a quarter the amount of red that we see, while
some see none. Others see less green and others less violet, but I have
met with no one that can see more than myself or General Festing, whose
color perceptions are almost identical. Hence we have called our curve of
illumination the "normal curve."

We have tested several eminent artists in this manner, and about one half
of the number have been proved to see only three quarters of the amount
of red which we see. It might be thought that this would vitiate their
powers of matching color, but it is not so. They paint what they see; and
although they see less red in a subject, they see the same deficiency in
their pigments; hence they are correct. If totally deficient, the case of
course would be different.

Let us carry our experiments a step further, and see what effect what is
known as a turbid medium has upon the illuminating value of different
parts of the spectrum. I have here water which has been rendered turbid
in a very simple manner. In it has been very cautiously dropped an
alcoholic solution of mastic. Now mastic is practically insoluble in
water, and directly the alcoholic solution comes in contact with the
water it separates out in very fine particles, which, from their very
fineness, remain suspended in the water. I propose now to make an
experiment with this turbid water.

I place a glass cell containing water in front of the slit, and on the
screen I throw a patch of blue light. I now change it for turbid water in
a cell. This thickness much dims the blue; with a still greater thickness
the blue has almost gone. If I measure the intensity of the light at each
operation, I shall find that it diminishes according to a certain law,
which is of the same nature as the law of absorption. For instance, if
one inch diminishes the light one half, the next will diminish it half of
that again, the next half of that again, while the fourth inch will cause
a final diminution of the total light of one sixteenth. If the first inch
allows only one quarter of the light, the next will only allow one
sixteenth, and the fourth inch will only permit 1/256 part to pass.

Let us, however, take a red patch of light and examine it in the same
way. We shall find that, when the greater thickness of the turbid medium
we used when examining the blue patch of light is placed in front of the
slit, much more of this light is allowed to pass than of the blue. If we
measure the light, w