The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1869-09-29


terrupted by flying clouds.
AIDS TO OBSERVATION
But, valuable as these observations we're n
is obvious that we should have had long to
wait before we could have become acquainted
with the usual behavior of these objects
and their possible relation to changes which
may be going on at the surface of the sun, if
we had been dependent on the rare and brief
phenomenon of a total solar eclipse for gather
ing information respecting them. But how,
the question might be asked, shall we ever be
able SO to subdue the overpowering glare of
our great luminary, and the dazzling illumi-
nation which it produces in our atmosphere
when we look nearly in its direction, as to
perceive objects which are comparatively so
faint Here again the science of OPTICS
comes in aid of astronomy.
When a line of light, such as a narrow slit
held in front of a luminous object, is viewed
through prism, the light is ordinarily spread
out into colored band, the length of which
may be increased at pleasure by substituting
two or more prisms for the single prism. As
the total quantity of light is not thereby in
creased, it is obvious that the intensity of the
light of the colored band will go on decreasing
as the length increases. Such is the case with
ordinary sources Of light, like the flame of a
candle or the sky, which give continuous
spectrum, or one generally continuous, though
interrupted by dark bands. But if the light
from the source be homogeneous, consisting,
that is, of light of one degree of refrangi-
bility only, the image of the slit will be
merely deviated by the prisms not widened
out into band and not consequently reduced
in intensity by the dispersion. And if the
source of light emit light of both kinds, it
will be easily understood that the images of
the slit corresponding to light of any definite
rafran nfiNi
will stand out, by their superior intensity, on
the weaker ground of the continuous spec
trum.
Preparations for observations of the kind
had long been in progress in the hands of our
countryman, Mr. Lockyer. His first attempts
were unsuccessful; but, undismayed by
failure, he ordered the construction of new
spectroscope of superior power, in which he
was aided by grant from the sum placed
annually by Parliament at the disposal of the
Royal Society for scientific purposes. The
execution of this instrument was delayed by
what proved to be the last illness of the emil
nent optician to whom it was entrusted, the
late MIR. Cooke; but when at last the instru-
ment was placed in his hands, Mr. Locllyer
was not long in discovering the object of his
two years search. On the 20th of October last
year, in examining the space Immediately sur
roundingthe edge of the solar disk, he obtained
evidence, by the occurrence of bright line
in the spectrum, that his slit was on the image
of one OF those prominences the nature of
which had SO long been an enigma. 1t fur
ther appeared from an observation made on
the 9th of November Gas indeed might be ex-
pected from the photographs of Mr.. De La
Rue, and the descriptions of those who had
observed total solar eclipses) that the promi-
nences were merely elevated portions of an
extensive luminous stratum of the same
general character, which, now that the neces-
sity of the interposition of the moon was
dispensed with, could be traced completely
round the sun. Notices of this discovery
were received from the author by the Royal
Society on October 21 and November 3, and
the former was almost Immediately published
in No. 105 of the Proceedings. These were
shortly afterwards followed by fuller paper
on the same subject ..
Meanwhile the same thing had been indie
pendently observed in another part of the
world After having observed the remarkable
spectrum of the prominences during the total
eclipse, it occurred to M. Jansen that the
same method might allow the prominences to
be detected at any time; and on trial he sue
ceeded in detecting them the very day after
the eclipse. The results of his observations
were sent by post, and were received shortly
after the account of Mr.. Lockyer's discovery
had been communicated by Mr.. De La Rue to
the French Academy.
In the way hitherto described prominence
is not seen as q whole, but the observer knows
when its image is intercepted by the slit; and
by varying a little the position of the slit
series of sections of the prominence are OBJ
tanned, by putting which together the form
of the prominence is deduced Shortly after
Mr.. Lockyers communication of his dis
covery Mr.. Higgins, who had been indepen-
IDENTIFY engaged in the attempt to render the
prominences visible by the aid of the spectre
scope, succeeded in seeing prominence as
whole by somewhat widening the slit, and
using red glass to diminish the glare of the
light admitted by the slit, the prominence
being seen by means Of the @ line in the red.
Mr.. Lockyer had design for seeing the pro-
minences as whole by giving the slit rapid
motion Of small extent, but this proved to be
superfuous, and they are now habitually seen
with their actual forms. Nor is our power of
observing them restricted to those which are
so situated that they are seen by projection
outside the sun s limb; such is the power of
the spectroscopic method of observation that
it has enabled Mr.. Lockyer and others to OBJ
serve them right on the disc Of the sun, an
important step for connecting them with other
solar phenomena.
REMARKABLE CHANGES IN PROGRESS.
One of the most striking results of the
habitual study of these prominences is the
evidence they afford of the stupendous
changes which are going on in the central
body of our system. Prominences the heights
of which are to be measured by thousands and
tens of thousands of miles, appear and disap-
pear in the course of some minutes. And q
study of certain minute changes of position in
the bright line f, which receive simple and