The Sun.
London, England, 1860-08-09 00:00:00

TILE ECLIPSE OF THE SUN.

Mr. Warren De La Rue gives the following in-
eresting account of the observations made at
Rivabellosa, a village near Miranda del Ebro :
The station selected was a threshing-floor situated
in lat. 42 deg. 42 min. north, and long. 11 min. 38
secs. west, at the height of 1,572 feet above the
mean level of the sea. The magnetic variation was
found to be 20 deg. 20 min. west. The locality,
being bounded by a beautiful panorama formed by
the distant Pyrenean range, was well situated for
observing the effect of the eclipse on the landscape.
My party consisted, besides myself, of Mr. R.
Beckley, the mechanical assistant of Kew ; Mr.
Downes, Mr. E. Beck, and Mr. Reynolds, Mr.
Clark, who at Mr. Vignolles's suggestion had acted
as our interpreter, also volunteered his services,
which proved mast valuable during the eclipse.
Each of my assistants had allotted to him aseparate
duty, and to their cheerful concurrence in carrying
out my wishes must be attributed the successful
result obtained.
My more important object was to endeavour to
obtain photographs of the various phases of the
eclipse by means of the Kew photoheliograph, an
instrument I designed, at the suggestion of Sir
John Herschel, for the special object of delineating
the sun's image by means of photography. Although
this was the primary object, I had also provided
myself with ample means of observing the eclipse
optically.
Our instruments and portable observatory,
weighing very little short of two tons, were by
previous arrangement conveyed in two days to my
station by the kindness of Mr. Vignolles, to whom
my thanks are especially due, and that of Mr.
Bartlett, of the firm of Messrs. Brassey and Co.,
contractors of the Bilboa and Tudela Railway.
By the 12th of July we had erected the ob-
servatory, and by the 14th had obtained the first
solar photograph in Spain. During these prelimi-
nary observations photographs were made of the
surrounding panorama by Mr. Downes, and the
geographical position of the station ascertained by
myself.
The climate proved to be most uncertain, and
much serious interruption was experienced by the
clouds, which frequently obscured the sun for the
whole day. Much inconvenience was also expe-
rienced from the dust, which rendered it necessary
to obtain large supplies of water, in addition to
that required for actual use, in order to keep the
station moistened; otherwise this enemy to photo-
graphy would have prevented any good result
from being obtained.
The Kew photoheliograph consists of a tube
having the form of a truncated pyramid, at the
smaller (upper) end of which is fixed the object-
glass, 3 4-10 inches in diameter, and 50 inches
focal length. The focal image of the sun formed
by the object-glass is 47-1000ths of an inch in
diameter, but, before it:is allowed to fall on the sen-
sitive plate, it is enlarged, by means of a secondary
combination of lenses, to four inches in diameter.
The sensitive plates, six inches square, are placed
at the large end of the pyramidal tube of the tele-
scope. The tube is what is termed equatorially
mounted, and is made to follow the diurnal motion
of the sun by means of clockwork.
In taking pictures of the sun the aperture of the
object-glass is usually reduced by means of stops to
two inches.
Even with this small aperture the duration of the
exposure of the sensitive plate to the action of the
sun is a very small fraction of a second of time. The
exposure of the plate to the sun's action is effected
by means of a sliding plate, having a very narrow
slit in it. The plate, which moves in the plane of
the primary focus of the telescope, is drawn down-
wards by means of an opening, and previous to
taking photographs is held up by a loop of thread
fastened to a hook.
When a picture is required to be taken the thread
is set on fire, and the plate flashes instantly across
the axis of the telescope, and allows the image of
the sun to pass momentarily through the slit on to
the sensitive plate.
The clockwork driving apparatus, although con-
venient, might actually be dispensed with in taking
sun pictures under ordinary circumstances, but at
the period of totality in a solar eclipse it is neces-
sary to expose the sensitive plate for some time to
the more feeble light of the luminous prominences
and corona, and then a clockwork driving appara-
tus becomes essential ; and it is also necessary to
employ the full aperture of the telescope.
The photoheliograph is provided with position
wires, which may be removed at will ; when left in
the tube they become depicted as dark lines cross-
ing the sun's disc whenever a sun picture is taken,
and serve to determine with great accuracy the po-
sition of any markings on the sun or of the cusps
with reference to a normal line, for instance, with a
circle of declination.
The day previous to the eclipse was completely
overcast, and the barometer was steadily falling.
Nevertheless four dozen plates were cleaned, so as
to be in readiness for the morrow; but the 18th
proved in the morning to be as cloudy as its pre-
decessor, and it was with very , faint hopes that we
went to our station. At 12 o'clock the sky began
to clear, and we obtained a faint picture of the sun
through the clouds ; about half-past 12 the clouds
melted away as if by magic, and we had a clear
blue sky ( without a cloud visible), except on the
distant mountains.
About 200 persons, who seemed to think that
the eclipse was only to be seen from our station,
rather interfered with our operations by their con-
versation,
which completely prevented our hearing
the beats of the chronometer ; but the majority were
persuaded after a little while to go on to a neigh-
bouring height, and the Alcalde Civilo Guinea, to
whom I wish publicly to tender my thanks, and the
Civic Guards induced the remainder to speak in a
lower tone.
Just before and after the eclipse sun pictures were
made, and during the. progress of the eclipse 31
photographs were obtained, the times of which are
carefully registered.
These will serve hereafter to determine the path
of the moon across the sun's disc and other data
with considerable accuracy.
The serrated edge of the moon is perfectly de-
picted in all the photographs, and in some of them
one cusp of the sun may be seen blunted by the pro-
jections of the lunar mountain, while the other re-
mains perfectly sharp. I continued during the
eclipse to observe the sun by means of a telescope
of three inches .aperture, by Dallmeyer, and I am
enabled to confirm the results obtained photogra-
phically. As I observed the progress of the eclipse,
I gave the signal from time to time for the taking
of a photograph, so that some have been procured
just as the moon passed across any conspicuous
solar spot.
When the sun was reduced to a small crescent,

