240 Proceedings of the Asiatic Society. [ Nov: 
chiefly practised on subjects in line in order to perfect myself in 
the manipulations, which were entirely new to me, but I know 
from the trials I have already made, that the process will also give 
excellent results in half tone as soon as I shall have been able to 
master the difficulties ‘of the printing, and to obtain proper appli- 
ances. I am, therefore, unwilling to delay the publication ofa 
process by which absolutely permanex.. photographic reproductions 
may be made from any class of subject with great perfection and 
economy by means of appliances which are within the reach of all. 
In principle my process is similar to that introduced in 1866 by 
Tessier du Mothay, which was afterwards modified and improved 
by Albert of Munich and other Germans, and still further perfected 
by Ernest Edwards of London, who has brought it into extensive 
use under the name of Heliotype. Many of the members present 
may probably have read descriptions of it in some of the English 
serials, or have seen specimens in a publication entitled “ Art, Pic- 
torial and Industrial,’ which is illustrated entirely by its means. 
The distinctive feature of all these processes is, that the printing 
surface is composed of gelatine, hardened in such a manner that it 
may stand the wear and tear of printing, and they all depend upon 
the well known property peculiar to a dried film of gelatine mixed 
with an alkaline bichromate of becoming insoluble after exposure 
to light, and repelling water in the parts exposed to light exactly 
in proportion to the amount of the action of the light upon them, 
and at the same time of acquiring a corresponding affinity for a 
greasy substance, such as printing ink. Although this property 
by itself has been most usefully applied in many photographic pro- 
cesses for the reproduction of subjects in line, it would be quite 
incapable of giving the required results in the processes now under 
notice, because the unexposed gelatine remains in a pulpy soft state 
incapable of withstanding the wear and tear of printing, and more- 
over it would be liable to dissolve entirely with any rise in temper- 
ature, the consequence of which would be the loss of all the lighter 
tones. It has been found, however, that the chromated gelatine 
film may be so hardened or oxidised by certain substances, such as 
the alums, especially chrome alum, tannin, chlorine, bichloride of 
mercury, permanganate of potash, and other suitable oxidising 
