Chemical or Photographic Rays of Solar Spectrum—Scheele, Ritter, and
Wollaston’s Discoveries—Wedgwood’s and Sir Humphry Davy’s Photographic
Pictures—The Calotype—The Daguerreotype—The Chromatype—The
Cyanotype—Collodion—Sir John Herschel’s Discoveries in the Chemical
Spectrum—M. Becquerel’s Discoveries of Inactive Lines in ditto—Thermic
Spectrum—Phosphoric Spectrum—Electrical Properties—Parathermic
Rays—Moser and Hunt’s Experiments—General Structure and antagonist
Properties of Solar Spectrum—Defracted Spectrum.
THE Solar Spectrum exercises an energetic action on matter, producing
the most wonderful and mysterious changes on the organised and
unorganised creation.
All bodies are probably affected by light, but it acts with greatest
energy on such as are of weak chemical affinity, imparting properties to
them which they did not possess before. Collodion and metallic salts,
especially those of silver, whose molecules are held together by an
unstable equilibrium, are of all bodies the most susceptible of its
influence; the effects, however, vary with the substances employed, and
with the different rays of the solar spectrum, the chemical properties
of which are by no means alike. As early as 1772 M. Scheele showed that
the pure white colour of chloride of silver was rapidly darkened by the
blue rays of the solar spectrum, while the red rays had no effect upon
it: and in 1801 M. Ritter discovered that invisible rays beyond the
violet extremity have the property of blackening argentine salts, that
this property diminishes towards the less refrangible part of the
spectrum, and that the red rays have an opposite quality, that of
restoring the blackened salt of silver to its original purity; from
which he inferred that the most refrangible extremity of the spectrum
has an oxygenising power, and the other that of deoxygenating. Dr.
Wollaston found that gum guaiacum acquires a green colour in the violet
and blue rays, and resumes its original tint in the red. No attempt had
been made to trace natural objects by means of light reflected from
them, till Mr. Wedgwood, together with Sir Humphry Davy, took up the
subject: they produced profiles and tracings of objects on surfaces
prepared with nitrate and chloride of silver, but they did not succeed
in rendering their pictures permanent. This difficulty was overcome in
1814 by M. Niepcé, who produced a permanent picture of surrounding
objects by placing in the focus of a camera-obscura a metallic plate
covered with a film of asphalt dissolved in oil of lavender.
Mr. Fox Talbot, without any knowledge of M. Niepcé’s experiments, had
been engaged in the same pursuit, and must be regarded as an independent
inventor of photography, one of the most beautiful arts of modern times:
he was the first who succeeded in using paper chemically prepared for
receiving impressions from natural objects; and he also discovered a
method of fixing permanently the impressions—that is, of rendering the
paper insensible to any further action of light. In the calotype, one of
Mr. Talbot’s applications of the art, the photographic surface is
prepared by washing smooth writing-paper, first with a solution of
nitrate of silver, then with bromide of potassium, and again with
nitrate of silver, drying it at a fire after each washing; the paper is
thus rendered so sensitive to light that even the passage of a thin
cloud is perceptible on it, consequently it must be prepared by
candle-light. Portraits, buildings, insects, leaves of plants—in short,
every object is accurately delineated in a few seconds; and in the focus
of a camera-obscura the most minute objects are so exactly depicted that
the microscope reveals new beauties.
Since the effect of the chemical agency of light is to destroy the
affinity between the salt and the silver, Mr. Talbot found that, in
order to render these impressions permanent on paper, it was only
necessary to wash it with salt and water, or with a solution of iodide
of potassium. For these liquids the liquid hyposulphites have been
advantageously substituted, which are the most efficacious in dissolving
and removing the unchanged salt, leaving the reduced silver on the
paper. The calotype picture is negative, that is, the lights and shadows
are the reverse of what they are in nature, and the right-hand side in
nature is the left in the picture; but if it be placed with its face
pressed against photographic paper, between a board and a plate of
glass, and exposed to the sun a short time, a positive and direct
picture, as it is in nature, is formed: engravings may be exactly copied
by this simple process, and a direct picture may be produced at once by
using photographic paper already made brown by exposure to light.
While Mr. Fox Talbot was engaged in these very elegant discoveries in
England, M. Daguerre had brought to perfection and made public that
admirable process by which he has compelled Nature permanently to
engrave her own works; and thus the talents of France and England have
been combined in bringing to perfection this useful art. Copper, plated
with silver, was successfully employed by M. Daguerre for copying nature
by the agency of light. The surface of the plate is converted into an
iodide of silver, by placing it horizontally with its face downwards in
a covered box, in the bottom of which there is a small quantity of
iodine which evaporates spontaneously. In three or four minutes the
surface acquires a yellow tint, and then, screening it carefully from
light, it must be placed in the focus of a camera obscura, where an
invisible image of external objects will be impressed on it in a few
minutes. When taken out, the plate must be exposed in another box to the
action of mercurial vapour, which attaches itself to those parts of the
plate which had been exposed to light, but does not adhere to such parts
as had been in shadow; and as the quantity of mercury over the other
parts is in exact proportion to the degree of illumination, the shading
of the picture is perfect. The image is fixed, first by removing the
iodine from the plate by plunging it into hyposulphite of soda, and then
washing it in distilled water; by this process the yellow colour is
destroyed, and in order to render the mercury permanent, the plate must
be exposed a few minutes to nitric vapour, then placed in nitric acid
containing copper or silver in solution at a temperature of 61-1/4° of
Fahrenheit for a short time, and lastly polished with chalk. This final
part of the process is due to Dr. Berre, of Vienna.
