LECTURE V.

RANGE OF VISION NOT COMMENSURATE WITH RANGE OF RADIATION THE ULTRA-VIOLET BAYS FLUORESCENCE THE RENDERING OF INVISIBLE RAYS VISIBLE VISION NOT THE ONLY SENSE APPEALED TO BY THE SOLAR AND ELECTRIC BEAM HEAT OF BEAM COMBUSTION BY TOTAL BEAM AT THE FOCI OF MIRRORS AND LENSES COMBUSTION THROUGH ICE-LENS IGNITION OF DIAMOND SEARCH FOR THE RAYS HERE EFFECTIVE SIR WILLIAM HERSCHEL'S DISCOVERY OF DARK SOLAR RAYS INVISIBLE RAYS THE BASIS OF THE VISIBLE DETACHMENT BY A RAY-FILTER OF THE INVISIBLE RAYS FROM THE VISIBLE COMBUSTION AT DARK FOCI CONVERSION OF HEAT-RAYS INTO LIGHT-RAYS CALORESCENCE PART PLAYED IN NATURE BY DARK RAYS IDENTITY OF LIGHT AND RADIANT HEAT INVISIBLE IMAGES REFLECTION, REFRACTION, PLANE POLARIZATION, DEPOLARIZATION, CIRCULAR
POLARIZATION, DOUBLE REFRACTION, AND MAGNETIZATION OF RADIANT HEAT.

§ 1. Range of Vision and of Radiation.

The first question that we have to consider to-night is this: Is the eye, as an organ of vision, commensurate with the whole range of solar radiation—is it capable of receiving visual impressions from all the rays emitted by the sun? The answer is negative. If we allowed ourselves to accept for a moment that notion of gradual growth, amelioration, and ascension, implied by the term evolution, we might fairly conclude that there are stores of visual impressions awaiting man, far greater than those now in his possession. Ritter discovered in 1801 that beyond the extreme violet of the spectrum there is a vast efflux of rays which are totally useless as regards our present powers of vision. These ultra-violet waves, however, though incompetent to awaken the optic nerve, can shake asunder the molecules of certain compound substances on which they impinge, thus producing chemical decomposition.

But though the blue, violet, and ultra-violet rays can act thus upon certain substances, the fact is hardly sufficient to entitle them to the name of 'chemical rays,' which is usually applied to distinguish them from the other constituents of the spectrum. As regards their action upon the salts of silver, and many other substances, they may perhaps merit this title; but in the case of the grandest example of the chemical action of light—the decomposition of carbonic acid in the leaves of plants, with which my eminent friend Dr. Draper (now no more) has so indissolubly associated his name—the yellow rays are found to be the most active.

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.

§ 2. Ultra-violet Rays: Fluorescence.