Common and others; but they are of course small, and have contributed
so far, much less to our astronomical knowledge than those already
mentioned. Very different are the results obtained in what, a short
time ago, appeared a less promising field. The image of a so-called
fixed star, in even the most powerful telescopes, presents itself as a
mere luminous point, and this is the case whether the star is one of
the brightest or one of the least conspicuous. The telescopic
appearance is simply a more or less brilliant point. The various
degrees of brightness which distinguish one star from another (_stella
enim a stellâ differt in claritate_), and which the unassisted eye
attributes to difference of size, led, long before the invention of
telescopes, to a classification of them accordingly. The brightest
stars are said to be of the 1st “magnitude,” those of the next
inferior degree of brilliancy, of the 2nd “magnitude,” and so on, down
to the 6th, which includes the faintest star discernible by an acute
eye under favourable circumstances. But stars too faint to be thus
seen came into view in the field of the telescope, and therefore those
of the 7th magnitude, and beyond, are termed _telescopic_ stars, and
each additional power given to the instrument brings others in view
that previously were invisible. The classification has been carried
down to the 18th or 20th magnitude, which expresses the limit of
visibility with the most powerful telescopes yet constructed. In the
methods hitherto employed for this classification, there is
necessarily much that is arbitrary and vague, and it is quite common
to find a different magnitude assigned to the same star by different
authorities. Now the photographic plate enables the astronomer to
determine the relative brightness of stars quite definitely. Everyone
knows that the time required to impress an image on the sensitive
plate is longer, as that image is less luminous. Hence, by finding the
time required for the images of different stars to be impressed, we
have a measure of their relative luminosities. Suppose the image of a
group of stars is allowed to act on a plate for, say, 5 seconds, we
should find only the brightest stars represented. If a second plate
have double the exposure given, it would be impressed by the images of
not only the brightest stars of the group, but also by those of the
next degree of brilliancy; and a third plate exposed for 20 seconds
would show more stars than the two former exposures. So that plate
after plate might be exposed under the same group for successively
longer and longer intervals indefinitely. Exposures extending over
hours have been made, notably by Mr. Common in England, and by Mr.
Gill at the Cape of Good Hope, showing not only how magnitude may be
determined to any extent, and the heavens most accurately mapped out,
but with this very remarkable result:—_thousands of stars, invisible
even in the most powerful telescopes, are portrayed in the
photographs_. Let us consider for a moment the significance of this
fact with regard to the new space-exploring powers it has placed in
the hands of science. The number of stars visible to the unassisted
eye in the whole expanse of the heavens has been variously estimated,
but the figures usually given lie between 3,000 and 4,000, and the
highest estimate for the most acute eyesight, under the most
favourable atmospheric conditions, places the limit at 5,000. The
brightest star in the heavens is Sirius, and Sir. J. Herschel
ascertained that its light is about 324, that of an average star of
the 6th magnitude. Taking the average luminosities of stars of the
first six magnitudes, Sir W. Herschel, from his own observations,
represents their relative brightness by the following figures: 100;
25; 12; 6; 2; 1. The different degrees of brightness seen is,
probably, due to the following three causes, combined in various
proportions: (1) the different sizes of these luminaries themselves;
(2) differences in their intrinsic luminosity; and, (3) differences in
their distances from us. And it is also extremely probable that the
last is generally by far the largest factor of the three. It has been
found by photometrical experiments, that the light we receive from the
sun is 20,000,000,000 (twenty thousand million) times more than that
of Sirius. If we suppose Sirius to be in reality only as large and as
bright as our sun, it follows that its distance from us must be no
less than 13,433,000,000,000 miles. The distance of stars of the 16th
magnitude has been estimated to be such that their light—travelling at
the rate of 185,000 miles per second—takes between five and six
thousand years to reach us. For a long time no sensible parallax could
be discovered in any of the fixed stars; that is, no change in their
positions was discernible when viewed from points 183,000,000 miles
apart, namely from the extremities of a diameter of the earth’s orbit.