Nothing can be more beautiful than the shading of these chiaroscuro
pictures when objects are at rest, but the least motion destroys the
effect; the method therefore is more applicable to buildings than
landscape. Colour is wanting; but the researches of Sir John Herschel
give reason to believe that even this will ultimately be attained.
The most perfect impressions of seaweeds, leaves of plants, feathers,
&c., may be formed by bringing the object into close contact with a
sheet of photographic paper, between a board and plate of glass; then
exposing the whole to the sun for a short time, and afterwards fixing it
by the process described. The colours of the pictures vary with the
preparation of the paper, by which almost any tint may be produced.
In the chromatype, a peculiar photograph discovered by Mr. Hunt,
chromate of copper is used, on which a dark brown negative image is
first formed, but by the continued action of light it is changed to a
positive yellow picture on a white ground; the farther effect of light
is checked by washing the picture in pure water.
In cyanotypes, a class of photographs discovered by Sir John Herschel,
in which cyanogen in its combinations with iron forms the ground, the
pictures are Prussian blue and white. In the chrysotype of the same
eminent philosopher, the image is first received on paper prepared with
the ammonia-citrate of iron, and afterwards washed with a neutral
solution of gold. It is fixed by water acidulated with sulphuric acid,
and lastly by hydriodate of potash, from which a white and purple
photograph results. It is vain to attempt to describe the various
beautiful effects which Sir John Herschel obtained from chemical
compounds, and from the juices of plants; the juice of the red poppy
gives a positive bluish purple image, that of the ten-week stock a fine
rose colour on a pale straw-coloured ground.
Pictures may be made by exposure to sunshine, on all compound substances
having a weak chemical affinity; but the image is often invisible, as in
the Daguerreotype, till brought out by washing in some chemical
preparation. Water is frequently sufficient; indeed Sir John Herschel
brought out dormant photographs by breathing on them, and some
substances are insensible to the action of light till moistened, as for
example, gum guaiacum. Argentine papers, however, are little subject to
the influence of moisture. The power of the solar rays is augmented in
certain cases by placing a plate of glass in close contact over the
sensitive surface.
All these various experiments, though highly interesting, have now been
superseded. It was found that paper did not always answer for
photography, on account of imperfections in its structure; silver plates
were too expensive; and glass was found to be unimpressable.
Nevertheless, M. Niepcé de Victor obtained beautiful results upon glass
coated with albumen mixed with sensitive substances, which suggested the
medium by means of which the art has been brought to its present
perfection, and that final step is due to Mr. Scott Archer. He coated a
plate of glass thinly with collodion, that is, gun-cotton dissolved in
ether and alcohol, which dries into a delicate transparent film of
extreme adhesiveness, and of such intense sensibility that the action of
light upon it is so instantaneous that it arrests a stormy sea or a
fleeting cloud before they have time to change. Now landscapes in
chiaroscuro are produced of great beauty, which by the slower methods
were mere masses of deep shade and broad light. Architecture is even
more perfectly obtained, but it fails to give a pleasing representation
of the human countenance.
Chemical action always accompanies the sun’s light, but the analysis of
the solar spectrum has partly disclosed the wonderful nature of the
emanation. In the research, properties most important and unexpected
have been discovered by Sir John Herschel, who imprints the stamp of
genius on all he touches—his eloquent papers can alone convey an
adequate idea of their value in opening a field of inquiry vast and
untrodden. The following brief and imperfect account of his experiments
is all that can be attempted here:—
A certain degree of chemical energy is distributed through every part of
the solar spectrum, and also to a considerable extent through the dark
spaces at each extremity. This distribution does not depend on the
refrangibility of the rays alone, but also on the nature of the rays
themselves, and on the physical properties of the analyzing medium on
which the rays are received, whose changes indicate and measure their
action. The length of the photographic image of the _same_ solar
spectrum varies with the physical qualities of the surface on which it
is impressed. When the solar spectrum is received on paper prepared with
bromide of silver, the chemical spectrum, as indicated merely by the
length of the darkened part, includes within its limits the whole
luminous spectrum, extending in one direction far beyond the extreme
violet and lavender rays, and in the other down to the extremest red:
with tartrate of silver the darkening occupies not only all the space
under the most refrangible rays, but reaches much beyond the extreme
red. On paper prepared with formobenzoate of silver the chemical
spectrum is cut off at the orange rays, with phosphate of silver in the
yellow, and with chloride of gold it terminates with the green, with
carbonate of mercury it ends in the blue, and on paper prepared with the
percyanide of gold, ammonia, and nitrate of silver, the darkening lies
entirely beyond the visible spectrum at its most refrangible extremity,
and is only half its length, whereas in some cases chemical action
occupies a space more than twice the length of the luminous image.
