The Redwood gazette. [volume] (Redwood Falls, Minn.) 1873-1940, 1877-05-03

are pure and simple which we obtain by
the prismatic refraction, namely, the
spectroscopic colors. The blue cobalt
glass, for instance, which is now called
mazarin glass, is proved by the spectre
scope not to owe its violet shade to the
very refrangible and chemically active
violet rays at the extreme end of the pris-
matte spectrum; but ON the contrary, this
part of the spectrum is totally absent from
light passed through blue glass. The
special shade of the mazarin glass is
caused by the fact that its blue is tenn
pered by a considerable quantity of the
less refrangible red rays at the other or
caloric extremity of the spectrum, and
even with trace of orange. Its blue is,
therefore, of less chemical activity than
the prismatic blue, and of course in all its
functions, such as heat, chemical action,
etc., is far below the original unchanged
solar light.
We have gone into the details of these
rather elementary matters for the purpose
of exposing the ignorance OF those who
ascribe to the glass special chemical or
curative influence. Some photographers
have used blue glass long ago, in order to
moderate the intensity of the light for the
eyes of the sitter, without robbing it of too
much of its chemical activity; and those
photographers who possess common sense
or experience know that, far from adding
to the effect of the light, the blue glass is
an impeachment, and the necessary time
of exposure is rather extended by its use
than otherwise. 1t is strange that such
errors can prevail for years, when sim
pie experiment can settle the matter. All
that is necessary is to photograph the
solar spectrum, to do the same with a
surface painted with number of various
colored pigments, and also to expose a
sensitized surface under a series Of col.
Ored strips of glass. The writer of this
artic did this more than thirty years
ago by the Daguerrean process, and salts
fed himself about the following points:
l. The chemical effect of the prismatic
spectrum extends, for iodide of silver,
from beyond the violet to the blue. Q.
When bromiHe is used in connection with
the iodine, it extends to within the green,
while the yellow and red rays appear to
have no effect on silver compounds, but
may possess it for other substances.
8. in photographing pigments there is
| the utmost diversity in the results, accord.
ing to the nature Of the pigment-much
greater than the differences in shade
would lead us to expect. As a general
thing, the pure reds, orange and yellow,
such as are produced by vcrmilion and
chromates of lead, are phOtographically
inert, and give blacks. The blues are the
most active, most of all being ultrama.
rine, next the violet lakes. But even the
red carmine takes we'll, as it has violet
shade; but among the blues, those border.
ing on green take least, and hence foliage
tends to give dark effects, which are only
slightly corrected by using brOlnine. f.
In using as negatives strips of coiored
glass to print in sunlight, much depends
on the shade and intensity of the color
| In general, the chemical effect follows the
prismatic series from red to blue; but the
aost effective blue glass is always found
to be far inferior to the sunlight alone,
pure and simple. And this fact is suffi
cient to settle the question about the spe
cial virtues claimed for blue glass: it can.
not possibly have any not already pos
scssed by sunlight. However, If peopl.
are induced by its pretended curativ,
properties to take sun baths, which they
otherwise might neglect, they may b.
often benefited by the salubrious intluenc,
of the radiation of the mighty orb, an in
fluence which cannot sufficiently he APP
preciatcd; but the blue glass would prob
ably get the credit which exclusively he
longs to glorious old Sol.-Scientift
American