552 
POPULAR SCIENCE REVIEW. 
You can then remove the tissue from the glass, and will find it very 
flexible, tough, and elastic, folding without creasing or breaking, and 
bearing general rough treatment without injury. On this the image is 
printed in the usual pressure-frame, with the collodion surface in contact 
with the negative, the image being printed not upon , but through the film. 
When removed from the pressure or printing-frame, after an exposure 
which it is difficult to regulate without experience, the tissue is again 
mounted, with a solution of India-rubber and damar in benzole, on paper 
or any similar surface (collodion, and also what is called <c india-rubber 
varnish,” thinned with benzole, answer the same purpose), and the process 
of development is merely that of immersing it in hot water, about 100° 
Falir., when portions of the gelatine dissolve out in proportion to the 
degree in which they have been acted upon by the light, and in the course 
of two or three hours the image is perfect, when a camel-hair brush is 
passed over them ; they are subjected to the action of a stream of hot 
water, dried, mounted, and rolled in the usual way. The image being 
printed through from the back is of course reversed, but -should this prove 
objectionable, either the original negative may be itself taken through 
instead of on the glass ; or, as is more commonly done, the print may be 
mounted face downward, and the paper attached prior to the process of 
development readily removed by the aid of a little benzole. 
Let us now glance at M. Fargier’s process. A plate of glass is coated 
with a mixture of carbon gelatine, and the bichromate is exposed in the 
pressure frame, and coated with collodion previous to development. The 
water then plays its part in detaching the film and developing the image 
by dissolving out the soluble portions at one and the same time, and last, 
though not least, you have to dodge that curling and diving film until you 
can capture and mount it in the usual way. 
Thus we see, how by a process very similar to that of Mr. Swan, results 
as perfect were attainable about four years ago ; but in addition to this 
fact, as affecting the safety and powers of Mr. Swan’s patent, we must 
remember that Pouncy of Dorchester, Davies of Edinburgh, Beattie of 
Dublin, M. C. Lea of America, and many others, exhibited specimens by 
processes closely resembling Mr. Swan’s, long before that gentleman’s 
patents were secured, and that, moreover, Mr. Burnet first pointed out, in 
the British Journal of Photography— if we remember rightly— the ad- 
vantage in carbon experiments of printing through the paper, although 
Mr. Swan says he is indebted for this to Mr. Blair. Again, the use of 
bichromates of potash and ammonia were originally suggested by Ponton. 
Fox Talbot first made a similar use of gelatine, and the idea of mixing a 
pigment therewith is the property of Poitevin ; while in America a patent 
was taken out some years ago for the application of sugar to effect a very 
similar purpose. All these things it seems to us may affect the working of 
Mr. Swan’s patent, and should therefore receive his attention. 
As most photographers are now engaged in testing Mr. Swan’s process, 
a few hints relative thereto may prove acceptable to such of our readers as 
are adepts in what is called “ the black art.” 
Different kinds of gelatine have been used, and, considering how variously 
this article is prepared, it will not surprise when we add, with various 
