Scientific American Supplement, No. 520, December 19, 1885 | Page 7

the collodion, condition of the bath, etc., this system is
very unsatisfactory. Another method, employed largely in England,
depends on the use of the Warnerke sensitometer. In this instrument the
light from a tablet coated with luminous paint just after being exposed
to a magnesium light is permitted to shine through a colored
transparent film of graduated density upon the plate to be tested. Each
degree on the film has a number, and, after a given exposure, the last
number photographed on the plate represents the sensitiveness on an
empirical scale. There are two or three objections to this instrument. In
the first place, the light-giving power of the luminous tablet is liable to
variations, and, if left in a warm, moist place, it rapidly deteriorates.
Again, it has been shown by Captain Abney that plates sensitized by
iodides, bromides, and chlorides, which may be equally sensitive to

white light, are not equally affected by the light emitted by the paint;
the bromides being the most rapidly darkened, the chlorides next, and
the iodides least of all. The instrument is therefore applicable only to
testing plates sensitized with the same salts.
In this investigation it was first shown that the plates most sensitive for
one colored light were not necessarily the most so for light of another
color. Therefore it was evident that the sun must be used as the ultimate
source of light, and it was concluded to employ the light reflected from
the sky near the zenith as the direct source. But as this would vary in
brilliancy from day to day, it was necessary to use some method which
would avoid the employment of an absolute standard of light. It is
evident that we may escape the use of this troublesome standard, if we
can obtain some material which has a perfectly uniform sensitiveness;
for we may then state the sensitiveness of our plates in terms of this
substance, regardless of the brilliancy of our source. The first material
tried was white filter paper, salted and sensitized in a standard solution
of silver nitrate. This was afterward replaced by powdered silver
chloride, chemically pure, which was found to be much more sensitive
than that made from the commercial chemicals. This powder is spread
out in a thin layer, in a long paper cell, on a strip of glass. The cell
measures one centimeter broad by ten in length. Over this is laid a sheet
of tissue paper, and above that a narrow strip of black paper, so
arranged so as to cover the chloride for its full length and half its
breadth. These two pieces of paper are pasted on to the under side of a
narrow strip of glass which is placed on top of the paper cell. The
apparatus in which the exposures are made consists of a box a little
over a meter in length, closed at the top by a board, in which is a
circular aperture 15'8 cm. in diameter. Over this board may be placed a
cover, in the center of which is a hole 0.05 cm. in diameter, which
therefore lets through 0.00001 as much light as the full aperture. The
silver chloride is placed a distance of just one meter from the larger
aperture, and over it is placed the photographic scale, which might be
made of tinted gelatines, or, as in the present case, constructed of long
strips of tissue paper, of varying widths, and arranged like a flight of
steps; so that the light passing through one side of the scale traverses
nine strips of paper, while that through the other side traverses only one
strip. Each strip cuts off about one-sixth of the light passing through it,

so that, taking the middle strip as unity, the strips on either side taken in
order will transmit approximately--
1 2 3 4 5 6 7 8 9 2.0 1.65 1.4 1.2 1.0 0.85 0.7 0.6 0.5
The instrument is now pointed toward the zenith for about eight
minutes, on a day when there is a bright blue sky. On taking the
apparatus into the dark room and viewing the impression by gaslight, it
will be found that the markings, which are quite clear at one end, have
entirely faded out by the time the middle division is reached. The last
division clearly marked is noted. Five strips cut from sensitized glass
plates, ten centimeters long and two and a half in width, are now placed
side by side under the scale, in the place of the chloride. By this means
we can test, if we wish, five different kinds of plates at once. The cover
of the sensitometer containing the 0.05cm. hole is put on, and the plates
exposed to sky light for a time varying anywhere between twenty
seconds