Probably in no department of science, certainly in no branch of
astronomical science, has photography been of such use as in the study
of solar eclipses. It is only when the sun is obscured by the moon that
we are able to see and properly photograph the corona or luminous
atmosphere around the sun. This solar corona, as has been said by Young,
“is visible only about eight days in the century in the aggregate, and
then only over narrow strips of the earth’s surface, and but from one to
five minutes at a time by any one observer.” Very little of the eight
days, however, can be utilized; indeed, as has been pointed out by Miss
Clerke in her admirable _History of Astronomy During the Nineteenth
Century_, the corona has only been observed by scientific men during
forty-five minutes in as many years. Opportunities of observing an
eclipse occur therefore at such comparatively long intervals, the
phenomena to be observed are so varied and extensive, and the time
during which the observations must be made is so very limited, that any
permanent records of the phenomena, such as photography enables us to
obtain, cannot fail to be of the greatest value. The most careful
drawings of the same eclipse by different observers at the same station
are so very dissimilar that it is generally unsafe to base any
conclusion on them; whereas in photographs we have truthful records of
the actual phenomena without personal equation of any kind, and with the
additional advantage that there is more detail in the photograph than it
is possible to insert in any drawing made during an eclipse, or even at
leisure after the three or four minutes’ observation of such an
indefinite and irregular object as the corona. The history of the
increase of our knowledge of the corona is practically the history of
the improvement of our photographic methods of attacking the phenomena
of an eclipse.
The first occasion on which photography was used at an eclipse of the
sun was on July 8, 1842, when Professor Majocchi, at Milan, attempted to
obtain Daguerreotype pictures of the corona. His account of the attempt
informs us that “a few minutes before and after totality an iodised
plate was exposed in a camera to the light of the thin crescent, and a
distinct image was obtained; but another plate exposed to the light of
the corona for two minutes during totality did not show the slightest
trace of photographic action. No photographic alteration was caused by
the light of the corona condensed by a lens for two minutes, during
totality, on a sheet of paper prepared with bromide of silver.” No
details are given of the apertures of the lenses employed, or of their
focal lengths. At the outset, therefore, astronomers were met with
failure, but the failure at Milan did not deter Dr. A. H. Busch and Herr
Berkowski from a similar attempt at Konigsberg on July 28, 1851. The
telescope used on this occasion had an aperture of 2.4 inches, and a
focal length of 30 inches. Commencing immediately after the beginning of
totality, a plate was exposed for 84 seconds in the focus of the
telescope, and on development an image of the corona was obtained. A
second plate exposed for from 40 to 45 seconds was fogged by the sudden
breaking out of the sunlight. The picture thus obtained--the first
photograph of the corona and prominences--is known as the Konigsberg
Daguerreotype, and is still preserved at the Strasburg Observatory. It
was lent by Professor Winnecke for the exhibition of scientific
instruments at South Kensington in 1876. On it the prominences, and the
lower portion of the corona extending about one-fourth of a solar
diameter from the moon’s limb, are distinctly shown, the encroaching of
the prominences on the dark disc of the moon, owing to irradiation,
being particularly evident.
Daguerreotype was again used for the annual eclipse of May 26, 1854, by
Mr. Campbell and Professor Loomis at New York; by Dr. Bartlett and
Victor Prevost, who obtained nineteen photographs, at West Point; and by
Professor Stephen Alexander and Mr. E. H. Old at Ogdensburg.
Liais in 1858 obtained photographs of the partial phases, using wet
plates. On one of these the moon can be seen projected on the corona
before totality. With the introduction of the collodion process more
sensitive plates were obtained, and a great advance was anticipated. At
the total eclipse of 1860, July 18, Mr. Warren de la Rue, at
Rivabellosa, in Spain, used wet plates. His instrument was one specially
devised by himself for photographing the sun’s disc for sun-spots, and
is known as the Kew heliograph. It is an ordinary equatorial mounting
with driving clock, carrying a photographic object-glass, 3.4 inches
clear aperture, and 50 inches focal length. The primary image is .466 of
an inch in diameter, but before the image falls on the plate it is
enlarged by an ordinary Huyghenian eyepiece to 3.8 inches diameter. The
exposing apparatus for the ordinary sun photographs is an instantaneous
shutter; this, of course, had to be abandoned for the eclipse
photographs. Two plates were exposed during totality, the exposure being
60 seconds in each case, but only slight traces of the corona were
obtained. At the same eclipse Father Secchi and Professor Monserat,
working at Desierto de las Palmas, obtained good photographs of the
corona, using an object-glass of .15 metre diameter, and 2.5 metres
focus, the primary image being 23 millimetres in diameter. The plates
were placed in the primary focus and according to Secchi, “all the
phases of the phenomena are represented on the photographs.” The
original negatives obtained at Desierto de las Palmas of this eclipse
have unfortunately been lost.
