POPULAR SCIENCE REVIEW. 
fection by Sir Henry James, in the Ordnance Office, South- 
ampton, where it is used for producing copies of maps. A 
mixture of gelatine and bichromate of potash is in this case 
also the foundation. A surface prepared with this mixture is 
exposed to the action of light behind a transparent photograph 
of the map, or other object to be copied, which is tightly 
pressed against it. The change which has been already 
described soon takes place, and a roller charged with litho- 
graphic ink is then passed over its surface. This blackens the 
whole surface; but when it is soaked in warm water, the portions 
of the sensitive coating which remain unchanged by the action 
of the light are dissolved out, and the lithographic ink is thereby 
removed from those parts of the picture. A flat prepared 
surface of zinc is then placed in contact with the inked picture, 
and the two are submitted to heavy pressure, when a complete 
transfer of the impression will be found on the zinc. After 
suitable preparation, any number of copies can be printed from 
this zinc plate in ordinary lithographic ink. This process is 
capable of giving very perfect results, and when applied to 
the copying of manuscripts, prints, or similar matter, it is 
impossible to conceive a more perfect reproduction. Indeed, 
it is no easy matter, when the original and the photozinco- 
grapli are placed side by side, to distinguish one from the 
other; and if the copy have been reduced in size by photo- 
graphic means, most persons would prefer it to the original, 
both in point of delicacy and sharpness. 
The last process which it is necessary to mention is the 
discovery of Mr. Dallas, and called by him photo-electric 
engraving. No explanatory details are given by the inventor, 
but there is little doubt, from the results already exhibited, 
that it is a modification of one or both of the photogly- 
phic and galvanographic processes. The great difficulty has 
always been to present the half-tones. Mr. Talbot’s process, 
it is true, solved this almost perfectly. Before us is a 
print representing a portion of the palace of the Tuileries. 
I he richness of the sculpture, the number of the statues, and 
i'i numerous fluted columns, render this an exceedingly 
difficult subject to engrave by a chemical process, owing to 
•V great variety of tints which it presents; and it affords 
uuiple evidence that this kind of photographic engraving is 
i-.'ipablo of rendering the most delicate gradations of tone, 
and the accurate delineation of details as perfectly as the 
bolder outlines of the picture. Confining the scrutiny to 
certain portions of the picture, the effect is quite equal to any 
photograph printed in the ordinary way, which is giving it 
ible praise; but when the picture is viewed 
. «ther, it appears patchy and unevenly developed. Owing 