In other words, if we suppose the line of the length just mentioned to
form the base of a triangle, having a star at its vertex, the angle
formed by the sides is so small that the most refined instruments
failed to measure it. In recent times, however, the parallax of a few
stars—about a dozen or so—has been detected and approximately
measured. The greatest observed parallax belongs to in α the
constellation of the Centaur, a star of the first magnitude, 30° from
the south pole of the heavens, and of this the parallax amounts to but
a little more than nine-tenths of a second of angular measurement,
corresponding with a distance of nearly 20,000,000,000,000 miles, a
space which takes light 3½ years to pass over. This star is,
therefore, believed to be the nearest of any to our system. The
smallest parallax that has been measured in any of these few stars is
a fraction of a second of angle corresponding with a distance twenty
times greater than the other, and requiring seventy years for light to
traverse it. Now, as the photographic plate shows us stars of
magnitudes indefinitely smaller even than the telescopic sixteenth, we
cannot but marvel at the manner in which the light travelling from
these suns in the immeasurable depths of space, and taking untold
thousands (nay, millions, it may be) of years in its journey is yet
able so to agitate the atoms of our silver compounds that images of
things that will themselves, probably, never be seen by mortal eyes
are presented to our view. A circumstance requiring explanation will
occur to the reader’s mind in connection with stellar photography; and
that is, how does it happen that, if the image of a star is a mere
point, it nevertheless impresses the plate as a visible dot? It is
probably because the point is a centre whence the photographic
influence radiates laterally on the plate to a small but yet sensible
distance.
Among the cosmic objects presented to our observation there are none
more fully charged with interest and instruction than the _Nebulæ_.
These are faintly luminous patches, in some few cases visible to the
naked eye, but for the most part telescopic. The milky way, which
extends round the celestial sphere, is a very conspicuous phenomenon of
the same kind. A few other hazy, cloudlike patches are seen in various
parts of the heavens, visible on a clear moonless night when the eye is
directed towards the proper quarter. The well known group of the
Pleiades sometimes presents this appearance, but most persons are able
by the unassisted vision to discern in it a group of six stars at least,
and an opera-glass or ordinary hand telescope easily resolves the object
into a cluster of 20 or 30 distinct stars. Telescopes of higher powers
bring more stars into view, and as many as 118 have been counted in the
group. There are several other groups of this kind perceptible to the
naked eyes merely as diffused patches of light, but resolvable by the
telescope into thickly clustered groups of minute stars; but in many of
the resolvable nebulæ the separate stars appear spread on a back-ground
of diffused luminosity. Again, there are other nebulæ which telescopes
of the highest powers we possess fail to resolve at all. Not only has
the photographic method shown stellar components of some of these last,
but it has depicted the form of nebulæ never seen at all, and whose
existence was previously unknown and unsuspected. For example, the
photograph has revealed the existence of a back-ground of nebulous
patches to the stars of the Pleiades—a thing that had never before been
suspected, although the group has been repeatedly observed by the most
powerful telescopes. Those who are at all acquainted with astronomy,
will understand the significance of this discovery for the science. The
results already obtained afford a marvellous support to the famous
speculation known as the nebular hypothesis. And as the forms of these
objects are accurately shown for us by their own light, changes in their
appearance may thus be detected as time goes on which may serve to lift
the above named theory into the region of demonstrated truth. The nebulæ
which neither telescope nor camera can resolve are such as the
spectroscope proves to be masses of glowing gas or vapour.