The next attempt at photographing the corona was on August 18, 1868,
this being remarkable as the first attempt to use a reflector for the
purpose. Colonel Tennant and Sergeant Phillips at Guntoor used a 9–inch
silver-on-glass mirror, by With, of 6 feet focal length, mounted
equatorially by Browning on the Newtonian plan.
Unfortunately the weather was unfavorable, but plates were exposed
through light clouds, the longest exposure being 10 seconds. The
photographs obtained show the prominences sharply defined, but only
slight traces of corona are visible. Mr. Sutton, at Mautawali Kiki, and
Drs. G. Fritsch, H. Vogel, and W. Zener, at Aden, were, from atmospheric
and other causes, unsuccessful with refractors.
At the eclipse of August 7, 1869, many attempts were made to photograph
the corona. In all cases where the image was enlarged before it fell on
the plate, slight traces of the corona were obtained; while Professor
Winlock and Mr. J. A. Whipple, at Shelbyville, with a 5½-inch lens of 7½
feet focal length, obtained seven pictures taken in the primary focus,
one with 40 seconds’ exposure, showing more detail than had previously
been photographed.
At this eclipse, Messrs. Hoover photographed the corona with a lens of
12 inches focus, and Professor Stephen Alexander also obtained
photographs at Ottumwa some of which give good ideas of the coronal
structure.
At the 1870 eclipse, December 22, a 4–inch Dallmeyer lens (rapid
rectilinear), stopped down to three inches aperture, and with a focal
length of thirty inches, was used by Mr. Brothers at Syracuse. Wet
plates were used, and the photographs were taken through light clouds,
the best of the pictures having had eight seconds’ exposure. Details in
the corona are very well shown in these photographs. In discussing his
results, Mr. Brothers says, “The photographs taken ... prove that the
light of the corona is very actinic, and that several photographs of
this beautiful phenomenon can be taken during the time of totality.” He
further adds, “That it is impossible to obtain satisfactory photographs
of the corona either with reflecting or refracting telescopes as
ordinarily used is, I think, now conclusively proved.”
Professor Winlock, at Jerez, during the same eclipse, obtained two good
photographs with ordinary telescopes; while Lord Lindsay, at Maria Louis
Observatory, with a 12–inch mirror of 6 feet focus, obtained plates so
much fogged as to be useless.
On December 21, 1871, splendid photographs were obtained at Baikul by
Mr. Davis (Lord Lindsay’s observer), and by Colonel Tennant, J. B.
Hennessey, Esq., and Captain Waterhouse, at Dodabetta. In each case
Dallmeyer 4–inch rapid rectilinear lenses of thirty-three inches focus
were used, the exposures varying from five to forty seconds. Herr
Dietsch, in Java, also obtained two good photographs with a “lens of
short focus,” with exposures of half and one-third second. Captain Hogg,
at Jaffna, also got fair results with cameras 16 inches and 23 inches
long. At the eclipse of April 6, 1875, Dr. Schuster, in Siam, obtained
good photographs, although small, with an ordinary camera.
The eclipse of 1878 marked another departure in photography. Dr. Draper
used wet plates, and got much detail in 165 seconds. Mr. Ranyard used
Mawson & Swan’s extra sensitive dry plates, with a 13–inch lens of 6
feet 2 inches focus, and obtained photographs extending 6′ (one-fifth of
a sun’s diameter) from the limb with exposures of one and three seconds.
Professor Harkness, the director of the American operations, arranged
two cameras, with 6–inch Dallmeyer lenses of 37.9–inch focus, and Mr. J.
