[4]
Now, as every ray of homogeneous light which we can perceive at all,
appears to us as a distinct color, it becomes probable that these
ultra-violet rays must make themselves apparent to animals as a distinct
and separate color (of which we can form no idea), but as different from
the rest as red is from yellow, or green from violet. The question also
arises whether white light to these creatures would differ from our white
light in containing this additional color.
These considerations cannot but raise the reflection how different the
world may--I was going to say must--appear to other animals from what it
does to us. Sound is the sensation produced on us when the vibrations of
the air strike on the drum of our ear. When they are few, the sound is
deep; as they increase in number, it becomes shriller and shriller; but
when they reach 40,000 in a second, they cease to be audible. Light is the
effect produced on us when waves of light strike on the eye. When 400
millions of millions of vibrations of ether strike the retina in a second,
they produce red, and as the number increases the color passes into
orange, then yellow, green, blue, and violet.
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