The point of maximum energy of chemical action varies as much for
different preparations as the scale of action. In the greater number of
cases the point of deepest blackening lies about the lower edge of the
indigo rays, though in no two cases is it exactly the same, and in many
substances it is widely different. On paper prepared with the juice of
the ten-week stock (Mathiola annua) there are two maxima, one in the
mean yellow and a weaker in the violet; and on a preparation of tartrate
of silver Sir John Herschel found three, one in the least refrangible
blue, one in the indigo, and a third beyond the visible violet. The
decrease in photographic energy is seldom perfectly alike on both sides
of the maximum. Thus at the most refrangible end of the solar spectrum
the greatest chemical power is exerted in most instances where there is
least light and heat, and even in the space where both sensibly cease.
Not only the intensity but the kind of action is different in the
different points of the solar spectrum, as evidently appears from the
various colours that are frequently impressed on the same analyzing
surface, each ray having a tendency to impart its own colour. Sir John
Herschel obtained a coloured image of the solar spectrum on paper
prepared according to Mr. Talbot’s principle, from a sunbeam refracted
by a glass prism and then highly condensed by a lens. The photographic
image was rapidly formed and very intense, and, when withdrawn from the
spectrum and viewed in common daylight, it was found to be coloured with
sombre but unequivocal tints imitating the prismatic colours, which
varied gradually from red through green and blue to a purplish black.
After washing the surface in water, the tints became more decided by
being kept a few days in the dark—a phenomenon, Sir John observes, of
constant occurrence, whatever be the preparation of the paper, provided
colours are produced at all. He also obtained a coloured image on
nitrate of silver, the part under the blue rays becoming a blue brown,
while that under the violet had a pinkish shade, and sometimes green
appeared at the point corresponding to the least refrangible blue. Mr.
Hunt found on a paper prepared with fluoride of silver that a yellow
line was impressed on the space occupied by the yellow rays, a green
band on the space under the green rays, an intense blue throughout the
space on which the blue and indigo rays fell, and under the violet rays
a ruddy brown appeared; these colours remained clear and distinct after
being kept two months.
Notwithstanding the great variety in the scale of action of the solar
spectrum, the darkening or deoxydizing principle that prevails in the
more refrangible part rarely surpasses or even attains the mean yellow
ray which is the point of maximum illumination; it is generally cut off
abruptly at that point which seems to form a limit between the opposing
powers which prevail at the two ends of the spectrum. The bleaching or
oxydizing effect of the red rays on blackened muriate of silver
discovered by M. Ritter of Jena, and the restoration by the same rays of
discoloured gum guaiacum to its original tint by Dr. Wollaston, have
already been mentioned as giving the first indications of that
difference in the mode of action of the chemical rays at the two ends of
the visible spectrum, now placed beyond a doubt.
The action exerted by the less refrangible rays beyond and at the red
extremity of the solar spectrum, in most instances, so far from
blackening metallic salts, protects them from the action of the diffused
daylight: but, if the prepared surface has already been blackened by
exposure to the sun, they possess the remarkable property of bleaching
it in some cases, and under other circumstances of changing the black
surface into a fiery red.
Sir John Herschel, to whom we owe most of our knowledge of the
properties of the chemical spectrum, prepared a sheet of paper by
washing it with muriate of ammonia, and then with two coats of nitrate
of silver; on this surface he obtained an impression of the solar
spectrum exhibiting a range of colours very nearly corresponding with
its natural hues. But a very remarkable phenomenon occurred at the end
of least refrangibility; the red rays exerted a protecting influence
which preserved the paper from the change which it would otherwise have
undergone from the deoxydizing influence of the dispersed light which
always surrounds the solar spectrum, and this maintained its whiteness.
Sir John met with another instance on paper prepared with bromide of
silver, on which the whole of the space occupied by the visible spectrum
was darkened down to the very extremity of the red rays, but an
oxydizing action commenced beyond the extreme red, which maintained the
whiteness of the paper to a considerable distance beyond the last
traceable limit of the visible rays, thus evincing decidedly the
existence of some chemical power over a considerable space beyond the
least refrangible end of the spectrum. Mr. Hunt also found that on the
Daguerreotype plate a powerful protecting influence is exercised by the
extreme red rays. In these cases the red and those dark rays beyond them
exert an action of an opposite nature to that of the violet and lavender
rays.
The least refrangible part of the solar spectrum possesses also, under
certain circumstances, a bleaching property, by which the metallic salts
are restored to their original whiteness after being blackened by
exposure to common daylight, or to the most refrangible rays of the
solar spectrum.
Paper prepared with iodide of silver, when washed over with ferrocyanite
of potash, blackens rapidly when exposed to the solar spectrum. It
begins in the violet rays and extends over all the space occupied by the
dark chemical rays, and over the whole visible spectrum down to the
extreme red rays. This image is coloured, the red rays giving a reddish
tint and the blue a blueish. In a short time a bleaching process begins
under the red rays, and extends upwards to the green, but the space
occupied by the extreme red is maintained perfectly dark. Mr. Hunt found
that a similar bleaching power is exerted by the red rays on paper
prepared with protocyanide of potassium and gold with a wash of nitrate
of silver.
