CELESTIAL PHOTOGRAPHY (CONTINUED).—SOME RESULTS.
The process used should therefore be the most rapid attainable; any work
on photography will give a number of processes of different degrees of
rapidity, but a process that suits one person’s manipulation may prove a
failure in another’s, and the general principles are the only rules
suitable for all. First, the glass plate should be carefully cleaned,
the collodion lightly coloured, the bath strong and neutral, certainly
not acid, and the developer fairly strong. Pyrogallic acid and silver
should not be used for intensifying; a good intensifier is made by
adding to a solution of iodide of potassium, strength one grain to the
ounce of water, a saturated solution of bichloride of mercury, drop by
drop, until the precipitate at first formed ceases to be re-dissolved;
use this after fixing.
Now let us inquire what has been done by this important adjunct to
ordinary means of observing. We may say that celestial photography was
founded in the year 1850 by Professor Bond, who obtained a daguerreotype
of the moon about that date. An immense advance has been made, but not
so great as there might have been if the true importance of the method
had been recognized as it ought to have been; and if we study the
history of the subject we find that till within the last few years we
have to limit ourselves to the works of two men who, after Bond, set the
work rolling. Several observers took it up for a time; but the work
requires much both of time and money, and different men dropped off from
time to time. There remained always steadfast one Englishman and one
American—Mr. De La Rue and Mr. Rutherfurd. The magnificent work Mr. De
La Rue has done was begun in 1852. He was so anxious to see whether
England could not do something similar to what had been done in America,
that, without waiting for a driving clock, he thought he would see
whether photographs of the moon could be taken by moving the telescope
by hand. He soon found that he was working against nature—that nature
refused to be wooed in this way; the moon in quite a decided manner
declined to be photographed, and we waited five years till Mr. De La Rue
was armed with a perfect driving clock. Mr. Rutherfurd was waiting for
the same thing in America.
At last, in 1857, Mr. De La Rue got a driving clock to his reflector of
thirteen inches aperture, and began those admirable photographs of the
moon which are now so well known. Since the above date the moon has been
photographed times without number, and Mr. De La Rue has made a series
which shows the moon in all her different phases. They are remarkable
for the beautiful way in which the details come out in all parts of the
surface. We must recollect that these pictures of which we have spoken,
some of them a yard in diameter, were first taken on glass about three
inches across, the image covering the central inch. At the same time the
British Association granted funds for the photographic registration of
sun-spots at the Kew Observatory, where the sun was photographed every
day for many years.
Encouraged by success, Mr. De La Rue, in 1858, attacked the planets
Jupiter and Saturn, and some of the stars. He discovered that
photographs of the moon can be combined in the stereoscope so that the
moon shows itself perfectly globular.
To accomplish this result it was necessary to photograph her at
different epochs, so that the libration, which gives it the appearance
of being turned round slightly and looking as it would do to a person
several thousand miles to the right or left of the telescope, should be
utilized. These two views when combined give the appearance of solidity
just as the image of a near object combined by the two eyes gives that
appearance. The reason of this appearance of solidity is easily seen by
looking at an orange or ball first with one eye and then with the other,
when it is noticed that each eye sees a little more of one side than the
other; and it is the combination of these slightly dissimilar images
that gives the solid appearance.
If we examine two of these photographs combined for the stereoscope, we
see that they have the appearance of being taken from two stations a
long distance apart. One shows a little more of the surface on one side
than the other. They are obtained in different lunations, when the moon,
in the same phase, has turned herself slightly round, showing more of
one side. In this way we have a distinct effect due to libration. In the
year 1859 Mr. De La Rue found that sun-pictures could be combined
stereoscopically in the same manner.
When we turn to the labours of Mr. Rutherfurd, we find him in 1857 armed
with a refractor of 11¼ inches aperture; the actinic focus, or rather
the nearest approach to a focus, was 7/10ths of an inch from the visual
focus. With this telescope, without any correction whatever, he, in 1857
and 1858, obtained photographs of the moon which, when enlarged to five
inches in diameter, were well defined. He also obtained impressions of
stars down to as far as the fifth magnitude, and also of double stars
some 3˝ apart—for instance, γ Virginis was photographed double. The ring
of Saturn and belts of Jupiter were also plainly visible, but
ill-defined. The satellites of Jupiter failed to give an image with any
exposure, while their primary did so in five or ten seconds. The actinic
rays, instead of coming to a point and producing an image of a
satellite, were spread over a certain area and thereby rendered too weak
to impress the plate.
In the summer of 1858 Mr. Rutherfurd combined his first stereograph of
the moon independently of Mr. De La Rue’s success in England.
Mr. Rutherfurd then commenced an inquiry of the greatest importance,
which will in time bring about a revolution in the processes employed.
In 1859 he attempted, by placing lenses of different curvatures between
the object-glass and the focus, to bring the chemical rays together,
leaving the visual rays out of the question; this had the effect of
shortening the focus considerably and improving the photographs; but he
found that, except for the middle of the field, this method would not
answer. He therefore in 1860 attempted another arrangement, and one
which he found answered extremely well for short telescopes.
Between the lenses of the object-glass of a 4½-inch refractor he put a
ring which separated the lenses by three-quarters of an inch, and
reduced the power of the flint-glass lens, which corrects the
crown-glass for colour, so that the combination became achromatic for
the violet rays instead of for the yellow. With this lens he was
successful to a certain extent: he obtained even better results than
with the 11¼ inch; but eventually he rejected this method, which we may
add has recently been tested by M. Cornu, who thinks very highly of it.
He next attempted a silver-on-glass mirror in 1861; in the atmosphere of
New York it only lasted ten days; he gave it up; and he then very
bravely, in 1864, attacked the project _de novo_, and began an
object-glass of a telescope which should be constructed so as to give
best definition with the actinic rays, just as ordinary object-glasses
are made to act best with the visual rays.
He found that in order to bring the actinic portion of the rays to a
perfect focus, it was necessary that a given crown-glass lens should be
combined with a flint, which will produce a combined focal length of
about ⅒ shorter than would be required to satisfy the conditions of
achromatism for the eye. This combination was of course absolutely
worthless for ordinary visual observation; his new lens when finished
was 11¼ inches aperture and a little less than 14 feet focal length.
With this he obtained impressions of ninth magnitude stars, and within
the area of a square degree in the Prœsepe in Cancer twenty-three stars
were photographed in three minutes’ exposure. Castor gave a strong
impression in one second, and stars of 2˝ distance showed as double. But
even with this method Mr. Rutherfurd was not satisfied. Coming back to
the 11¼-inch object-glass which he had used at first, he determined to
see whether or not the addition of a meniscus lens outside the front
lens would not give him the requisite shortness of the focus and bring
the actinic rays absolutely together. By this arrangement he got a
telescope which can be used for all purposes of astronomical research,
and he has also eclipsed all his former photographic efforts.