216 
POPULAR SCIENCE REVIEW. 
was made by M. Becquerel for the late Sir David Brewster : — A polished 
daguerreotype plate having been connected with the positive pole of a 
Grove’s battery, and a piece of platina foil having also been connected with 
the negative pole, these are immersed in a solution composed of one part of 
hydrochloric acid in eight parts of water. After a brief immersion, the 
silver surface of the plate becomes first violet and then black from the 
formation of subchloride. In this state the plate, after being gently rubbed 
and washed, is ready to receive the natural colours. It has been suggested 
that papers prepared with the sub-chromate of silver will be more likely to 
yield satisfactory results in heliochromy than those which owe their sensi- 
tiveness to the subchloride. 
A Poor Man's Photography . — Under this apparently whimsical title, 
Professor Piazzi Smyth, the Astronomer Royal for Scotland, has published 
a lecture, delivered by him before the Edinburgh Photographic Society. 
The object of the lecture is to show how a poor man (the Professor himself) 
went to the Pyramids of Egypt, armed with a small pocket camera, by 
which he obtained a number of negatives, only an inch square, but so sharp 
as to bear considerably enlarging, and how these minute pictures were sub- 
sequently more valuable as scientific data connected with the great Pyramid 
than those taken by the ordnance survey, four years afterwards, under cir- 
cumstances possessing peculiar facilities for carrying out such an under- 
taking. Several of Professor Smyth’s original negatives of the Pyramid 
have been exhibited at the London Photographic Society, and have elicited 
much admiration on account of their great sharpness, clearness, and delicacy. 
It may be interesting here to observe, that untouched reproductions from 
these negatives of one inch square have been exhibited by means of the 
magic lantern, on a scale of sixteen feet. 
Photo-engraving . — The details of another process for making engraved 
plates by means of photography have recently been published by Mr. R. H. 
Courtenay. While it is in its main features similar to that of Paul Pretsch, 
it differs in some of its details, and in these points of difference, according to 
the inventor, are to be found its superiority to any other published process 
of a similar nature. Its leading features are these : — A plate of glass having 
received a sensitive coating composed of gelatine and an alkaline bichromate, 
is exposed to light under a negative that differs from one of the ordinary 
land in having finely-ground silica interspersed throughout it. These atoms 
give the requisite granular condition for holding ink in the copperplate sub- 
sequently obtained. After exposure to light the gelatinised glass is immersed 
in water, by which the surface previously plane now starts into a series of 
depressions and ridges — it swells out or remains depressed precisely in ac- 
cordance with the action of the light. This surface, when partially dried, is 
rendered conducting by means of silver bronze powder or analogous means, 
and is then used for the purpose of producing an electrotype cast capable of 
yielding impressions, as in the case of an ordinary engraved copperplate. 
Mr. Woodbury is also engaged in perfecting a process of photo-engraving 
which, according to the Photographic News , was discovered partly by acci- 
dent. Observing that in one of his gelatine reliefs some colour which had 
been added was granulated, he investigated the cause and has since utilised 
this defect. From a granulated picture in gelatine he obtains an impression 
