyrogallol was found to possess the same property in a more 
energetic degree. Mr. Robert Hunt discovered that ferrous 
sulphate acted somewhat more powerfully than pyrogallol, 
provided that a soluble salt of silver was present. This was 
more than twenty years ago. Since then, no useful addition 
has been made to this list of so-called “developers,” though 
several of simply scientific interest. I was enabled to add to 
It hematoxyline about ten years since. Morphia has been 
found to have limited developing powers. When pyrogallol 
or gallic acid was employed in the absence of a soluble salt of 
Silver, it was found that the activity of these bodies could be 
greatly increased by the presence of an alkali (alkaline develop- 
ment), for which purpose ammonium carbonate has been gener- 
ally employed. It has been recently affirmed that in the 
ammonia -pyrogallol process, pyrogallol could be replaced 
(though with less energetic action, and therefore not advan- 
tageously) by a decoction of coffee berries, probably by reason 
of the quinic and caffetannic acids contained; and possibly 
by one or two other substances. This I believe represents the 
!nformation acquired up to the present time. 
In the following investigation I A age to show: 
1. That the number of bodies en _pe 
developing the latent image, so far from being very limited, as 
form of development, when no soluble salt of silver is present, 
's that which depends on the use of free alkali. I propose to 
show that there exists a form of development which under 
* Unless possibly after greatly prolonged exposure. 
Am. Jour. Sct.—Turp Serres, Vow. XIV, No. 79.—Juxy, 1877. 
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