Weekly expositor. (Brockway Centre, Mich.) 1882-1894, 1889-05-23
NERuLR EXPLAINED.
Instructive Information for Stu
dents OF Astronomy.

4
stars, of which he gavetfi five on his
map, writes j. Norman Lockyer in
Harper's Magazine; and Tycho Brahe,
whose work was done before the inven-
tion OF the telescope, although he did
not notice any bodies which we now
class as nebulas, was firmly convinced
that that Hebllous luminosity which
we call the Milky-Way"was something
entirely different in itsnature from the
stars. He imagined 1T TO be what he
called I'm ethereal " Ssence, a sort OF
fire-mist, so that when in his time, in
the year 1872, new star appeared, he
supposed it to be considerable ag-
glomeration of this ethereal fluid.
| Calico was able to show that the
Milky-Way. the ethereal substance"
of Tycho, was only TN appearance due
to enormous numbers of. stars lying in
the same visual ray; she stars of which
it is composed can indeed be seen with
very small optical power.
1t was not till 1912, a few years after
the introduction of the telescope, that
we got the first real definition of a body
which we now call nebula. The first
observation We owe to Simon Marius,
who stated that some of the bodies Wis
ible in his telescope exactly resembled
the appearance produced by the flame
of candle seen through horn. 1t was
not till 1906 that the nebula in Orion
| was discovered although NOW TO the
trained eye ii is very easily visible, so
that il seems rather wonderful that it
was not discovered before. In 17l4, in
England, attention began do be paid TO
these bodies, but it was not until the
time of Sir William Hersehsl that the
most magnificent revelations were
made. He was the FIRST to construct
very large telescopes, by means of
which objects which appear Gc,the eye
as excessively dim, or objects entirely
invisible to the naked eye, ,may be
brought into full visibility. Inthis way
Sir William Herschel convinced him
self OF the existence of true nehnlous
fluid differing from stars.
After not only Sir William Herschel,
but his son, Sir John Herschel, had ac-
cumulated vast stores of facts, Lord
Rosse took up the story, and made "a
telescope very much more powerful
than any which had been employed by
the Herschels. His telescope has n
light-gEasping power compared with
the eye of 130,0O0 The chief result OF
Lord Roses work TO which we need
here refer is the idea that in a great
many bodies which had been classed
as nebul9 this enormous increase of
optical power suggested that we were
only dealing with very distant clusters
of stare.
Lord Rose was able to get the sug-
gestiOn of "resolvability" in so many
bodies which had been classed as
nebul@ by Sir William Herschel and
others that gradually the idea came to
be held that the most nebulous nebula,
II we could get sufficient optical power
to bear upon it, would be broken up
into stars just as certainly as the
Milky-Way had been by Calico.
This would mean that the nebule
were simply clusters Of stars so Infinite
ly remote from our ken that even with
the power of Lord Rosse's instrument
they retained the appearance of an
ethereal essence.
This was the general opinion in
1864, in the early days of spectrum
analysis, when DR.. Higgins turned his
spectroscope one night 10 one of the
planetary nebul2. At first he thought
that something had gone wrong in the
apparatus, because he could only see
bright line instead of the usual sort of
spectrum obtaiped from q star. Further
work on other nebule showed him,
however, that the spectroscope was do
ing its level best, and that the cause,
the anomely, was really that the nebula
gave out monochromatic light, while
stars gave out light OF all colors.
In some cases another line was seen,
easily proved 10 be due to hydrogen,
and in another planetary nebula other
observers have since shown that there
is another hydrogen line visible.
1t became DIR. Higgins duty to find
out the origin of the first line observ-
! ed, and he came to the conclusion, af-
! ter considerable labor, that this line
was very near, if not exactly, in the
! position of the chief line seen in the
spectrum of nitrogen, and the sugges-
! ton was therefore made that these ne
! bul8 were masses of nitrogen and n.y.
drogen mixed, or if not nitrogen, some
constituent of nitrogen mixed with hy-
! drogen. That result made the idea of
Lord Rose concerning the possibility
OF the resolvabllity OF neaul1 into stars
untenable. We had to consider from
that time that the light of the nebule
came from gas or vapor, and hence
it was held that the nebula were mass
es of gas.
Another explanation of the origin OF
the green line has already been given.
If We study the spectrum of magne-
stun, we find a very bright futing
with its less refrangible edge abso-
lutely in the position of the green line,
in the nebule and in comets the same
line appears, not nearly, as in the case
of the line of nitrogen. but absolutely.
But not only so. We find another line
of magnesium also visible in the plan
etary nebule. Again, in. the spectrum
of magnesium burning in the Bunsen
burner can be photographed a line
having the exact wave-lehgth OF a line
also seen in the nebula of Orion, so
that there is considerable amount of
cumulative evidence that magnesium
is the true origin of this spectrum, the
luminosity being produced by meteori-
ties, the chief constituent OF which is a