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
each side, so instead of opening and closing the sectors, they are set at
fixed intervals, and the slit is moved in front of the spectrum, just
making the shadow cast by the reflected beam too dark or too light, and
oscillating between the two till equality is discovered. The scale number
is then noted, and the curve constructed as before. It must be remembered
that, on each side of the yellow, equality can be established.
This method of securing a comparison light is very much better for sun
work than any other, as any variation in the light whose spectrum is to
be measured affects the comparison light in the same degree.
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