upon surfaces prepared with such
substances, we reveal both the existence and the extent of the
ultraviolet spectrum.

Sec. 2. _Ultra-violet Rays: Fluorescence_.

The method of exhibiting the action of the ultraviolet rays by their
chemical action has been long known; indeed, Thomas Young photographed
the ultra-violet rings of Newton. We have now to demonstrate their
presence in another way. As a general rule, bodies either transmit
light or absorb it; but there is a third case in which the light
falling upon the body is neither transmitted nor absorbed, but
converted into light of another kind. Professor Stokes, the occupant
of the chair of Newton in the University of Cambridge, has
demonstrated this change of one kind of light into another, and has
pushed his experiments so far as to render the invisible rays visible.

A large number of substances examined by Stokes, when excited by the
invisible ultra-violet waves, have been proved to emit light. You know
the rate of vibration corresponding to the extreme violet of the
spectrum; you are aware that to produce the impression of this colour,
the retina is struck 789 millions of millions of times in a second. At
this point, the retina ceases to be useful as an organ of vision; for,
though struck by waves of more rapid recurrence, they are incompetent
to awaken the sensation of light. But when such non-visual waves are
caused to impinge upon the molecules of certain substances--on those
of sulphate of quinine, for example--they compel those molecules, or
their constituent atoms, to vibrate; and the peculiarity is, that the
vibrations thus set up are _of slower period_ than those of the
exciting waves. By this lowering of the rate of vibration through the
intermediation of the sulphate of quinine, the invisible rays are
brought within the range of vision. We shall subsequently have
abundant opportunity for learning that transparency to the visible by
no means involves transparency to the invisible rays. Our bisulphide
of carbon, for example, which, employed in prisms, is so eminently
suitable for experiments on the visual rays, is by no means so
suitable for these ultra-violet rays. Flint glass is better, and rock
crystal is better than flint glass. A glass prism, however, will suit
our present purpose.

Casting by means of such a prism a spectrum, not upon the white
surface of our screen, but upon a sheet of paper which has been wetted
with a saturated solution of