5.  The particles of the blue cloud are immeasurably small, but they increase gradually in size, and at a certain period of their growth cease to discharge perfectly polarized light.  For some time afterwards the light that reaches the eye, through the Nicol in its position of least transmission, is of a magnificent blue, far exceeding in depth and purity that of the purest sky; thus the waves that first feel the influence of size, at both limits of the polarization, are the shortest waves of the spectrum.  These are the first to accept polarization, and they are the first to escape from it.

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.

Sec. 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.