A. Rogers and Mr. Clark with these, using specially prepared dry plates
made by Mr. Rogers, obtained two good series of photographs. In the
report on the eclipse operations published from the United States Naval
Observatory, Mr. J. A. Rogers not only discusses the value of
photographs as compared with drawings, but enters fully into all the
details of eclipse photography, concluding by strongly advocating the
adoption of dry plates. Mr. O. L. Peers during this eclipse obtained a
wet plate photograph showing greater extension of the corona than any of
the dry plate ones, but there seems some doubt about the apparatus he
used. He used either a 2⅛-inch or 3⅛-inch Voigtlander portrait lens, and
exposed either for twelve or for twenty-three seconds. Mr. Peers says he
used a 2⅛-inch lens, and twelve seconds’ exposure, while Voigtlander
declares he makes only 3⅛-inch lenses of the focus 1:8 Mr. Peers used,
and on examination of the photograph it is found that the trail of the
moon on the plate indicates an exposure of twenty-three seconds. After
the 1878 eclipse dry plates were universally adopted by eclipse
observers.
The photographic arrangements of the expedition to Sohag, in Egypt, for
the eclipse on May 17, 1882, were made by Captain Abney, the chief
objects of the expedition being to photograph the spectra of the corona
and prominences. Arrangements were also made by Captain Abney for corona
photographs with a 4–inch lens of sixty inches focus belonging to him.
The spectrum photographs taken show as many as thirty lines in the
prominences, while the photographs of the corona obtained by Dr.
Schuster with exposures of from three to thirty-two seconds show great
extension of the corona with the most exquisite detail. These plates are
also remarkable for the discovery of a comet in the photographs,
although the comet was not seen by observers. Captain Abney and Mr. J.
Norman Lockyer were responsible for the methods of photographic attack
adopted by the English observers, Messrs. Lawrence and Woods, at the
Caroline Islands, on May 6, 1883. The spectroscopic results and the
corona photographs taken with the 4–inch lens of Captain Abney,
previously used in 1882, were most successful. Janssen on this occasion
used two objectives, one 6–inch and one 8–inch diameter, and using long
exposures, photographed the corona extending two diameters from the sun,
this being much further than it could be traced with a telescope.
Photography was again used on September 8, 1885, at the total eclipse in
New Zealand.
At the eclipse of August, 1886, visible at Granada, Captain Darwin used
a chronograph as devised by Dr. Huggins, consisting of a mirror inclined
in a tube in such a manner as to enable photographs to be taken in the
primary focus without the intervention of a flat. Good results were
obtained. Dr. Schuster and Mr. Maunder used 4–inch lenses of 60–inch
focus, and obtained good results. Their spectrum photographs were also
successful. Professor Pickering, of Harvard, used a heliostat and a
photo-heliograph of 38 feet focus, supported horizontally, but no
results were obtained with this apparatus, although he was partially
successful with his other instruments.
Very few photographs were obtained of the eclipse of August 19, 1887, in
Russia, owing to the unfavorable weather. The English observers intended
to use similar instruments to those employed in 1886, but the weather
did not permit.
The eclipse of January 1, 1889, was very successfully photographed by
the American observers, the largest aperture used being thirteen inches.
On some of the plates used during this eclipse the standard intensity
scale recommended by Captain Abney several years ago was fixed, and for
the first time definite conclusions as to the brightness of the corona
were obtained.
The expedition sent out by the Royal Astronomical Society for the
eclipse of December 22, 1889, were each fitted with a 4–inch
photographic lens, belonging to Captain Abney, mounted on the usual
equatorial plan, and intended to continue the series so well begun by
Dr. Schuster in 1882, and also with a 20–inch mirror of 45 inches focus,
specially constructed and mounted for eclipse work, and designed to
photograph the outer portions of the corona too faint for ordinary
instruments. The plates for use with the 4–inch lenses were specially
prepared by Captain Abney, and on each of them he had placed a scale of
standard intensity squares for measuring the brightness of the corona.
Small squares on each of the plates were exposed to a standard light for
various times; these squares were then covered with a strip of black
paper, and the plates taken out to the Eclipse Station and exposed on
the corona. When the plates were developed the image of the corona and
the squares were, of course, developed to the same extent, the squares
thus serving as standards for absolutely measuring the photographic
intensity of the light of the corona. The density of the deposit in any
part of the picture of the corona can be compared with the density of
the most similar of the squares on the same plate by Captain Abney’s
photometer, and as this photometer depends upon the method of limiting
apertures, it gives absolute readings.
