UNIVERSITY OF CALIFORNIA
AT LOS ANGELES
THE GIFT OF
MAY TREAT MORRISON
IN MEMORY OF
ALEXANDER F MORRISON
Cbe Wfctorfan Era Series
Recent Advances in Astronomy
Recent Advances
in
Astronomy
By
ALFRED H. FISON, D.Sc.
HERBERT S. STONE & COMPANY
CHICAGO & NEW YORK
M DCCC XCIX
,
Q641-
Preface
In the following pages I have endeavoured to
give a simple account of some of the more interest-
ing " Recent Advances in Astronomy". To har-
monize with the general scheme of the series of which
this work forms a volume, it was at first suggested
that I should develop recent progress in Astronomy
jf historically. The difficulties in the way of treating
^ any branch of science in such a manner are, how-
ever, very considerable ; especially when, as in the
present instance, it is desired to present the subject
in such a manner as to be readily followed by those
who have but slight familiarity with its techni-
calities. I am only acquainted with one entirely
S satisfactory "History of Astronomy", and that one
^ scarcely appeals to other than professional astro-
^ nomers. It has upon the whole appeared best to
<g effect a compromise between an historical and a
purely descriptive method; and I have, therefore,
o while dealing with what have appeared to me to
be a few among the more interesting problems of
modern Astronomy in a series of separate essays,
followed in each the historical method as far as
possible. It has been found practicable to adhere
to this scheme more rigidly in the latter part of the
work.
429055
vi Preface
Every writer of a popular work on Astronomy, or
any other branch of science, must become largely
indebted to those who have devoted their labour
to the compilation of works of reference; and I
would acknowledge my deep obligation to the ex-
tensive accumulation of accurate knowledge con-
tained in Miss Clerke's two works A History of
Astronomy during the Nineteenth Century, and The
System of the Stars.
A. H. FISON.
September, 1898.
Contents
CHAPTER I
Page
The Life of a Star -------/
APPENDIX TO CHAPTER I
The Measurement of Stellar Distances - - -JO
CHAPTER II
The Milky Way and the Distribution of Stars - -59
CHAPTER III
The Recent Study of Mars ------ roi
CHAPTER IV
The Analysis of Sunlight - - - - - 144.
CHAPTER V
The Analysis of Starlight 193
CHAPTER VI
The Red Flames of the Sun ------ 219
INDEX 239
Recent Advances in
Astronomy.
Ghapten J. v W |' j .. : j /,\
The Life of a Star.
" Great is the mystery of Space, greater is the mystery of
Time. Either mystery grows upon man, as man himself
grows; and either seems to be a function of the godlike which
is in man. In reality, the depths and the heights which are
in man, the depths by which he searches, the heights by which
he aspires, are but projected and made objective externally in
the three dimensions of space which are outside of him."
DE QUINCEY.
With our present knowledge of the sun -like
nature of the stars, and the colossal part that they
play in the scheme of the physical universe, it ap-
pears strange that, in spite of the bold spirit of
speculation that characterized the ancient philo-
sophy, a philosophy that recognized the possibility
of the development of higher forms of life from
lower ; that saw in the Sun, Moon, and Earth differ-
ent forms of air in different stages of condensation ;
and in the universe itself the working of a fortuitous
concourse of atoms, no worthy speculation should
have been recorded as to the nature of the stars.
: ., I ( M 520 ) A
2 Recent Advances in Astronomy.
Alike to the philosophers of Ancient Greece, and to
the early astronomers of Greece and Alexandria
whose lives were spent in tracing with deepest
thought and rarest skill the movements of the
heavenly bodies, it was sufficient that the stars were
points of fire, each set in its place in the concave of
the firmament, and eternally borne by it in diurnal
revolution round the central Earth.
It is unnecessary '.to jdc/plore than very briefly re-
view the sfcps,. initiated in the* bold speculations of
CoperoTeui>:'tb.wkrds':tha'. middle of the sixteenth
century, by which our present knowledge of the
sun-like nature of the stars has been attained.
Copernicus had shown it to be probable that the
Earth is one of the planets, a group of small bodies
revolving, each in its own period or year, round the
central Sun; and had recognized, as the logical
consequence of his scheme, that to remain, as they
appeared to remain, unaffected in their apparent
positions upon the celestial vault during the sup-
posed annual sweep of the Earth in its orbit, the
stars must be vastly more remote than the Sun;
but to him the material vault of heaven had merely
been thrown farther back, and the stars were still
points of fire studding its concave surface. The
bolder and direct deduction that to appear un-
affected in direction through all seasons of the year,
in spite of the enormous displacement in the posi-
tion of the observer upon the moving Earth, the
stars must be so remote, that, to be visible at all,
the majority of them must be bodies of the same
order of light-giving power with the Sun itself was
The Life of a Star.
recognized, though with hesitation, by John Kepler,
and was for the first time fully accepted by Galileo.