the shadows of all objects were depicted with won-
derful sharpness and blackness, and as I cast my
eyes on the now silent crowd they and the land-
scape appeared as if illuminated by the electric
light, so brilliant were the lights, so sharp and
black were the shadows.

The sky began to assume an indigo tint, and the
landscape to be tinged with a bronze hue. But a
few moments, however, could be spared for these
observations, and when I placed my eye again to
the telescope, and removed the dark glass, some
minutes before the totality, I distinctly saw the
whole of the lunar disc, and perceived a luminous
prominence on the east of the zenith. This was quite
visible, while the sun's image was reflected by a
glass surface fixed at an angle of 45 deg., in the
eye piece, and its intensity therefore much dimi-
nished. The upper surface of the glass diagonal re-
flector I had, however, silvered to the extent of one-
half, and as I brought into action the silvered half
just previous to totality, I perceived a large sheet
of prominences on the east. A little to the east of
the zenith a brilliant cloud, quite detached from the
sun, and at some distance from the moon, came into
view. A few degrees to the west of the zenith a
minute point was perceived at the commencement of
the totality. South of the vertical only one small
prominence was perceived.
The brilliancy of these prominences far exceeded
that of the corona. Much detail was visible in the
protuberances both of light and shadow and con-
figuration. No appearance of Baily's beads was seen.
Casting the eye for a few seconds away from the
telescope to behold the corona and the surrounding
landscape, I was much surprised to find that the
darkness was not so intense as I had anticipated.
Thelideep indigo of the upper part of the sky
shading through a sepia tint into red and orange as
it approached the horison, the deep blue of the
mountains as contrasted with the orange sky, and
the peculiar light cast on the spectator, impressed
me with a feeling of solemnity never to be effaced,