Taking a slide with a wide and long slot
in it, a portion is occupied by a right angled prism, one of the angles
of 45 deg. being toward the center of the slot. By sliding this prism in
front of the spectrum I can deflect outward any portion of the spectrum I
like, and by a mirror can reflect it through a second lens, forming a
patch of light on the screen overlapping the patch of light formed by the
undeflected rays. If the two patches be exactly equal, white light is
formed. Now, by placing a rod as before in front of the patch, I have two
colored stripes in a white field, and though the background remains of
the same intensity of white, the intensities of the two stripes can be
altered by moving the right angled prism through the spectrum. The two
stripes are now apparently equally luminous, and I see the point of
equality is where the edge of the right angled prism is in the green.
Placing a narrow cell filled with our turbid medium in front of the slit,
I find that the equality is disturbed, and I have to allow more of the
yellow to come into the patch formed by the blue end of the spectrum, and
consequently less of it in the red end. I again establish equality.
Placing a thicker cell in front, equality is again disturbed, and I have
to have less yellow still in the red half, and more in the blue half.
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