paratively few years hand engravings on wood or metal • were the
only practical means employed for the illustrating of books but
within the last decade a remarkable change has come about and
where once few or no illustrations were to be seen, because of the.
great cost, books now abound with them. This revolution in
illustrating has been brought about by photography arid, owing
to the beauty, correctness, rapidity and cheapness with which
illustrations can now be reproduced, our best magazines are now
greatly enhanced and many others, which sell solely upon their
appearances, are enabled to do a paying business.
There are several methods of engraving by the aid of photog
raphy but that class of work which we meet with most frequently
is line and half tone process work, known as photo-engraving, and
in this paper only this class of work will be treated.
In general the photo-engraving process may be outlined as fol
lows: first, negative making; second, reversing the negative;
third, printing the negative image upon metal, and fourth, etching.
Negative Making. -The art of photography is based upon the
fact that certain salts of silver undergo a change of molecular
structure when acted upon by light. This change of structure is.
caused by certain chemical rays—actinic rays—which are largely
present in sunlight, magnesium light and electric arc light. The
salts of silver which have ben found susceptible to acting rays,
are the iodide, bromide and chloride; each having the same gen
eral property but varying slightly in their respective qualities.
To obtain certain results desirable in negatives these salts are
often combined.
In practice a glass plate is coated on one side with these salts.
and we have what is known as a photographic plate. It is
essential, however, that some medium be used to hold these salts
in a film upon the surface of the glass and here arise the distinc
tions collodion plates and gelatin or " dry " plates.
In the collodion plate the glass is coated with a film of collodion.
holding in solution certain halogen salts such as iodide of am
monium, iodide of cadmium, chloride of calcium and chloride of
strontium. This plate is then submitted to the action of a solu
tion of silver nitrate which causes the formation of the sensitive•
salts on the collodion film. If, for instance, iodide of ammonium
be present in the collodion it will be converted, by the action of
the silver nitrate, into iodide of silver, a sensitive salt, and am-