sts a black shadow surrounded with a colored background.
Now I may cast another shadow from a candle or an incandescence lamp, and
the two shadows are illuminated, one by the light of the colored patch
and the other by the light from an incandescence lamp which I am using
tonight. [Shown.] Now one stripe is evidently too dark. By an arrangement
which I have of altering the resistance interposed between the battery
and the lamp, I can diminish or increase the light from the lamp, first
making the shadow it illuminates too light and then too dark compared
with the other shadow, which is illuminated by the colored light.
Evidently there is some position in which the shadows are equally
luminous. When that point is reached, I can read off the current which is
passing through the lamp, and having previously standardized it for each
increment of current, I know what amount of light is given out. This
value of the incandescence lamp I can use as an ordinate to a curve, the
scale number which marks the position of the color in the spectrum being
the abscissa. This can be done for each part of the spectrum, and so a
complete curve can be constructed, which we call the illumination curve
of the spectrum of the light under consideration.

Now, when we are working in the laboratory with a steady light, we may be
at ease with this method, but when we come to working with light such as
the sun, in which there may be constant variation, owing to passing, and
may be usually imperceptible, mist, we are met with a difficulty; and in
order to avoid this, General Festing and myself substituted another
method, which I will now show you. We made the comparison light part of
the light we were measuring. Light which enters the collimating lens
partly passes through the prisms and is partly reflected from the first
surface of the prism; that we utilize, thus giving a second shadow. The
reflected rays from P_{1} fall on G, a silver on glass mirror. They are
collected by L_{5}, and form a white image of the prism also at F.

The method we can adopt of altering the intensity of the comparison light
is by means of rotating sectors, which can be opened or closed at will,
and the two shadows thus made equally luminous. [Shown.] But although
this is an excellent plan for some purposes, we have found it better to
adopt a different method. You will recollect that the brightest part of
the spectrum is in the yellow, and that it falls off in brightness on