The rays
are collected by a lens, L_{4}, tilted at an angle as shown, to form a
white image of the near surface of the second prism on F.
Passing a card with a narrow slit, S_{2}, cut in it in front of the
spectrum, any color which I may require can be isolated. The consequence
is that, instead of the white patch upon the screen, I have a colored
patch, the color of which I can alter to any hue lying between the red
and the violet. Thus, then, we are able to get a real patch of very
approximately homogeneous light to work with, and it is with these
patches of color that I shall have to deal. Is there any way of measuring
the brightness of these patches? was a question asked by General Festing
and myself. After trying various plans, we hit upon the method I shall
now show you, and if any one works with it he must become fascinated with
it on account of its almost childish simplicity--a simplicity, I may
remark, which it took us some months to find out. Placing a rod before
the screen, it casts 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.
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