The Visible and Invisible Parts of the Spectrum--Methods for showing
the Existence of the Invisible Portions--Phosphorescence--Photography
of the Dark Rays--Thermo-Electric Currents.
We are apt to forget, when looking at the spectrum, that what the eye
sees is not all that is to be found in the prismatic analysis of light.
The spectrum, it must be recollected, is not limited to those rays which
the eye perceives. There are rays both beyond the extreme violet and
below the extreme red, which exist and which exercise a marked effect on
the world's economy. Thus, rays beyond the violet are those which with
the violet and the blue rays principally affect vegetation, enabling
certain chemical changes to take place which are necessary for its
growth and health; whilst the rays below the red are those possessing
the greatest amount of energy, and if they fall upon bodies which absorb
them, as very nearly all bodies do to a certain extent, they heat them.
The warmth we feel from sunlight is principally due to the dark rays
which lie below the red of the spectrum.
The existence of both kinds of these dark rays may be demonstrated in a
very simple manner by the effect that they produce on certain bodies.
For instance, there is a yellow dye with which cheap ribbon is dyed,
which if placed in the spectrum and beyond the violet causes a visible
prolongation of the spectrum. The light in the newly-seen and once
invisible part of the spectrum is yellow, the colour of the ribbon
itself. In fact, the whole of that part of the spectrum, which on the
white screen is seen as blue and violet, becomes yellow, the red and
green remaining unchanged. This change in colour is due to fluorescence,
a phenomenon of light which Sir G. Stokes found was caused by an
alteration in the lengths of the waves of light when reflected from
certain bodies. It is not meant to imply by this that the wave-length of
any ray falling on a body can be altered by reflection, but only that
the body itself on which the rays fall emits rays of light which are not
of the same wave-length as those which fall upon it. Now it is a fact
that the rays that lie beyond the violet, and which are ordinarily
invisible, are shorter than the violet rays, and that these are shorter
than the yellow rays. It follows therefore that when, what we may now
call, the ultra-violet rays fall on the yellow dyed ribbon, the waves
emitted by it are so lengthened that they appear yellow to the eye
instead of dark, violet, or blue.
We can also brush a solution of quinine on the screen, and immediately
the place where the ultra-violet rays fall is illuminated by a violet
light. We do not see the ultra-violet rays themselves, but only the rays
of increased wave-length, which are emitted by their effect on the
sulphate of quinine. Common machine oil as used for engines also emits
greenish rays when excited by the ultra-violet rays, and a very
beautiful colour it is. Fluorescence then is one means of demonstrating
the existence of the ultra-violet rays--or Ritter's rays as they were
formerly called, after their discoverer--in a very simple manner. The
method of rendering the effects of the infra-red rays visible to the eye
is also interesting. All, or at all events most, of our readers have
seen Balmain's luminous paint. A glass or card coated with this
substance, which is essentially a sulphide of calcium, when exposed to
the light of the sun, or of the electric arc, and then taken into
comparative darkness, is seen to shine with a peculiar violet-coloured
light. If when thus excited we place it in a bright spectrum for some
little time, we shall find on shutting off the light that where the
ultra-violet and blue fell on it, the violet light is intenser than the
light of the main part of the screen; where the yellow fell there is
neither increase or diminution in brightness; but that in the red it
becomes darker, and also beyond the limit of the visible spectrum,
indicating the existence of rays beyond, which through their greater
length have not the power of affecting the eye. If the spectrum be shut
off, however, very soon after it falls on the plate, it has been
asserted that the red and infra-red rays have increased the brightness
of that particular part of the plate on which they fell. At first these
two observations seem to contradict one another; they do not in reality.
We may expose a tablet of Balmain's paint to light, and place a heated
iron in contact with the back of the plate; we shall then find that the
iron produces a bright image of its surface on a less bright background.
This bright image will gradually fade away, and the same space will
eventually become dark compared with the rest of the plate. The reason
of this is clear. When light excites the paint a certain amount of
energy is poured into it, which it radiates out slowly as light. When
the hot iron is placed in contact with it, the heat causes the light to
radiate more rapidly, and consequently with greater intensity, at the
part where its surface touches, and the energy of that particular
portion becomes used up. When the energy of radiation of this part
becomes less than that of the rest of the tablet, its light must of
necessity be of less brightness than that of the background, with which
the heated iron has had no contact. For this reason the image of the
iron subsequently appears dark. We shall see presently, and as before
stated, that the principal heating effect of the spectrum lies in the
red and infra-red, and it is owing to the heating of the paint by these
rays that the image might be at first slightly brighter than the
background, and subsequently darker.
There is another way in which the existence of both the ultra-violet and
infra-red rays can be demonstrated, and that is by means of photography.
