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