lity; and some of the
late books treating upon this very subject represent a solar spectrum as
being made up of a heat spectrum, a light spectrum, and an actinic or
chemical spectrum, and the idea has often been made to do duty as an
analogy in trinitarian theology; nevertheless it is utterly wrong and
misleading. There is no such thing as an actinic spectrum; that is,
there are no such rays as special chemical rays; any given ray will do
chemical work if it falls upon the proper kind of matter. For instance,
while it is true that for such salts of silver as the chloride, the
bromide, etc., the shorter waves are most efficient; by employing salts
of iron one may get photographic effects with wave lengths much too long
for any eye to perceive. Capt. Abney has photographed the whole solar
spectrum from one end to the other, which is sufficient evidence that
there are no special chemical rays. As to the eye itself, certain of the
wave lengths are competent to produce the sensation we call light, but
the same ray will heat the face of a thermopile or produce photographic
effects if permitted to act upon the proper material, so there is no
more propriety in calling it a light ray than in calling it a heat ray
or an actinic ray. What the ray will do depends solely upon what kind of
matter it falls upon, and all three of these names, _light_, _heat_, and
_actinism_, are names of _effects of radiant energy_. The retina of the
eye is itself demonstrably a photographic plate having a substance
called purpurine secreted by appropriate glands spread over it in place
of the silver salts of common photography. This substance purpurine is
rapidly decomposed by radiant energy of certain wave lengths, becoming
bleached, but the decomposition is attended by certain molecular
movements; the ends of the optic nerves, which are also spread over the
retina, are shaken by the disrupting molecules, and the disturbance is
the origin of what we call the sensation of light. But the sensation is
generally a compound one, and when all wave lengths which are competent
to affect the retina are present, the compound effect we call white or
whiteness. When some of the rays are absent, as, for instance, the
longer ones, the optical effect is one we call green or greenness; and
the special physiological mechanism for producing the sensation may be
either three special sets of nerves, capable of sympathetic vibration to
waves of about 1-39,000, 1-45,000, an