It has been already mentioned that the light from these immeasurably
distant stars and nebulæ is so faint that the most sensitive
photographic plates have to be exposed for hours. This would be a matter
of no difficulty if the clockwork mechanism by which the apparatus is
made to follow the apparent motion of the heavens could be constructed
with absolute perfection. But as this is not obtainable, even with the
most careful workmanship, and the smallest jar or irregularity would
distort and confuse the images, this source of disturbance is eliminated
in the following manner: attached to the photographing apparatus and
driven with it is a telescope, provided with cross wires, and through
this an observer views some star during the whole period of the
exposure, his business being to keep the image of the star accurately on
the cross wire, which he is enabled to do by having the means of
slightly modifying the movement of the clockwork. In the Paris
Exhibition of 1889 were shown many very fine large photographic prints
of nebulæ (notably of great nebula in Orion), which have recently been
obtained in this manner, and those nebulæ that had been photographically
resolved had the stellar components marked with wonderful distinctness.
Comets and meteorites have been photographed, and even the _aurora
borealis_ and the lightning’s path have been brought within the camera’s
ken.
Space would fail us to describe the many applications now found for
photography in microscopy, in medicine and surgery, in anthropology, in
commerce, and in the arts. It is obvious also from the improvements that
are continually made, that many of these applications have not yet
received their full developments. Photography has been enlisted into the
service of the army and navy, and regular courses of instruction in the
art are given in their training schools. A well equipped photographic
waggon now accompanies every army corps, and in almost every ship of
war, some proficient operator is to be found. By an ingenious
combination of photography, aerostatics and electricity, it is possible
to obtain with perfect safety accurate information of the disposition of
an enemy’s forces and fortifications. A small captive balloon is sent
up, to which is attached a camera. At a height of a few hundred yards,
the balloon is practically safe from any projectiles, and in its cable
are interwoven two electric wires by which currents are conveyed to
electro-magnets, which produce all the movements required for any number
of exposures. Jurisprudence has found its account in recognizing the
art, for the photograph is received in evidence for proving identity,
etc. The administration of the criminal law takes advantage of the art
to secure the likeness of prisoners for future identification, and the
modern instantaneous process renders unnecessary the subjects’
concurrence with the operation. Again, if the “hue and cry” has to be
raised for an individual “wanted” for any offence, and a photographic
likeness of him is procurable, thousands of copies can be made of it in
a few hours, by night as easily as by day, and distributed to every
police station in the whole country.
Modern processes now enable us to obtain prints from negatives in as
many seconds as a few years ago hours were required, and this by
artificial light. A process of printing lately introduced and yielding
artistic results which deserve to find more general favour, is that
called the _platinotype_. Instead of the ordinary print produced on
lightly glazed paper by the reduction of silver compounds, and of
questionable permanency, the image is formed in the paper by metallic
platinum, the most changeless of all possible substances under ordinary
influences. The pictures are of a rich velvety black, with soft
gradations, and the surface is without glaze or glare. The print has, in
fact, the appearance and all the best qualities of the most highly
finished mezzotint engraving, combined with the minute fidelity
characteristic of the photograph. The problem of producing a photograph
in colours, permanently showing nature’s tints in all their gradations,
has still a great fascination for some experimenters, and startling
announcements are made from time to time of some discovery in this
direction. It does not appear, however, that any success has really been
arrived at, beyond the results long ago obtained by Becquerel as
described on page 614; and, indeed, as our knowledge of the science of
the subject increases, the less likely does the possibility of
photographing colours appear. It is, however, never safe to lay down the
limits of discovery in science.[14] Note that precisely in the matter of
rendering colour even in its due gradation of tone or luminous
intensity, the photograph is quite untruthful. Everybody has noticed how
unnaturally dark and heavy the foliage of trees appears in the prints;
if we suppose a lady in a blue dress, with yellow trimmings, to sit for
her portrait, the photograph will show her in a white dress with black
trimmings; a sitter with light yellow or auburn hair will appear of
quite a dark complexion; if you photograph a lemon and a plum together,
the latter will probably come out lighter than the former; or if a
daffodil be the subject, the flower will be drawn in tones much darker
than the leaves. This incorrectness of tone relations can, however, be
greatly lessened by the device of reducing the quantity of the blue
rays, by interposing a piece of optically plane yellow-tinted glass, by
using the sensitive plates tinted with certain coal-tar dyes, which are
now prepared and sold under the name of “ortho-chromatic plates,” or by
both methods combined.
