n exposure the results will vary with the light
and the width of the slit, as well as being influenced by the character
of the instrument itself. Further, all such experiments should be made
with a normal developer, and development continued for a definite time.
The only exception to this rule would be in the event of wishing to
ascertain the utmost that could be got out of a plate, but, under
ordinary circumstances, the developer ought never to vary, nor yet the
duration of development. To try the effect of various developers, or
varying time in development, a departure must be made of such a nature
as would operate to bring out upon each plate, or piece of a plate, the
utmost it would develop short of fog, against which caution must be
adopted in all spectrum experiments.

On development, say for one, two, or three minutes, wash off and fix.
You will recognize the H violet lines and the others to the left, and
this experiment shows what is the sensitiveness of this particular plate
to the various regions of the spectrum with this particular apparatus,
and with a normal exposure and development. So far, this teaches very
little; it merely indicates that this particular plate is sensitive or
insensitive to certain rays of colored light. To make this teaching of
any value, we must institute comparisons. Accordingly, instead of simply
exposing one plate, suppose we cut a strip from two, three, four, or
even half a dozen different plates, and arrange them side by side,
horizontally, in the dark slide, so that the spectrum falls upon the
whole when they are placed in the camera and exposed. There is really no
difficulty in cutting strips a quarter of an inch wide, the lengthway of
a quarter plate. Lay the gelatine plate film up, and hold a straight
edge on it firmly, so that when we use a suitable diamond we can plow
through the film and cut a strip which will break off easily between the
thumb and finger. A quarter plate can thus be cut up into strips to
yield about a dozen comparative experiments. When cut and snapped off,
mark each with pencil with such a distinguishing mark as shall be
clearly seen after fixing. The cut up strips can be kept in the maker's
plate box.

* * * * *

The deep down underground electric railway in London has so far proved
an unprofitable concern for its stockholders. It is 31/2 miles long,
touches some of the greatest points of traffic, but somehow or other