2 
POPULAR SCIENCE REVIEW. 
potash, is poured on to a steel plate and allowed to dry. If 
one lialf of the plate be covered with a piece of card so as to 
obstruct all light from it, and the other half exposed to the 
action of sunshine for a minute or two, it will be found, on 
examining the plate in a dark room by the light of a candle, 
that the portion which has been exposed to the sun has 
become of a brown colour, whilst the shaded part of the plate 
remains of the original yellow tint. This is a well-known photo- 
graphic property of bichromate of potash, and was long ago 
applied by Mr. Ponton to the purpose of printing photographs 
on paper. But, besides a change of colour, another alteration 
will be found to have taken place. When dipped into water, the 
gelatine and bichromate of potash which have not been acted 
upon by the light will gradually dissolve, leaving the steel 
surface quite clean; but the other portion, which has been 
turned brown by exposure to the smfis rays, will scarcely 
dissolve at all. If, instead of a piece of card, the leaf of a fern, 
a piece of lace, or the light feathery flowers of a grass, be 
pressed in contact with the prepared surface of steel by means 
of a thick piece of plate-glass, the finest line, even the 
minutest fibre or thread, will be copied on to the steel surface, 
and, after being washed with water, will show an eminently 
beautiful white image impressed upon a yellowish-brown 
ground. The next step in the process consists in etching this 
steel plate in such a manner that an impression of the object 
can be struck off in printing ink. This is a matter of more 
difficulty than would at first sight be imagined ; many chemical 
agents are known which are capable of attacking the exposed 
surface of the steel plate, whilst they will have no action on 
the parts protected by the altered gelatine ; but a plate so 
etched will not give a good impression, except under very 
favourable circumstances. If the negative bq a piece of 
black lace, the finished and etched plate will have a perfect 
representation of the lace eaten into its surface to a con- 
siderable depth, by the action of the corrosive liquid, and if 
this be given into the hands of a copperplate printer, he will, 
in all probability, produce from it very beautiful and perfect 
prints, which at a little distance could not be distinguished 
from the original lace. 
The perfection of this kind of subject is due to the lines 
of etching formed by the threads of lace being of such a 
diameter that the ink is properly held by them ; but if, instead 
of a piece of lace, a photographic picture were used, a very 
different result w T ould be obtained. The steel plate, it is 
true, would be impressed with an exquisitely beautiful image, 
and upon applying the etching liquid, the picture would be 
bitten in with tolerable accuracy; but when tested by the 
