There are substances, however, on which the violet and ultra-violet waves exert a special decomposing power; and, by permitting the invisible spectrum to fall 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 the sulphate of quinine and afterwards dried, an obvious extension of the spectrum is revealed.  We have, in the first instance, a portion of the violet rendered whiter and more brilliant; but, besides this, we have the gleaming of the colour where, in the case of unprepared paper, nothing is seen.  Other substances produce a similar effect.  A substance, for example, recently discovered by President Morton, and named by him Thallene, produces a very striking elongation of the spectrum, the new light generated being of peculiar brilliancy.