The African expedition was entirely unsuccessful, owing to clouds, but
the expedition to Salut Isles, under charge of the late Father Perry,
obtained successful photographs, which are at present under examination.
From them Captain Abney will be able to measure the absolute
photographic intensity of the light of the corona.
An American expedition was sent to Cayenne with instruments used on
January 1, 1889, and obtained successful photographs, while an American
expedition to Southwest Africa was unsuccessful, for the reason already
given. This expedition, under the direction of Professor David P. Todd,
was located at Cape Ledo, about half a mile from the English Eclipse
Station.
Several new departures in eclipse photography were introduced. Chief
amongst these was the remarkable apparatus by means of which no less
than twenty-three objectives and two mirrors were accurately pointed at
the sun and caused to follow it by one large clock. A large duplex polar
axis (the old English form as used for the 12.5 inch reflector at
Greenwich) was mounted on solidly constructed stone piers and very
carefully adjusted. This axis is constructed of 6–in. wrought-iron
tubing, the total weight being about 2000 lbs. In it the cameras were
fixed by set screws, the optic axis of the instruments being adjusted
parallel to each other, and at an angle equal to the south polar
distance of the sun at the time of totality. The carefully regulated and
very powerful clockwork attached to the instrument caused the polar axis
to rotate, and thus the whole battery of instruments followed the sun.
Each lens was fitted with a pneumatic shutter regulated to give the
required exposure in each case. The cameras themselves were enclosed in
a dark-room, the lenses only being exposed to the sun, so that dark
slides were not required, the plates being held on open rotating frames,
these frames being rotated at the proper time by pneumatic arrangements.
When the cameras were once pointed, and the clock driving properly, all
the operations of exposure and changing of plates were performed without
personal superintendence by means of the pneumatic apparatus, and a
chronograph attached to the valve system of this apparatus recorded the
exact time at which each exposure was begun and ended.
It is to be regretted that this ingenious and elaborate apparatus did
not have a satisfactory trial, owing to the dense clouds; but Professor
Todd assures us that he was thoroughly satisfied with the success of the
pneumatic movements during the three minutes ten seconds he brought it
into operation at the time of totality.
It is not improbable that (in spite of the great strength and weight of
the axis and the solidity of the supporting piers) with this plan of
fixing a large number of cameras and spectroscope on one polar axis, the
constant opening and shutting of shutters, and the changing of the
plates, may produce so much shake that none of the long exposure
photographs will be satisfactory. This, of course, can only be
ascertained by the use of the instrument on the corona, and several
years must elapse before the trial can be made.
Another unusual instrument was a photo-heliograph of five inches
aperture and forty feet focus, mounted on a combination of the
equatorial stand and tripod.
The long tube was made of iron, coiled spirally and strongly riveted,
the necessary rigidity being attained by strong wires extending from end
to end, and tightly stretched by a disc in the middle of the tube. Close
to one end of the tube the polar axis was attached by a universal joint;
the other end of the tube being supported by two rods, one on the east
and one on the west side, these rods being also attached by universal
joints. By means of these rods the proper inclination was given to the
tube. The east rod was the declination rod, and was capable of sliding
along the polar axis. The west rod was for giving motion in right
ascension, being terminated at the free end in the form of a piston of a
sand clock fixed in an inclined position. The rate at which the sand
escaped from the cylinder could be accurately regulated, so that the
rate of descent of the piston was completely under control, and was, of
course, such as would cause the instrument to follow the sun.
This instrument was erected at Cape Ledo, close to a hill of such
inclination that the sun could be followed during the whole of the
eclipse, while the long tube could be manipulated with greater advantage
than would have been possible if the instrument had been erected on
level ground. The hot air rising from the heated hill probably affected
the definition in the photographs, but under the circumstances that
could scarcely be avoided.
This form of mounting certainly solved the question of the possibility
of using long-focus lenses mounted as direct photo-heliographs, but the
apparatus is certainly unwieldly, and was only got into the fit state
that it was on the eclipse day by the very great care and patience of
Professor Bigelow. As it was intended principally to photograph the
partial phases of the eclipse with this instrument, instantaneous
exposures were arranged for, but Professor Bigelow succeeded so well in
the adjustment of the instrument and the regulation of the sand clock,
that he would have tried to obtain photographs of the lower corona with
it had the weather permitted.
