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. Thus,
suppose I interpose an artificial cloud before the slit of the
spectroscope, having adjusted the two shadows, it will be seen that the
passage of steam in front of the slit does not alter the relative
intensities; but this result must be received with caution. [The lecturer
then proceeded to point out the contrast colors that the shadow of the
rod illuminated by white light assumed.]

I must now make a digression. It must not be assumed that every one has
the same sense of color, otherwise there would be no color blindness.
Part of the researches of General Festing and myself have been on the
subject of color blindness, and these I must briefly refer to. We test
all who come by making them match the luminosity of colors with white
light, as I have now shown you. And as a color blind person has only two
fundamental color perceptions instead of three, his matching of
luminosities is even more accurate than is that made by those whose eyes
are normal or nearly normal. It is curious to note how many people are
more or less deficient in color perception. Some have remarked that it is
impossible that they were color blind and would not believe it, and
sometimes we have been staggered at first with the remarkable manner in
which they recognized color to which they ultimately proved deficient in
perception. For instance, one gentleman when I asked him the name of a
red color patch, said it was sunset color. He then named green and blue
correctly, but when I reverted to the red patch he said green.

On testing further, he proved totally deficient in the color perception
of red, and with a brilliant red patch he matched almost a black shadow.
The diagram shows you the relative perceptions in the spectrum of this
gentleman and myself. There are others who only see three-quarters,
others half, and others