The application of a moderately strong hydriodate of potash to darkened
photographic paper renders it peculiarly susceptible of being whitened
by further exposure to light. If paper prepared with bromide of silver
be washed with ferrocyanate of potash while under the influence of the
solar spectrum, it is immediately darkened throughout the part exposed
to the visible rays down to the end of the red, some slight interference
being perceptible about the region of the orange and yellow. After this
a bleaching action begins over the part occupied by the red rays, which
extends to the green. By longer exposure an oval spot begins again to
darken about the centre of the bleached space; but, if the paper receive
another wash of the hydriodate of potash, the bleaching action extends
up from the green, over the region occupied by the most refrangible rays
and considerably beyond them, thus inducing a negative action in the
most refrangible part of the spectrum.
In certain circumstances the red rays, instead of restoring darkened
photographic paper to its original whiteness, produce a deep red colour.
When Sir John Herschel received the spectrum on paper somewhat
discoloured by exposure to direct sunshine, instead of whiteness, a red
border was formed extending from the space occupied by the orange, and
nearly covering that on which the red fell. When, instead of exposing
the paper in the first instance to direct sunshine, it was blackened by
the violet rays of a prismatic spectrum, or by a sunbeam that had
undergone the absorptive action of a solution of ammonia-sulphate of
copper, the red rays of the condensed spectrum produced on it, not
whiteness, but a full and fiery red, which occupied the whole space on
which any of the visible red rays had fallen; and this red remained
unchanged, however long the paper remained exposed to the least
refrangible rays.
Sunlight transmitted through red glass produces the same effect as the
red rays of the spectrum in the foregoing experiment. Sir John Herschel
placed an engraving over a paper blackened by exposure to sunshine,
covering the whole with a dark red-brown glass previously ascertained to
absorb every ray beyond the orange: in this way a photographic copy was
obtained in which the shades were black, as in the original engraving;
but the lights, instead of being white, were of the red colour of venous
blood, and no other colour could be obtained by exposure to light,
however long. Sir John ascertained that every part of the spectrum
impressed by the more refrangible rays is equally reddened, or nearly
so, by the subsequent action of the less refrangible; thus the red rays
have the very remarkable property of assimilating to their own colour
the blackness already impressed on photographic paper.
That there is a deoxydating property in the more refrangible rays, and
an oxydating action in the less refrangible part of the spectrum, is
manifest from the blackening of one and the bleaching effect of the
other; but the peculiar action of the red rays in the experiments
mentioned shows that some other principle exists different from
contrariety of action. These opposite qualities are balanced or
neutralized in the region of the mean yellow ray. But, although this is
the general character of the photographic spectrum, under certain
circumstances even the red rays have a deoxydating power, while the blue
and violet exert a contrary influence; but these are rare exceptions.
The photographic action of the two portions of the solar spectrum being
so different, Sir John Herschel tried the effect of their united action
by superposing the less refrangible part of the spectrum over the more
refrangible portion by means of two prisms; and he thus discovered that
two rays of different refrangibility, and therefore of different lengths
of undulation, acting simultaneously, produce an effect which neither,
acting separately, can do.
Some circumstances that occurred during the analysis of the chemical
spectrum seem to indicate an absorptive action in the sun’s atmosphere.
The spectral image impressed on paper prepared with nitrate of silver
and Rochelle salt commenced at, or very little below, the mean yellow
ray, of a delicate lead colour; and when the action was arrested, such
was the character of the whole photographic spectrum. But, when the
light of the solar spectrum was allowed to continue its action, there
was observed to come on suddenly a new and much more intense impression
of darkness, confined in length to the blue and violet rays; and, what
is most remarkable, confined also in breadth to the middle of the sun’s
image, so far at least as to leave a border of the lead-coloured
spectrum traceable, not only round the clear and well-defined convexity
of the dark interior spectrum at the less refrangible end, but also
laterally along both its edges; and this border was the more easily
traced, and less liable to be mistaken, from its striking contrast of
colour with the interior spectrum, the former being lead gray, the
latter an extremely rich deep velvety brown. The less refrangible end of
this interior brown spectrum presented a sharply terminated and
regularly elliptical contour, the more refrangible a less decided one.
“It may seem too hazardous,” Sir John continues, “to look for the cause
of this very singular phenomenon in a real difference between the
chemical agencies of those rays which issue from the central portion of
the sun’s disc, and those which, emanating from its borders, have
undergone the absorptive action of a much greater depth of its
atmosphere; and yet I confess myself somewhat at a loss what other cause
to assign for it. It must suffice, however, to have thrown out the hint,
remarking only, that I have other, and I am disposed to think decisive,
evidence of the existence of an absorptive solar atmosphere extending
beyond the luminous one.” M. Arago observed that the rays from the
centre of the sun have a greater photographic power than those from the
edges, and the photographic images of the sun, taken on glass by M.
