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
and three m