e and below these two
sections, and that consequently there is much light that we cannot
see, and there are many sounds to which our ears are deaf. In the case
of light the action of these higher and lower vibrations is easily
perceptible in the effects produced by the actinic rays at one end of
the spectrum and the heat rays at the other.

As a matter of fact there exist vibrations of every conceivable degree
of rapidity, filling the whole vast space intervening between the slow
sound waves and the swift light waves; nor is even that all, for there
are undoubtedly vibrations slower than those of sound, and a whole
infinity of them which are swifter than those known to us as light. So
we begin to understand that the vibrations by which we see and hear
are only like two tiny groups of a few strings selected from an
enormous harp of practically infinite extent, and when we think how
much we have been able to learn and infer from the use of those
minute fragments, we see vaguely what possibilities might lie before
us if we were enabled to utilize the vast and wonderful whole.

Another fact which needs to be considered in this connection is that
different human beings vary considerably, though within relatively
narrow limits, in their capacity of response even to the very few
vibrations which are within reach of our physical senses. I am not
referring to the keenness of sight or of hearing that enables one man
to see a fainter object or hear a slighter sound than another; it is
not in the least a question of strength of vision, but of extent of
susceptibility.

For example, if anyone will take a good bisulphide of carbon prism,
and by its means throw a clear spectrum on a sheet of white paper, and
then get a number of people to mark upon the paper the extreme limits
of the spectrum as it appears to them, he is fairly certain to find
that their powers of vision differ appreciably. Some will see the
violet extending much farther than the majority do; others will
perhaps see rather less violet than most, while gaining a
corresponding extension of vision at the red end. Some few there will
perhaps be who can see farther than ordinary at both ends, and these
will almost certainly be what we call sensitive people--susceptible in
fact to a greater range of vibrations than are most men of the present
day.

In hearing, the same difference can be tested by taking some sound
which is just not too high to be audible--on the very v