Niepcé, were blood-red, much deeper in the centre, and on one occasion
the image was surrounded by an auriol. Several circumstances concur in
showing that there are influences also concerned in the transmission of
the photographic action which have not yet been explained, as, for
example, the influence which the time of the day exercises on the
rapidity with which photographic impressions are made, the sun being
much less effective two hours after passing the meridian than two hours
before. There is also reason to suspect that the effect in some way
depends on the latitude, since a much longer time is required to obtain
an image under the bright skies of the tropics than in England; and it
is even probable that there is a difference in the sun’s light in high
and low latitudes, because an image of the solar spectrum, obtained on a
Daguerreotype plate in Virginia, by Dr. Draper, differed from a spectral
image obtained by Mr. Hunt on a similar plate in England. The inactive
spaces discovered in the photographic spectrum by M. E. Becquerel,
similar to those in the luminous spectrum, and coinciding with them, is
also a phenomenon of which no explanation has yet been given; possibly
the chemical rays may be absorbed by the atmosphere with those of light.
Although chemical action extends over the whole luminous spectrum, and
much beyond it, in gradations of more or less intensity, it is found by
careful investigation to be by no means continuous; numerous inactive
lines cross it, coinciding with those in the luminous image as far as it
extends; besides, a very great number exist in the portions that are
obscure, and which overlap the visible part. There are three
extraspectral lines beyond the red, and some strongly marked groups on
the obscure part beyond the violet; but the whole number of those
inactive lines, especially in the dark spaces, is so great that it is
impossible to count them.
Notwithstanding this coincidence in the inactive lines of the two
spectra, photographic energy is independent of both light and heat,
since it exerts the most powerful influence in those rays where they are
least, and also in spaces where neither sensibly exist; but the
transmission of the sun’s light through coloured media makes that
independence quite evident. Heat and light pass abundantly through
yellow glass, or a solution of chromate of potash; but the greater part
of the chemical rays are excluded, and chlorine gas diluted with common
air, though highly pervious to the luminous and calorific principles,
has the same effect. Sir John Herschel found that a slight degree of
yellow London fog had a similar effect with that of pale yellow media:
he also remarked that a weak solution of azolitmine in potash, which
admits a great quantity of green light, excludes chemical action; and
some years ago the author, while making experiments on the transmission
of chemical rays, observed that green glass, coloured by oxide of copper
about the 20th of an inch thick, excludes the photographic rays; and, as
M. Melloni has shown that substance to be impervious to the most
refrangible calorific rays, it has the property of excluding the whole
of the most refrangible part of the solar spectrum, visible and
invisible. Green mica, if not too thin, has also the same effect,
whereas amethyst, deep blue, and violet-coloured glasses, though they
transmit a very little light, allow the chemical rays to pass freely.
Thus light and photographic energy may be regarded as distinct parts of
the solar beam, and both being propagated by vibrations of the etherial
medium they are merely motion. Excellent images have been obtained of
the moon in its different phases by Professor Secchi, at Rome;
candlelight is nearly deficient of the chemical rays. How far they may
influence crystallization and other molecular arrangements is unknown,
but their power is universal wherever the solar beam falls, although
their effect only becomes evident in cases of unstable molecular
equilibrium.
It is not by vision alone that a knowledge of the sun’s rays is
acquired: touch proves that they have the power of raising the
temperature of substances exposed to their action. Sir William Herschel
discovered that rays which produce the sensation of heat exist in the
solar spectrum independent of those of light; when he used a prism of
flint glass, he found that the warm rays are most abundant in the dark
space a little beyond the red extremity of the spectrum, that from
thence they decrease towards the violet, beyond which they are
insensible. It may be concluded therefore, that the calorific rays vary
in refrangibility, and that those beyond the extreme red are less
refrangible than any rays of light. Since Sir William Herschel’s time it
has been discovered that the calorific spectrum exceeds the luminous one
in length in the ratio of 42 to 25, but the most singular phenomenon is
its want of continuity. Sir John Herschel blackened the under side of a
sheet of very thin white paper by the smoke of a lamp, and, having
exposed the white side to the solar spectrum, he drew a brush dipped in
spirit of wine over it, by which the paper assumed a black hue when
sufficiently saturated. The heat in the spectrum evaporated the spirit
first on those parts of the paper where it fell with greatest intensity,
thereby restoring their white colour, and he thus discovered that the
heat increases uniformly and gradually throughout the luminous spectrum,
and that it comes to a maximum and forms a spot at a considerable
distance beyond the extreme red. It then decreases, but again increasing
it forms a second maximum spot, after which it ceases altogether through
a short space, but is again renewed and forms two more insulated spots,
and even a fifth may be traced at a little distance from the latter.
These circumstances are probably owing to the absorbing action of the
atmospheres of the sun and earth. “The effect of the former,” says Sir
John, “is beyond our control, unless we could carry our experiments to
such a point of delicacy as to operate separately on rays emanating from
the centre and borders of the sun’s disc; that of the earth’s, though it
cannot be eliminated any more than in the case of the sun’s, may yet be
varied to a considerable extent by experiments made at great elevations,
under a vertical sun, and compared with others where the sun is more
oblique, the situation lower, and the atmospheric pressure of a
temporarily high amount. Should it be found that this cause is in
reality concerned in the production of the spots, we should see reason
to believe that a large portion of solar heat never reaches the earth’s
surface, and that what is incident on the summits of lofty mountains
differs not only in quantity but also in quality from what the plains
receive.”
