HF The History of Photography 600pág

Photochemical investigations 3 3
end of the spectrum had a far quicker blackening effect than the other rays, proving
that the violet rays are much more actinic (chemically active) than the other colours
of the spectrum. This discovery was only incidental in tests to prove that light cannot
be considered as a simple substance or element, and Scheele attached no particular
importance to this fact, which proved a source of constant difficulty to early photographers,
resulting in a distorted rendering of the colours of nature in black-andwhite.
JEAN SENEBIER, chieflibrarian of Geneva, made valuable photometric observations
by exposing chloride of silver under varying thicknesses of paper, glass, and other
substances. Extending Scheele's experiment with the solar spectrum he measured the
time it took each coloured light to darken chloride of silver, and found that the red
rays needed as many minutes as the violet rays seconds. His findings (published in
1782) for the whole spectrum showed the following effect :14
Under violet light
purple
blue
green
yellow
orange
red
within r 5 seconds
25
29
37
st minutes
12
20
Senebier also made important investigations into the effect of light on resins and
found that some lose their solubility in turpentine on exposure to light, i.e. they
harden-a property later used by Nicephore Niepce in his photographic experiments.
At the time Scheele and Senebier were publishing their researches, DR JOSEPH
BLACK, F.R.s. - whom we shall shortly meet as a member of the Wedgwood circlewas
giving his famous chemical lectures in Edinburgh.15 In one of them he discusses
the action of light in darkening chloride and nitrate of silver, coming to the same
conclusion as Scheele, whom he cites, that by the action of light the silver salt is
restored to its metallic state.
In 1800 (srn) WILLIAM HERSCHEL, F.R.S., the great astronomer, observed during
some thermometric experiments with the solar spectrum 16 that there were heat
rays beyond the visible red rays, and these were subsequently called infra-red rays.
This led J. w. RITTER, a chemist of Jena, to investigate the solar spectrum_ further,
and, following the method of Scheele, he discovered the invisible ultra-violet rays
at the other end of the spectrum. (Herschel's thermometrical method would have
been useless in investigating the ultra-violet rays, which are cold.)
On the 22nd February [1801] I came upon solar rays on the violet side of the
colour spectrum and beyond it, and indeed proved it by means of horn silver
[chloride of silver]. They reduced even more strongly than the violet light itself,
and the extent of these rays is very great.17
Ritter exposed to the solar spectrum in a dark room a strip of white paper coated
with freshly precipitated silver chloride, and observed that the darkening action
began rapid! y beyond the visible violet rays, and then proceeded to the violet, etc.
In other words, Ritter not only discovered the ultra-violet rays, but he observed
that they are chemically more powerful than the luminous rays.
About this time, and independently of Ritter, DR WILLIAM HYDE WOLLASTON,
F.R.S., followed up Herschel's and Scheele's researches and also observed the action