Wedgewood and Davy, in 1802, made prints on paper by means of silver
salts, but it was not until 1830 that Niepce and Daguerre founded
photography, which Arago, in an address to the French Chamber, at once
suggested might subsequently be used to record the positions of stars.
In 1839 we find Sir John Herschel carrying out a series of experiments
so important for our correct knowledge of the sequence of steps in the
early stages of photography that I have no hesitation in quoting from
one of Herschel’s manuscripts relating to a deposit on a glass plate
of “muriate” [chloride] of silver from a mixed solution of the nitrate
with common salt. The manuscript states: “After forty-eight hours [the
chloride] had formed a film firm enough to bear draining the water off
very slowly by a siphon. Having dried it, I found that it was very
little affected by light, and by washing it with nitrate of silver,
weak, and drying it, it became highly sensitive. In this state I took a
camera picture of the telescope on it.”
The original of the above-mentioned photograph, the first photograph
ever taken on glass, is now in the science collection at the Victoria
and Albert Museum, South Kensington.
In the early days of photography colored glasses were first used
to investigate the action of different colors on the photographic
plate. Sir John Herschel was among the first to propose that such
investigations should be made direct with a spectrum, and he, like Dr.
J. W. Draper, stated that he had found a new kind of light beyond the
blue end of the spectrum, as the photographic plate showed a portion of
the spectrum there which was not visible to the eye. Advance followed
advance, and in 1842 Becquerel photographed the whole solar spectrum,
in colors, with nearly all the lines registered by the hand and eye
of Fraunhofer, not only the blue end, but the complete spectrum, from
Draper’s “latent light,” as he called the ultra-violet rays, to the
extreme red end.
The first photograph of a celestial object was one of the moon, secured
by Dr. J. W. Draper in 1840; we had to wait till 1845, so far as I
know, before a daguerreotype was taken of the sun; this was done by
Foucault and Fizeau, while the first photograph of a star—Vega—was
taken at Harvard in 1850. After the introduction of the wet-collodion
process regular photography of the sun’s surface was commenced, at Sir
John Herschel’s recommendation, at Kew in 1858, and the total solar
eclipse of 1860 was made memorable by the photographs of De La Rue, who
before that time had secured most admirable photographs of the moon, as
also had Rutherfurd.
Photography now began to pay the debt she owed to spectrum analysis.
The first laboratory photograph of the spectra of the chemical elements
was taken by Dr. W. A. Miller in 1862.
Rutherfurd was the first to secure a photograph of the solar spectrum
with considerable dispersion by means of prisms.
In 1863 Mascart undertook a complete photographic investigation of
the ultra-violet portion of the solar spectrum, a work of no mean
magnitude. He, however, did not employ a train of prisms for producing
the spectrum, but a diffraction grating, using the light reflected
from the first surface. The first photograph of the spectrum of a star
was secured by Henry Draper, the son of Dr. J. W. Draper, one of the
pioneers in photography in 1872.
It was not till the introduction of dry plates in 1876 that the
photography of the fainter celestial objects or of their spectra was
possible, as a long exposure was naturally required. Stellar spectra
were photographed by Huggins in 1879, and in the next year Draper
photographed the nebula of Orion. As the dry plates became more rapid,
and as longer exposures were employed, revelation followed revelation;
the nebulæ as seen by the naked eye, and even some stars, were found
by the Henrys, Roberts, Max Wolf, Barnard, and others, to be but the
brighter kernels of large nebulous patches.
This new application of photography, depending upon long exposures (the
longest one I know of has extended to forty hours), had an important
reflex action on the mechanical parts of the telescope; it was not
only necessary to keep the faintest star exactly on the same part of
the plate during the whole of the exposure, but night after night the
stellar image must be brought on to the same part of the plate so that
the exposure might be continued.
A system of electric control of the going of the driving-clock of the
telescope by means of a sidereal clock was introduced, the simplest one
being designed by Russell, of Sydney; a most elaborate one by Grubb, of
Dublin.
Another application of the method of long exposures has been the
discovery of minor planets by the trails impressed by their motion
among the stars on the photographic plates on which the images of both
are impressed.
A complete spectroscopic survey of the stars by means of photography
was commenced in 1886 at Harvard College, as a memorial to Draper, who
died while he was laboring diligently and successfully in securing
advances in astrophysical inquiries. To carry on this work at Harvard,
Professor Pickering wisely reverted to the method first employed by
Fraunhofer, and utilized by Respighi and another in 1871, of placing
prisms in front of the object-glass.
In the photographing of stellar spectra by means of objective prisms,
the driving-clock of the telescope must _not_ go exactly at sidereal
rate, but at certain speeds depending on the brightness and position of
the star under examination.
This is necessary because the image of the spectrum of a star on
the photograph is only a thin line in which it is impossible to see
the spectral lines; the spectrum must be broadened, and this is
accomplished by making the star image “trail” to a certain degree on
the plate. This trailing is accomplished by means of the clock, the
rate of which is made to vary. In this way the trail of a spectrum of
a star on the photographic plate is always obtained of the same width,
while the density of the image is made fairly constant by increasing
the rate for bright stars and decreasing it for fainter ones. In this
way spectra of the brighter stars rivalling in perfection and detail
those obtained of the spectrum of the sun itself thirty years ago
have been obtained. Such photographs have rendered a minute chemical
classification of the stars possible.
One of the most interesting applications of photography to spectrum
analysis during the latter part of the century has been the utilization
by Messrs. Deslandres and Hale of a suggestion made by Janssen, that
by employing photography images of the sun and its surroundings can
be obtained in light on one wave length. In this way we can study
the distribution of any one of the chemical constituents of the sun
separately, and note its behavior, not only on the sun itself, but in
the atmosphere which enfolds the disk.
It is strange that, in spite of the suggestions of Faye, and others
after him, one of the great advantages of the employment of photography
in astronomical work, namely, the abolition of “personal equation,” has
so far been almost entirely neglected. What “personal equation” is can
be perhaps illustrated by considering an observer who is observing the
transit of a star over the wires in a transit instrument.
His object is to note the exact time, to a fraction of a second, when
a star passes each wire; and this is done by listening to the beats
of a clock near at hand and estimating the fractions. Some observers
constantly note the time either a little in advance or a little later
than the actual time, and this small distance between the observer
and the true times is more or less constant for each observer. This
difference has to be taken into account for every observation. Even
the use of the chronograph in transit work, by which the observation
is electrically recorded, does not entirely eliminate the error.
The photographic method of transit work has been experimented on,
but, so far as I know, it has not yet been used at more than one or
two observatories. It will doubtless eventually rid us of “personal
equation” entirely, for the star image may be photographed and the time
recorded by the same current of electricity.
At the end of the century we could almost say that except in relation
to the work of the meridional observatories, photographic methods of
recording observations had become exclusively used. One of the cases
in which its utility is most in evidence is in the matter of eclipse
observations. Spectra of the sun’s surroundings containing a thousand
lines are taken in a second of time, thus replacing five or six
doubtful eye observations by wealth of results which have enabled the
recent vast progress to be secured.