If on opening a text book on geology one should find stated the view
concerning the creation and age of the earth that was held a hundred
years ago, and this view gravely put forward as a possible or
alternative hypothesis with the current one deducible from the nebula
theory, one would be excused for smiling while he turned to the title
page to see who in the name of geology should write such stuff.
Nevertheless this is precisely similar to what one will find in most
treatises on physics for schools and colleges if he turns to the subject
of light. For instance, I quote from a book edited by an eminent man of
science in England, the book bearing the date 1873.
"There are two theories of light; one the _emissive_ theory; ... the
other, the _vibratory_ theory;" just as if the emissive or corpuscular
theory was not mathematically untenable sixty years ago, and
experimentally demonstrated to be false more than forty years ago.
Unless one were treating of the history of the science of optics there
is no reason why the latter theory should be mentioned any more than the
old theory of the formation of the earth. It is not to be presumed that
any one whose opinion is worth the asking still thinks it possible that
the old view may be the true one because the evidence is demonstrable
against it, yet while the undulatory theory prevails there are not a few
persons well instructed otherwise who still write and speak as though
light has some sort of independent existence as distinguished from
so-called radiant heat; in other words, that the heat and light we
receive from the sun are specifically different.
A brief survey of our present knowledge of this form of energy will
help to show how far wrong the common conception of light is. For
fifteen years it has been common to hear heat spoken of as a mode of
molecular motion, and sometimes it has been characterized as
_vibratory_, and most persons have received the impression that the
vibratory motion was an actual change of position of the molecular in
space instead of a _change of form_. Make a ring of wire five or six
inches in diameter, and, holding it between the thumb and finger at the
twisted ends, pluck it with a finger of the other hand; the ring will
vibrate, have three nodes, and will give a good idea of the character of
the vibration that constitutes what we call heat. This vibratory motion
may have a greater or less amplitude, and the energy of the vibration
will be as the square of that amplitude. But the vibrating molecule
gives up its energy of vibration to the surrounding ether; that is to
say, it loses amplitude precisely as a vibrating tuning fork will lose
it. The ether transmits the energy it has received in every direction
with the velocity of 186,000 miles per second, whether the amplitude be
great or small, and whether the number of vibrations be many or few. It
is quite immaterial. The _form_ of this energy which the ether transmits
is _undulatory_; that is to say, not unlike that of the wave upon a
loose rope when one end of it is shaken by the hand. As every shake of
the hand starts a wave in the rope, so will every vibration of a part of
the molecule start a wave in the ether. Now we have several methods for
measuring the wave lengths in ether, and we also know the velocity of
movement. Let v = velocity, l = wave length, and n = number of
vibrations per second, then n = v/l, and by calculation the value of n
varies within wide limits, say from 1 × 10^{14} to 20 × 10^{14}. But all
vibrating bodies are capable of vibrating in several periods, the
longest period being called the fundamental, and the remainder, which
stand in some simple ratios to the fundamental, are called _harmonics_.
Each of these will give to the ether its own particular vibratory
movement, so that a single molecule may be constantly giving out rays of
many wave lengths precisely as a sounding bell gives out sounds of
various pitches at one and the same time.
Again, when these undulations in the ether fall upon other molecules the
latter may reflect them away or they may absorb them, in which case the
absorbing molecules are themselves made to vibrate with increased
amplitude, and we say they have been heated. Some molecules, such as
carbon, appear to be capable of stopping undulations of all wave lengths
and to be heated by them; others are only affected by undulations of
particular wave lengths, or of wave lengths between special limits. In
this case it is a species of sympathetic vibration. The distinction
between the molecular vibrations, and the undulations in ether that
result from them, must be kept in mind, as must also the effect of the
undulations that fall upon other molecules. To one the name _heat_ is
applied, to the other the name of _radiant energy_ is given; and it
matters not whether the undulations be long or short, the same molecule
may give out both.
Now let a prism be placed in the path of such rays of different wave
length from a single molecule, and what is called the dispersive action
of the prism will separate the rays in the order of their wave lengths,
the longer waves being less refracted than the shorter ones; but the
energy of any one of these will depend upon the _amplitude of
undulation_, which in turn will depend upon the amplitude of vibration
of the part of the molecule that originated it, but in general the
longer waves have greater amplitude, though not necessarily so.
Consequently, if a thermopile be so placed as to receive these various
rays, and their energy be measured by its absorption on the face of the
pile, each one would be found to heat it, the longer waves more than the
shorter ones, simply because the amplitude is greater, but for no other
reason, for it is possible, and in certain cases is the fact, that a
short wave has as much or more energy than a longer one. If the eye
should take the place of the thermopile it would be found that some of
these rays did not affect it at all, while some would produce the
sensation of light. This would be the case with any waves having a wave
length between the limits of, say, 1-37,000 of an inch and 1-60,000 of
an inch; any shorter waves will not produce the sensation of light. If
instead of the eye a piece of paper washed in a solution of the chloride
of silver should be placed where the dispersed rays should fall upon it,
it would be found that only the shorter waves would affect it at all,
and among these shorter ones would be some of those rays which the eye
could not perceive at all.
It was formerly inferred from these facts that the heat rays, the light
rays, and the chemical rays were different in quality; 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, and 1-55,000 of an inch in length, as
Helmholtz has suggested, or, as seems to the writer more probable, the
substance purpurine is a highly complex organic substance made up of
molecules of different sizes and requiring wave lengths of different
orders to decompose them, so that a part of the substance may be quite
disintegrated, while other molecules may be quite entire throughout the
visual space. This will account for most of the chromatic effects of
vision, for complementary colors, and for color blindness, by supposing
that the purpurine is not normally constituted. This is in accordance
with experimental photography, for it has been found that the long waves
will act only upon heavier molecules. It is true vision may be good when
there is no purpurine, but there is no doubt but that this substance is
secreted in the eye, and that it is photographic in its properties, and
so far must be taken as an element in any theory of vision; but the
chief point here considered is that objectively light does not exist
independent of the eye, that light is a physiological phenomenon, and to
speak of it otherwise is to confound a cause with an effect. It is,
hence, incorrect to speak of the velocity of light; it has no velocity.
It is _radiant energy_ that has the velocity of 186,000 miles a second.
It is incorrect to say we receive heat from the sun. What we do receive
is radiant energy, which is here transformed into heat. This is not
hypercritical, but is in accordance with the knowledge we have to-day.
The old nomenclature we use, but without definite meaning; the latter is
left to be inferred from the connection or context. If a man should
attach to the water main in a city a properly constructed waterwheel,
the latter will rotate; but it would not be proper to say that he
received rotation from the reservoir. What he received was water with a
certain pressure; in other words, a certain form of energy, which he
transforms into rotation by the appropriate means; but by substituting
other means he can make the same water pressure maintain a vibratory
motion, as with the hydraulic ram valve, or let it waste itself by open
flow, in which case it becomes ultimately molecular vibration that is
heat. The analogy holds strictly. The trouble all comes from neglecting
to distinguish between different forms of energy--energy in matter and
energy in the ether.
* * * * *