188 
NATURE 
[ Fan. 5, 1871 

so that when dry, the tissue, as it is called, assumed the 
form of a thin, black cake with a paper backing. This 
sensitive tissue was placed under a negative to print in the 
ordinary manner, the light penetrating in parts to a greater 
or less degree, and thereby rendering the surface partially 
insoluble. On removing the tissue from the printing frame, 
it might, if it were desired, be forthwith washed to remove 
the soluble portions (as in the case of the fern-leaf experi- 
ment), but by so doing the picture would be hard and 
deficient in detail, and therefore a slight modification is 
here instituted. Instead of washing away from the face 
of the tissue, the operation is pursued from the back, the 
film being in the first place cemented face downwards 
upon a sheet of india-rubber, and in this condition put 
into a tank of warm water. The original paper backing 
of the tissue is in this way at once washed off, as is also 
every part of the gelatine mixture not rendered insoluble, 
which latter, constituting the image itself, remains attached 
to the india-rubber sheet before mentioned. The picture 
is now sufficiently developed, and indeed quite perfect, 
except that it is reversed to our view, for we are looking 
at it, it must be remembered, from the back ; this defect 
is, however, easily remedied by attaching to the image 
another sheet of paper by means of gum or gelatine, and 
then dissolving off the india-rubber facing by means of 
benzole or turpentine, when the finished image is obtained 
resting upon a support of white paper. The object of 
washing the carbon tissue from the reverse side and not 
from the front, or surface exposed to the sun, is to secure 
the finer details in the picture by fixing at once to a basis 
such portions of the tissue as may have been but very 
slightly acted upon, and thus prevent them from being ruth- 
lessly washed away when placed in warm water. 
The actual composition of pictures produced in this 
manner consists of gelatine, pigment, and a stable chro- 
mium compound, the gelatine being in a fixed or tanned 
condition, by a subsequent immersion of the prints ina 
solution of alum, and thus there is every reason to believe 
in the permanent character of such prints. A more simple 
and ready method of carbon printing has been lately 
invented by Mr. Johnson, and termed the Autotype process, 
but the principles involved therein are nearly the same. 
Passing from printing in permanent pigments, in which, 
as in silver printing, the aid of light is necessary for the 
production of each separate picture, we come next to 
photo-mechanical methods. Of these there may be said 
to be two kinds partaking of the nature of lithographic 
and engraving methods. Of the first description we may 
mention three modes of working, all of which are capable 
of yielding very creditable specimens of printing: these 
are Albert-type, the Lichtdriick process, and Edwards’s 
collographic method. The three inventions, which differ 
from each other and from minor plans of a similar nature 
only in a few details, are all based on the same principles. 
A sheet of patent plate glass is in the first place coated 
with a thick solution of bichromate of potash and gelatine; 
this film on drying is placed face downwards upon a sheet 
of black paper in the sun, and in this way the light rays 
penetrate the glass and act upon the sensitive compounds 
adherent to its under side. The bichromated gelatine 
becomes insoluble and firmly cemented to the glass, except 
on the exterior surface, for the black paper upon which 
this has rested absorbs the rays and leaves the outer film of 
gelatine still in a soluble condition. A second coating of 
the sensitive gelatine mixture is now applied to the former 
one, to which it adheres perfectly, from the fact af the first 
surface being unchanged, and upon the second coating 
an image is printed by means of a negative in the ordinary 
manner. After printing, the progress of which, by-the-bye, 
may be watched through the glass, instead of washing the 
surface and dissolving out all the soluble parts, a sponge 
dipped in cold water is simply rubbed over it, the moisture 
being absorbed by the gelatine where it has not been acted 
upon by light, and is capable therefore of swelling out ; 


those portions of the film, on the other hand, which have 
been rendered quite insoluble and hard, are unable to 
take up any water whatever, and remain untouched there- 
fore by the action of the sponge, while other parts again, 
slightly exposed to light, absorb water just to that degree 
to which they have remained soluble. In this condition 
an inked roller is passed over the surface, in the same 
manner precisely as in lithography, the greasy ink adher- 
ing to all the insoluble surfaces (where no water is), and 
to the other parts in a greater or less degree according to 
the amount of water present in those places. Thus the 
gelatinised glass is treated in every sense like a lithographic 
stone, being moistened, inked, and pressed in the same 
manner ; the resulting print, however, is generally finer 
than that obtained in ordinary lithography, as the graining 
of a stone surface is always somewhat coarse, while in the 
present instance the breaking up of the ink by the minute 
pores of the gelatine impregnated with moisture is of an 
exceedingly fine character. Many thousand prints may 
be pulled off a printing block of this kind before it is La 
destroyed, as the double layer of gelatine imparts a yield- 
ing nature to the plate which is not easily damaged ; in 
Germany, in England, and also, we believe, in America, 
this process of photographic printing is extensively prac- 
tised. 
But by far the most important of all methods yet dis- 
covered is the Woodbury engraving process. So simple, 
and at the same time so perfect in its work, a casual ob- 
server cannot but fail at once to appreciate its value. A 
thin sheet of gelatine is sensitised by impregnation with 
bichromate solution, and exposed to light under a nega- 
tive ; subsequent immersion in warm water removes the 
soluble portions from the surface, and we have then a 
thin gelatine plate upon which the image is represented, 
more or less, in relief. This matrix, as it is called, is 
hardened by treatment with alum,and placed when dry 
in a hydraulic press, in contact with a plate of type metal. 
Subjected to considerable pressure the metal plate takes 
the impression of the relief, and thus becomes in every 
sense an engraved plate, in which the darkest shadows! 
are represented by the deepest hollows, the half-tones by 
slight undulations, while in the high lights there is no 
depression at ail. The printing off of copies from this 
engraved plate is very ingeniously contrived. A little 
pool of transparent gelatinous ink is poured upon a sheet 
of white paper, and the metal plate is brought down upon 
the same with some pressure ; all superfluous ink is at 
once pressed out, and after a pause of a few seconds to 
allow the warm ink to cool and to become set, the plate 
is again raised, and a beautifully shaded print is the result, 
in which the shadows and half-tones are formed by layers 
of ink of different thicknesses. For inasmuch as the ink 
is of a transparent character, and there is more or less of 
it deposited upon the paper according to the depths of the 
hollows in the engraved plate, so the halftones are ren- _ 
dered with perfect gradation and fidelity, while in the high 
lights almost all the ink having been pressed away and 
removed, there remains nothing but the white paper 
which forms the basis of the print. 
By printing at once from many plates (for a gelatine 
matrix will yield several dozen of them), photographs may 
be printed at the rate of some thousands daily, without of 
course the assistance of light in any way. Moreover, the 
productions are of so perfect and delicate a nature as to 
be confounded actually with silver prints, being at the same 
time absolutely permanent. We are glad to say that this 
method is also being worked practically and extensively 
in this country, as also in France and America, and will, 
without doubt, be the process of the future ; for it is in- 
deed the only mechanical process by means of which 
photographs may be rapidly produced, possessing the 
same degree of excellence as the beautiful, but alas! too 
fleeting, albumenised pictures. 
HOB. Ps 