A remarkable phosphorescent property was discovered by M. E. Becquerel
in the solar spectrum. Two luminous bands separated by a dark one are
excited by the solar spectrum on paper covered with a solution of gum
arabic, and strewed with powdered sulphuret of calcium or Canton’s
phosphorus. One of the luminous bands occupies the space under the least
refrangible violet rays, and the other that beyond the lavender rays, so
that the dark band lies under the extreme violet and lavender rays. When
the action of the light is continued, the whole surface beyond the least
refrangible violet shines, the luminous bands already mentioned
brightest; but all the space from the least refrangible violet to the
extreme red remains dark. If the surface, prepared either with the
sulphuret of calcium or Bologna stone, be exposed to the sun’s light for
a little time, it becomes luminous all over; but when, in this state, a
solar spectrum is thrown upon it, the whole remains luminous except the
part from the least refrangible violet to the extreme red, on which
space the light is extinguished; and when the temperature of the surface
is raised by a lamp, the bright parts become more luminous and the dark
parts remain dark. Glass stained by the protoxide of copper, which
transmits only the red and orange rays, has the same effect with the
less refrangible part of the spectrum; hence there can be no doubt that
the most refrangible and obscure rays of the spectrum excite
phosphorescence, while all the less refrangible rays of light and heat
extinguish it.
Paper prepared with the sulphuret of barium, when under the solar
spectrum, shows only one space of maximum luminous intensity, and the
destroying rays are the same as in the sulphuret of calcium. Thus the
obscure rays beyond the extreme violet produce light, while the luminous
rays extinguish it.
The phosphoric spectrum has inactive lines which coincide with those in
the luminous and chemical spectra, at least as far as it extends; but in
order to be seen the spectrum must be received for a few seconds upon
the prepared surface through an aperture in a dark room, then the
aperture must be closed, and the temperature of the surface raised two
or three hundred degrees; the phosphorescent parts then shine
brilliantly and the dark lines appear black. Since the parts of similar
refrangibility in different spectra are traversed by the same dark
lines, rays of the same refrangibility are probably absorbed at the same
time by the different media through which they pass.
It appears from the experiments of MM. Becquerel and Biot, that
electrical disturbances produce these phosphorescent effects. There is
thus a mysterious connexion between the most refrangible rays and
electricity which the experiments of M. E. Becquerel confirm, showing
that electricity is developed during chemical action by the violet rays,
that it is feebly developed by the blue and indigo, but that none is
excited by the less refrangible part of the spectrum.
A series of experiments by Sir John Herschel have disclosed a new set of
obscure rays in the solar spectrum, which seem to bear the same relation
to those of heat that the photographic or chemical rays bear to the
luminous. They are situate in that part of the spectrum which is
occupied by the less refrangible visible colours, and have been named by
their discoverer Parathermic rays. It must be held in remembrance that
the region of greatest heat in the solar spectrum lies in the dark space
beyond the visible red. Now, Sir John Herschel found that in experiments
with a solution of gum guaiacum in soda, which gives the paper a green
colour, the green, yellow, orange, and red rays of the spectrum
invariably discharged the colour, while no effect was produced by the
extra-spectral rays of heat, which ought to have had the greatest effect
had heat been the cause of the phenomenon. When an aqueous solution of
chlorine was poured over a slip of paper prepared with gum guaiacum
dissolved in soda, a colour varying from a deep somewhat greenish hue to
a fine celestial blue was given to it; and, when the solar spectrum was
thrown on the paper while moist, the colour was discharged from all the
space under the less refrangible luminous rays, at the same time that
the more distant thermic rays beyond the spectrum evaporated the
moisture from the space on which they fell; so that the heat spots
became apparent. But the spots disappeared as the paper dried, leaving
the surface unchanged; while the photographic impression within the
visible spectrum increased in intensity; the non-luminous thermic rays,
though evidently active _as to heat_, were yet incapable of effecting
that peculiar chemical change which other rays of much less heating
power were all the time producing. Sir John having ascertained that an
artificial heat from 180° to 280° of Fahrenheit changed the green tint
of gum guaiacum to its original yellow hue when moist, but that it had
no effect when dry, he therefore tried whether heat from a hot iron
applied to the back of the paper used in the last-mentioned experiment
while under the influence of the solar spectrum might not assist the
action of the calorific rays; but, instead of doing so, it greatly
accelerated the discoloration over the spaces occupied by the less
refrangible rays, but had no effect on the extra-spectral region of
maximum heat. Obscure terrestrial heat, therefore, is capable of
assisting and being assisted in effecting this peculiar change by those
rays of the spectrum, whether luminous or thermic, which occupy its red,
yellow, and green regions; while, on the other hand, it receives no such
assistance from the purely thermic rays beyond the spectrum acting under
similar circumstances and in an equal state of condensation.
The conclusions drawn from these experiments are confirmed by that which
follows: a photographic picture formed on paper prepared with a mixture
of the solutions of ammonia-citrate of iron and ferro-sesquicyanite of
potash in equal parts, then thrown into water and afterwards dried, will
be blue and negative, that is to say, the lights and shadows will be the
reverse of what they are in nature. If in this state the paper be washed
with a solution of proto-nitrate of mercury, the picture will be
discharged; but if it be well washed and dried, and a hot smoothing-iron
passed over it, the picture instantly reappears, not blue, but brown; if
kept some weeks in this state in perfect darkness between the leaves of
a portfolio, it fades, and almost entirely vanishes, but a fresh
application of heat restores it to its full original intensity. This
curious change is not the effect of light, at least not of light alone.
