exposed to sky light for a time varying anywhere between twenty seconds
and three minutes, depending on the sensitiveness of the plates. The
instrument is then removed to the dark room, and the plates developed by
immersing them all at once in a solution consisting of four parts
potassium oxalate and one part ferrous sulphate. After ten minutes they
are removed, fixed, and dried. Their readings are then noted, and
compared with those obtained with the silver chloride. The chloride
experiment is again performed as soon as the plates have been removed,
and the first result confirmed. With some plates it is necessary to make
two or three trials before the right exposure can be found; but if the
image disappears anywhere between the second and eighth divisions, a
satisfactory result may be obtained.

The plates were also tested using gaslight instead of daylight. In this
case an Argand burner was employed burning five cubic feet of gas per
hour. A diaphragm 1 cm. in diameter was placed close to the glass
chimney, and the chloride was placed at 10 cm. distance, and exposed to
the light coming from the brightest part of the flame, for ten hours.
This produced an impression as far as the third division of the scale.
The plates were exposed in the sensitometer as usual, except that it was
found convenient in several cases to use a larger stop, measuring 0.316
cm. in diameter.

The following table gives the absolute sensitiveness of several of the
best known kinds of American and foreign plates, when developed with
oxalate, in terms of pure silver chloride taken as a standard. As the
numbers would be very large, however, if the chloride were taken as a
unit, it was thought better to give them in even hundred thousands.

SENSITIVENESS OF PLATES.