Super-sensible Vibrations
Moreover, as every text book on science informs us, there are sounds too
low as well as those too high for the human ear to register, but which
are registered by delicate instruments. Again, there are colors beyond
the place of red, at one end of the visible spectrum; and others beyond
the place of violet at the other end of that spectrum, which the human
eye is unable to register and detect, but which our apparatus in the
laboratory plainly register. The ray of light which registers on the
photographic plate, and which causes sunburn on our skin, is too high a
rate of vibration for our eyes to perceive. Likewise the X-Rays, and
many other of the finer rays of light known to science are imperceptible
to the unaided human vision--they are actually "dark rays" so far as the
human eye is concerned, though man has devised instruments by means of
which they may be caught and registered.