A certain temperature must be attained, and that suffices in total
darkness; yet, on exposing to a very concentrated spectrum a slip of the
paper used in the last experiment, after the uniform blue colour has
been discharged and a white ground left, this whiteness is changed to
brown over the whole region of the red and orange rays, _but not beyond_
the luminous spectrum.
Sir John thence concludes:—1st. That it is the heat of these rays, not
their light, which operates the change; 2ndly. That this heat possesses
a peculiar chemical quality which is not possessed by the purely
calorific rays outside of the visible spectrum, though far more intense;
and, 3rdly. That the heat radiated from obscurely hot iron abounds
especially in rays analogous to those of the region of the spectrum
above indicated.
Another instance of these singular transformations may be noticed. The
pictures formed on cyanotype paper rendered more sensitive by the
addition of corrosive sublimate are blue on a white ground and positive,
that is, the lights and shadows are the same as in nature, but, by the
application of heat, the colour is changed from blue to brown, from
positive to negative; even by keeping in darkness the blue colour is
restored, as well as the _positive character_. Sir John attributes this,
as in the former instance, to certain rays, which, regarded as rays of
heat or light, or of some influence _sui generis_ accompanying the red
and orange rays of the spectrum, are also copiously emitted by bodies
heated short of redness. He thinks it probable that these invisible
parathermic rays are the rays which radiate from molecule to molecule in
the interior of bodies, that they determine the discharge of vegetable
colours at the boiling temperature, and also the innumerable atomic
transformations of organic bodies which take place at the temperature
below redness, that they are distinct from those of pure heat, and that
they are sufficiently identified by these characters to become
legitimate objects of scientific discussion.
The calorific and parathermic rays appear to be intimately connected
with the discoveries of Messrs. Draper and Moser. Daguerre has shown
that the action of light on the iodide of silver renders it capable of
condensing the vapour of mercury which adheres to the parts affected by
it. Professor Moser of Königsberg has proved that the same effect is
produced by the simple contact of bodies, and even by their very near
juxtaposition, and that in total darkness as well as in light. This
discovery he announced in the following words:—“If a surface has been
touched in any particular parts by any body, it acquires the property of
precipitating all vapours, and these adhere to it or combine chemically
with it on these spots differently from what they do on the untouched
parts.” If we write on a plate of glass or any smooth surface whatever
with blotting-paper, a brush, or anything else, and then clean it, the
characters always reappear if the plate or surface be breathed upon, and
the same effect may be produced even on the surface of mercury; nor is
absolute contact necessary. If a screen cut in a pattern be held over a
polished metallic surface at a small distance, and the whole breathed
on, after the vapour has evaporated so that no trace is left on the
surface, the pattern comes out when it is breathed on again.
Professor Moser proved that bodies exert a very decided influence upon
each other, by placing coins, cut stones, pieces of horn, and other
substances, for a short time on a warm metallic plate: when the
substance was removed, no impression appeared on the plate till it was
breathed upon or exposed to the vapour of mercury, and then these
vapours adhered only to the parts where the substance had been placed,
making distinct images, which in some cases were permanent after the
vapour was removed. Similar impressions were obtained on glass and other
substances even when the bodies were not in contact, and the results
were the same whether the experiments were performed in light or in
darkness.
Mr. Grove found, when plates of zinc and copper were closely
approximated, but not in contact, and suddenly separated, that one was
positively and the other negatively electric; whence he inferred that
the intervening medium was either polarised, or that a radiation
analogous, if not identical, with that which produces Moser’s images
takes place from plate to plate.
Mr. Hunt has shown that many of these phenomena depend on difference of
temperature, and that, in order to obtain good impressions, dissimilar
metals must be used. For example, gold, silver, bronze, and copper coins
were placed on a plate of copper too hot to be touched, and allowed to
remain till the plate cooled: all the coins had made an impression, the
distinctness and intensity of which were in the order of the metals
named. When the plate was exposed to the vapour of mercury the result
was the same, but, when the vapour was wiped off, the gold and silver
coins only had left permanent images on the copper. These impressions
are often minutely perfect, whether the coins are in actual contact with
the plate or one-eighth of an inch above it. The mass of the metal has a
material influence on the result; a large copper coin makes a better
impression on a copper plate than a small silver coin. When coins of
different metals are placed on the same plate they interfere with each
other.
When, instead of being heated, the copper plate was cooled by a freezing
mixture, and bad conductors of heat laid upon it, as wood, paper, glass,
&c., the result was similar.
Mr. Hunt, observing that a black substance leaves a stronger impression
on a metallic surface than a white, applied the property to the art of
copying prints, woodcuts, writing, and printing, on copper amalgamated
on one surface and highly polished, merely by placing the object to be
copied smoothly on the metal, and pressing it into close contact by a
plate of glass: after some hours the plate is subjected to the vapour of
mercury, and afterwards to that of iodine, when a black and accurate
impression of the object comes out on a grey ground. Effects similar to
those attributed to heat may also be produced by electricity. Mr.