Footnote 14:
See page 630.
If any illustration were needed of the great popularity now attained by
the practice of photography, reference might be made to the large number
of periodicals devoted to the subject, and appearing weekly,
fortnightly, quarterly or annually, in every civilised country, and also
to the multitudes of societies that have been formed for the promotion
of the art. In Great Britain alone there are now at least 150 such
societies in active operation, and they are correspondingly numerous
elsewhere. If, when we consider all that has been accomplished up to the
present time, with the jubilee year of photography scarcely passed, and
observe the increasing numbers of its cultivators guided by the
explanations of its phenomena that science is beginning to furnish, we
can expect a corresponding progress in the next fifty years, then the
centenary may be reached with a roll of achievements that could we know
them now we should think marvellous.
As already remarked elsewhere, the practical side of photography has
outstripped the theoretical one, for so far its progress has been much
less indebted for processes and technic to the direct guidance of
science than almost any other of our Nineteenth Century acquisitions,
such as telegraphy, electric lighting, etc. The materials employed, and
the mode of manipulation, have certainly _not_ been deduced from
previous knowledge of the nature of light or from the laws of chemistry,
although when, by repeated trials and happy guesses, the right direction
had been found, the field into which it led could be more easily
explored under the direction of chemistry and physics. But even yet the
fundamental principle, or the precise nature of the action of light on
certain compounds, has not been definitely made out, and although some
theories on the subject have been proposed, no one has been generally
accepted as an adequate explanation of the known facts, and still less
have any quantitative relations been established for these actions. The
photographer cannot compose a formula for the composition of his
emulsions and developers from assured data like those that enable the
chemist to weigh out with accuracy the constituents that go to produce a
required compound.
The attainment of permanency in its products, which, by several
processes, photography can now boast of, is one of its triumphs, and
will tend greatly to enlarge the sphere of its utility. For example, we
have a public institution, known as the National Portrait Gallery, in
which it is sought to gather together and preserve the likenesses of the
most eminent Englishmen, and presentments of such of far less fidelity
than photographic portraits are eagerly sought after. It has been
suggested that something like a National Gallery of _permanent
photographic portraits_ of the chief men of their time would be a
fitting and acceptable legacy to the public of the future. This idea has
much to recommend it, particularly as authentic likenesses would thus be
secured for the nation beyond the chance of loss.
Photography has been applied in preparing blocks in relief for printing
along with letterpress in the same way as woodcut blocks. The process
has the great advantage of producing in a wonderfully short time a
perfect facsimile of the artist’s drawing without the intervention of
any engraver. A plate of zinc, brass, or copper, coated with a dried
film of bichromated albumen, is exposed to light under the transparent
negative of a drawing in pure line, that is, one having in it only lines
of uniform colour throughout. The parts of the film reached by the
light, which correspond with the lines of the original design, are
rendered insoluble, while the rest can readily be removed by water.
These unprotected parts have then to be removed by the action of acids,
but these are used alternately with the application to the plate of
certain compositions, the purpose of which is to prevent lateral erosion
of the lines in relief before the requisite depth of the metal has been
removed. Fig. 147_f_ is the reproduction of a pen-and-ink sketch by this
or some similar process. But nature and the ordinary photograph show us
graduated tones which ordinary printers’ ink cannot really reproduce,
inasmuch as it is incapable of gradation, and can give the _effect_ of
gradation only by such devices as are mentioned on page 642 (last
sentence). Now, the photograph cannot yield a printing-block until its
continuous tones are broken up into lines or dots. Not a few methods of
doing this have been contrived, but that which is by far the most
commonly used, and is most successfully practised on the commercial
scale, is simple in principle, although in actual working it calls for
much experience and skill. The negative is taken upon a wet collodion
plate, in front of which, within the camera, and at a very short
distance (say 1/30th inch) from the film, is a transparent _screen_,
bearing two sets of parallel opaque lines at right angles to each other.