If we place an ordinary photographic plate in the spectrum and develop
it, we shall find that besides being affected by the blue and violet
rays, it is also affected by the rays beyond the violet, the energy of
these rays being capable of causing a decomposition of the sensitive
silver salt. If quartz prisms and lenses be used, and the electric light
be the source of illumination, the ultra-violet spectrum will extend to
an enormous extent. A more difficult, but perhaps even more interesting
means of illustrating the existence of the infra-red rays, and first due
to the writer, can be made by means of photography. It is possible to
prepare a photographic plate with bromide of silver, which is so
molecularly arranged that it becomes capable of being decomposed not
only by the violet and blue rays, but also by the red rays, and by those
rays which have wave-lengths of nearly three times that of the red rays.
It would be inappropriate to enter into a description of the method of
the preparation of these plates. Those who are curious as to it will
find a description in the Bakerian lecture published in the
Philosophical Transactions of the Royal Society for 1881. With plates so
prepared it has been found possible to obtain impressions in the dark
with the rays coming from a black object, heated to only a black heat.
That these dark rays possess greater energy or capacity for doing work
of some kind than any other rays of the spectrum, can be shown by means
of a linear thermopile (Fig. 4), if it be so arranged as to allow only a
narrow vertical slice of light to reach its face.
Fig. 4.--The Thermopile.
The principle of the thermopile we need not describe in detail. Suffice
it to say that the heating of the soldered junctions of two dissimilar
metals (there are ten pairs of antimony and bismuth in the above
instrument) produces a feeble current of electricity, which, however, is
sufficient to cause a deflection to the suspended needle of a delicate
galvanometer. To the needle is attached a mirror weighing a fraction of
a grain, and the deflections are made visible by the reflection from it
of a beam of light issuing from a fixed point along a scale. The greater
the heating of the junctions of the thermopile, within limits which in
these cases are never exceeded, the greater is the current produced, and
consequently the greater is the deflection of the mirror-bearing
needle, and of the beam of light along the scale. In order to get a
comparative measure of the energies of the different rays, it is
necessary that they should be completely absorbed. Now the junctions
themselves of the pile being metal, and therefore more or less bright,
will not absorb completely, but if they be coated with a fine layer of
lamp-black, the rays falling on the pile will be absorbed by this
substance, and their absorption will cause a rise in temperature in it,
and the heat will be communicated to the thermopile.
If we make a bright spectrum, and one not too long, say three inches in
length, and pass the linear thermopile through its length, we shall find
that when the galvanometer is attached, the galvanometer needle will be
differently deflected in its various parts. The deflection will be
almost insensible in the violet, but sensible in the blue, rather more
in the green, still more in the yellow, and it will further increase in
the red. When, however, the slit of the thermopile is placed beyond the
limit of the visible spectrum, the deflection enormously increases, and
will increase till a position is reached as far below the red as the
yellow is above it. After this maximum is reached, by moving the pile
still further from the red, the galvanometer needle will travel towards
its zero, and finally all deflection will cease. At this point we may
suppose we have reached the limit of the spectrum, but if rock-salt
prisms and lenses be used, the limit will be increased. What the real
limit of the spectrum is, is at present unknown; Mr. Langley with his
bolometer, and rock-salt prisms, an instrument more sensitive than the
thermopile, must have nearly reached it.
Fig. 5.--Heating effect of different Sources of Radiation.
The above figure is a graphic representation of the heating effect of
the spectrum of the electric light, sunlight, and the incandescence
electric light, on the lamp-black coating of the thermopile, as shown by
the galvanometer. The vast difference between the heating effect of the
visible rays of the first two sources compared with the last is clearly
indicated.
Since every ray may be taken as totally absorbed, the heating of the
lamp-black is a measure of the energy or the capacity of performing work
of some description, which they possess. Waves of the sea do work when
they beat against the shore, and they do work when they lift a vessel.
If we notice a ship at anchor we shall find that behind the vessel and
towards the shore the waves are lowered in height or amplitude; the
energy which they have expended in raising the vessel of necessity
causes this lowering. In the same way the waves of light, after falling
on matter whose molecules or atoms are swinging in unison with them, are
destroyed, and the energy is spent in either decomposing the matter into
a simpler form at first--though the subsequent form may be more
complex--or in raising its temperature. As lamp-black or carbon is in
its simplest form, the only work done upon it by the energy of radiation
is the raising of its temperature, and it is for this reason that this
material is so excellent for covering the junctions of the pile. The eye
evidently does not absorb all rays, since only a limited part of the
spectrum is visible, and it would be useless to take a measure of the
heating effect of lamp-black for the visible part of the spectrum as a
measure of its luminosity, since the latter fades off in the red--the
very place in which the heat curve rises rapidly.