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.
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