The photographic apparatus on this instrument has a very ingeniously
constructed revolving plate holder, carrying round plates of twenty-two
inches diameter. The exposing apparatus and the apparatus for rotating
the plate between the exposures were moved by pneumatic arrangements,
exposures being made at intervals of six seconds, the exact time of each
being recorded on a chronograph. As no dark slides were used, it was
necessary to enclose the whole of the photographic apparatus in a
dark-room. One hundred and ten exposures were made with this telescope
during the partial phases of the eclipse, all the photographs taken
having to be obtained through clouds.
In several expeditions previous to this, where more than one kind of
observation has been required, two or more objectives have been mounted
on the same stand and driven by the same clock; but this plan is always
open to the objection that any accidental disturbance in the
manipulation of one of the pieces of apparatus will most probably spoil
the results for both. With the American plan of many objectives on one
heavy axis, and a pneumatic apparatus to manage all the actual
operations of exposures and changing of plates, this objection of
possible accidental disturbance is to some extent overcome; but the
shake of the many operations taking place on the one axis introduces
another risk. Beside this, the apparatus is very heavy, and exceedingly
difficult to transport and erect, even in a civilized country.
Such is a very summarized account of the instruments hitherto employed,
and it seems to me that the time has now come when much can be gained by
the employment of fixed instruments and a moving large plane mirror.
This idea of using a heliostat is, of course, not new, for it has been
used several times on a small scale, and for special purposes. There is
nothing beyond the difficulty of making a plane mirror sufficiently
large for the work to prevent the adoption of this method in the future;
and this difficulty now has ceased, as it is only a matter of time and
labor to make plane mirrors of sufficient size. With a large plane
mirror, twenty inches or upwards in size, mounted on a heliostat
mounting, and so arranged as to reflect sunlight into a series of
instruments rigidly supported in a horizontal position, the difficulties
of eclipse observers will be very considerably lessened. The one driving
clock will keep the pencil of light constantly in the same direction,
and this can be used partly for photographing the corona, partly for
spectroscopic work, partly for polariscopic observations, and so on for
any other purposes, the whole of the instruments being fixed in the best
possible positions for the observers. Practically, with a large flat
mounted in the manner indicated, we can fix any portion of the sky we
require to observe, and to do it we can point as many instruments as we
can crowd into the pencil, each instrument being quite independent of
the others. The length of focus of an objective would not introduce any
difficulties on this plan, for the length of the tube is of little
importance when it can be fixed in an horizontal position. The observers
at the Eclipse Station only have one astronomical adjustment to make,
_i.e._, that of the position of the heliostat, and only one driving
clock to regulate. This clock, since it has only to move the weight of
the plane mirror and its mounting, can be more accurately made and
regulated than is possible with a clock when it has to carry the weight
of the tube and heavy axis of an ordinary telescope. The positions of
the observers are more easy and natural during the precious seconds of
totality; or, if personal superintendence is to be abolished in favor of
the American pneumatic apparatus, this suggested arrangement of the
instrument is better fitted for the pneumatic attachments than the old
plan is. The whole of the photographic apparatus can be fixed up in a
dark hut or under a dark tent with far less trouble and risk of stray
light than is possible with the old manner of mounting.
The cost of a good heliostat mounting is about the same as that of a
good telescope, and with one heliostat we can do the work of at least
half a dozen of the usual instruments.
A. A. COMMON, F.R.S., and A. TAYLOR, A.R.S.M.
* * * * *
THE Supreme Court at St. Paul, Minnesota, handed down a decision on July
1st in the case of Ida Moore, of Minneapolis, against Photographer Rugg.
Rugg sold a copy of Mrs. Moore’s picture, which was put on exhibition in
improper places, much to the discredit of the lady, and she brought suit
for damages. The Supreme Court holds that it is a case in which there is
ground for the recovery of damages; that the photographer has no right
to dispose of pictures which are the sole property of the sitter. The
decision is an important one. Similar cases have arisen once or twice
previously in other parts of the country.