Karsten, by placing a glass plate upon one of metal, and on the glass
plate a medal subjected to discharges of electricity, found a perfect
image of the medal impressed on the glass, which could be brought into
evidence by either mercury or iodine; and, when several plates of glass
were interposed between the medal and the metallic plate, each plate of
glass received an image on its upper surface after the passage of
electrical discharges. These discharges have the remarkable power of
restoring impressions that have been long obliterated from plates by
polishing—a proof that the disturbances upon which these phenomena
depend are not confined to the surface of the metals, but that a very
decided molecular change has taken place to a considerable depth. Mr.
Hunt’s experiments prove that the electro-negative metals make the most
decided images upon electro-negative plates, and _vice versâ_. M.
Matteucci has shown that a discharge of electricity does not visibly
affect a polished silver plate, but that it produces an alteration which
renders it capable of condensing vapour.
The impression of an engraving was made by laying it face downwards on a
silver plate iodized, and placing an amalgamated copper plate upon it;
it was left in darkness fifteen hours, during which time an impression
of the engraving had been made on the amalgamated plate _through the
paper_.
An iodized silver plate was placed in darkness with a coil of string
laid on it, and with a polished silver plate suspended one-eighth of an
inch above it: after four hours they were exposed to the vapours of
mercury, which became uniformly deposited on the iodized plate, but on
the silver one there was a sharp image of the string, so that this image
was formed in the dark, and even without contact. Coins or other objects
leave their impressions in the same manner with perfect sharpness and
accuracy, when brought out by vapour without contact, in darkness, and
on simple metals.
Red and orange coloured media, smoked glass, and all bodies that
transmit or absorb the hot rays freely, leave strong impressions on a
plate of copper, whether they be in contact or one-eighth of an inch
above it. Heat must be concerned in this, for a solar spectrum
concentrated by a lens was thrown on a polished plate of copper, and
kept on the same spot by a heliostat for two or three hours: when
exposed to mercurial vapour, a film of the vapour covered the plate
where the diffused light which always accompanies the solar spectrum had
fallen. On the obscure space occupied by the maximum heating power of
Sir William Herschel, and also on the great heat spot in the thermic
spectrum of Sir John Herschel, the condensation of the mercury was so
thick that it stood out a distinct white spot on the plate, while over
the whole space that had been under the visible spectrum the quantity of
vapour was much less than that which covered the other parts, affording
distinct evidence of a negative effect in the luminous spectrum and of
the power of the hot rays, which is not always confined to the surface
of the metal, since in many instances the impressions penetrated to a
considerable depth below it, and consequently were permanent.
Several of these singular effects appear to be owing to the mutual
action of molecules in contact while in a different state, whether of
electricity or temperature: others clearly point at some unknown
influence exerted between surfaces at a distance, and affecting their
molecular structure: possibly it may be the parathermic rays, which have
a peculiar chemical action even in total darkness. In the last
experiment the effect is certainly produced by the positive portion of
one of those remarkable antagonist principles which characterise the
solar spectrum.
Thus it appears that the prism resolves the pure white sunbeam into
three superposed spectra, each varying in refrangibility and intensity
throughout its whole length; the visible part is overlapped at one end
by the chemical or photographic rays, and at the other by the thermic,
but the two latter so much exceed the visible part, that the linear
dimensions of the three—the luminous, thermic, and photographic—are in
proportion to the numbers 25, 42·10, and 55·10, so that the whole solar
spectrum is twice as long as its visible part. The two extremities exert
a decided antagonist energy. The least refrangible luminous rays
obliterate the action of the photographic rays, while the latter produce
phosphorescent light, which is extinguished by the least refrangible
luminous rays. According to Mr. Hunt’s experiment, the hot rays condense
mercurial vapour on a polished metallic plate, while the luminous rays
prevent its formation. Electricity is excited by the chemical rays,
while the parathermic are found in the less refrangible rays alone. Each
of the spectra is crossed by coloured and rayless lines peculiar to
itself, and these are traversed at right angles by innumerable dark
lines of various breadths, the whole forming an inexpressibly wonderful
and glorious creation.
The arrangement varies a little according to the material of the prism
and the manner of producing the spectrum, as in that obtained by
Professor Draper from diffracted light. It was formed by a beam
diffracted by passing through a netting of fine wire, or by reflection
from a polished surface of steel, having fine parallel lines drawn on
it. This diffracted spectrum is divided into two equal parts in the
centre of the yellow; and as in the prismatic spectrum, one half is
antagonist to the other half, the red or negative end undoing what the
positive or violet end has done. The centre of the yellow is the hottest
part, and the heat decreases to both extremities. A line of cold is
supposed to exist on this spectrum answering to Fraunhofer’s dark line
H.
The undulations of the ethereal medium which constitute a sunbeam must
be infinitely varied, each influence having a vibration peculiar to
itself. Those of light are certainly transverse to the direction of the
ray; while Professor Draper believes that those of heat are normal, that
is, in the direction of the ray, like those of sound. A doubt exists
whether the vibrations of polarised light are perpendicular to the plane
of polarisation or in that plane. Professor Stokes of Cambridge has come
to the conclusion, both from the diffracted spectrum and theory, that
they are perpendicular to the plane of polarisation, but M. Holtzmann is
of opinion that they are in that plane, so the subject is still open to
discussion.