HF The History of Photography 600pág

Permanent photographs 337
that the stronger the toning the greater the permanence-and this is certainly borne
out by the photographs which have come down to us. Yet inspection of the work of
photographers of reputation shows that in spite of all precautions, some of their
pictures have deteriorated although produced under apparently the same conditions
as those which have remained unchanged. This uncertainty led inventive minds to
direct their attention to the production of permanent photographs by other means,
which we shall briefly enumerate in the following pages. A great stimulus in this
direction was given by the Due de Luynes, who in June 1856 offered awards for
methods of producing cheap permanent printing methods. The duke, a member of
the Institut de France and of the Societe Franc;:aise de Photographie, was an archaeologist,
and he was disappointed by the lack of permanence of photographs-the very
medium by which he had hoped that records of antiquities, which time was destroying,
might be preserved for posterity. He gave rn,ooo francs (then £400) -8,000
francs to encourage photomechanical printing, and 2,000 francs for the invention of
permanent photographic prints.
PIGMENT PRINTING WITH CHROMIC SALTS
One of the most important discoveries in the history of photography was the observation
made in the spring of 1839 by MUNGO PONTON that bichromate of potas- Pl 215
sium9 spread on paper is light-sensitive. Ponton was trying to make photogenic
drawings with chromate of silver when he discovered that 'when paper was immersed
in the bichromate of potash alone, it was powerfully and rapidly acted on by the
sun's rays. It accordingly occurred to me to try paper so prepared to obtain drawings,
though I did not at first see how they were to be fixed. The result exceeded my
expectations.' With a suitable object laid upon the paper, those parts exposed to sunlight
were hardened while the parts protected by the object could be dissolved away
with water, leaving the image of the object white upon an orange ground, and quite
permanent. The action of bichromate of potash is thus very similar to that of bitumen
of Judea in Nicephore Niepce's heliography.
Ponton communicated his discovery to the Society of Arts for Scotland, of which
he was a Vice-President, on 29 May 1839. The full text of his 'Notice of a cheap and
simple method of preparing paper for photographic drawing, in which the use of
any salt of silver is dispensed with' will be found in The Edinburgh Neri' Philosophical
Journal, July 1839.10 The cheapness of the process seems to have appealed most to
Ponton, both as a Scot and as Secretary of the Bank of Scotland, for he drew particular
attention to the fact that while half-a-crown would buy only t oz. of nitrate of
silver it would buy a pound of bichromate of potash. Neither he nor his audience
could then have been aware of the far-reaching implications of the discovery, which
seemed to be merely a variation of Talbot's, rather than the basis for an entirely new
approach to making pictures by light. Indeed, Ponton's observation was hardly less
important than Wedgwood's, a milestone in the history of photography and photomechanical
printing.
When the French physicist EDMOND BECQUEREL experimented with variations of
Ponton's process on different kinds of paper in 1840, he established that it was not the
bichromate of potash as such which was very light sensitive, but that the size in the
paper (starch) greatly increased its sensitivity. That fact was not lost on JOSEPH
DIXON of Massachusetts, who in the following year made the first practical-though
misguided-application of Ponton's and Becquerel's observations, when he counterfeited
dollar notes by coating lithographic stone with bichromated gum. However,
nothing was heard of this until the inventor was free to tell his story,11 and credit for