These lines are mechanically ruled with the utmost regularity, and are
separated by only very small intervals. There may be from 80 to 200 of
them in the space of one inch, according to the class of work required.
The effect of this is that the light reaches the photographic film
through a series of minute transparent squares, the sides of which may
be only from the 1/140th to the 1/400th of an inch in length. Now it is
found that the brighter lights from the original positive, after passing
these small apertures, spread so as to more or less cover the opposite
parts of the negative, while the feebler lights, from the shades of the
original, impress the plate to a less degree, the developed image in
these showing, perhaps, merely a small dot or, in the very darkest
parts, a blank. In this way, then, may the photographic negative be
obtained with a granulated texture following in graduation the tones of
the original. After this, the rest is easy, for the process of exposing
a metal plate, coated with a sensitive film under the negative, and of
etching it with acids, etc., is essentially the same as in the
foregoing. Such is the _half-tone process_, which is now so largely
superseding wood and other engraving. It is unnecessary to describe
technical details here, such as the employment of _bitumen of Judæa_ as
the coating for the metal plate, or how the image must be reflected into
the lens from a mirror to avoid a reversal in the final print, etc.
There are endless modifications of the processes briefly mentioned
above, and some of these are guarded as valuable trade secrets. Several
of the illustrations in this work are prepared by the half-tone process,
of which plates I., IV., V., etc., are examples, and they should be
examined with a strong lens, in order that the different rendering of
the light and the dark parts may be compared.
_PHOTOGRAPHY IN COLOURS._
It is the statement as to the futility of assigning limits to scientific
discovery that has been justified by facts. The preceding edition of
this work was not long in the hands of its readers before the solution
of the problem of photography in colours was announced from Paris,
where, at the close of 1890, the physicist M. Lippmann had succeeded in
photographing the solar spectrum in its natural colours, and at the
beginning of 1891, he was able to exhibit at the Academy of Science
untouched photographs of a stained glass window in three colours, of a
dish of oranges and red flowers, and of a gorgeously coloured parrot,
all in their natural tints. The method employed had no apparent relation
to that of Becquerel, but was of the simplest, and, moreover, one which
any reader who has followed the first few pages of our section on the
“Causes of Light and Colours” will have little difficulty in completely
understanding, if he has devoted a little attention to Fresnel’s
interference experiment. M. Lippmann took a photographic plate, coated
to a greater depth than usual with a gelatine film containing the
sensitive salts of silver, and in the camera this plate was exposed with
the glass towards the lens, while at the other side of the film was a
metallic reflecting surface, namely, quicksilver. Supposing a ray of red
light to enter the glass and traverse the film, it would be reflected
from the metallic surface, and would meet the direct ray within the
substance of the film, with a difference of length of path that would
produce the interferences already described, and so give rise to
alternate lines or bands of darkness and brightness. It would, of
course, be in the lines of maximum brightness that the silver would be
first deposited by the photographic action, and these microscopically
fine lines or striæ of silver would give back, from ordinary light, a
colour corresponding to the waves of red light that produced them.
Similarly with the other colours. Anyone may observe the production of
colour from ordinary white light in the iridescent tints of
mother-of-pearl, where the effects are due to the varying distances of
fine edges of the layers of the substance. If an impression is taken
from a piece of mother-of-pearl by solid paraffin, or by white wax, or
even by common red sealing-wax, the colours will seem to be adhering to
the impression, but the operation may be repeated times without number.
It is the distance apart of the lines or striæ that determinates the
colour, and this is always some definite multiple of the wave lengths,
given on p. 411, for the various colours. M. Lippmann’s products are
true colour photographs, and they form a new and elegant experimental
demonstration of the doctrine of luminiferous undulations.