It will be advisable to present the principle under-
lying this deduction in a more definite form, since
the thorough comprehension of what has already
been achieved by it, and what may reasonably be
expected from its application in the future, is of
fundamental importance in the astronomy of the
stars. It involves directly the only method that has
so far been successfully
applied to the measure-
ment of the distance of
a star.
Let the curve in fig.
i be regarded as repre-
senting the orbit of the
Earth round the Sun,
an oblique view of the
nearly circular orbit,and
suppose that, the Earth
being in the position in-
dicated by the point P,
a star is observed, and
Fig. i. Illustrating Stellar Parallax.
that the direction in
which it is seen is recorded with all possible ac-
curacy. Let the straight line PA indicate this
direction. The star must lie somewhere in the
direction PA, but there is nothing in the observation
to indicate its distance from A, the point of observa-
tion. Six months later, however, the Earth will
have reached the position indicated by Q, having
traversed in that time one-half of its complete orbit.
4 Recent Advances in Astronomy.
Let the direction in which the star now appears be
observed, and let it be recorded by the line QB.
The star lies therefore in the direction QB, but there
is nothing in the last observation to indicate its
exact position in this direction. Since, however,
the pair of observations have located the star in the
directions PA and QB, it must be situated at s, their
point of intersection, and the geometry of the figure
at once gives the proportion between the distance
of the star and the dimensions of the Earth's orbit.
The principle of this, the only method by which
the distance of a star has so far been determined,
cannot but appear extremely simple, but a difficulty
in the interpretation of its application appeared,
when to Copernicus, as to his followers for nearly
three hundred years, the lines of sight PA and QB
appeared to be parallel, showing no tendency what-
ever to meet. Thus, the first observation having
established PA as the direction of the star, the second
would give the parallel line QC, and not a direction
such as QB, sensibly inclined to the first.
Assuming the fact of the Earth's journey round
the Sun, the only possible interpretation of the
apparent parallelism of the lines of sight was that
the stars are so remote that, although the direc-
tions PA and QC are inclined toward each other,
each being directed to the star, the inclination is
so slight that it was incapable of detection. That
this was the true explanation appeared more and
more certain as the truth of the Copernican system
became more firmly established; and in the con-
viction that success was possible, and as instruments
The Life of a Star. 5
were devised by the employment of which it became
possible to determine with an ever-increasing degree
of accuracy the direction of a star, the search after
the inclination of the lines of sight towards a star
from opposite extremities of the Earth's orbit, or the
"parallax of a star", became increasingly keen. 1
It is a matter of history that after close upon three
centuries of arduous toil toil occasionally rewarded
by unexpected discoveries of the greatest interest
and of the farthest-reaching importance, though
resulting in failure so far as the immediate object
of the search was concerned success was at last
achieved. In 1838, Bessel of Konigsberg demon-
strated, as the result of the critical examination of a
great number of observations, that a certain small
star in the constellation of the Swan did appear to
experience a displacement in its position upon the
heavens during the progress of the year, the inclina-
tion of the lines of sight toward it from the two most
favourably situated positions of the Earth in its orbit
being estimated at nearly two-thirds of a second of
arc. The star, in itself an insignificant member of
the orbs of heaven, thus destined from its associa-
tion with Bessel's discovery to acquire an honour-
able place in the history of astronomy, is known as
6 1 Cygni ; and its distance as deduced from Bessel's
measurements was 600,000 times that of the Sun.
Bessel's estimate has, however, been reduced by
1 The parallax of a star is more exactly defined in astronomy as one-half
of the greatest observable inclination of the lines of sight, or as the inclina-
tion toward each other of two straight lines to the star, one directed from
the Sun and the other from the Earth at a time when the direction of the
Earth, as seen from the Sun, makes a right angle with that of the star.
6 Recent Advances in Astronomy.
more recent measurements, carried out with finer
instrumental appliances, and with the advantages
arising from accumulated experience, to 440,00x5
times the distance of the Sun.
It would be scarcely advisable to digress here into
a critical examination of the difficulties that have
been experienced in the search after stellar parallax,
and the methods by which they have been in part
overcome; but the subject is of such high impor-
tance in the astronomy of the stars that I have ven-
tured to give, in the form of an appendix to the
present chapter, a rather more detailed account of
Bessel's discovery, as well as of the leading features
of more recent work.
Within a few months of the date of Bessel's dis-
covery, Professor Henderson, of Edinburgh, an-
nounced the fact of his having succeeded in detect-
ing parallax in the bright southern star a Centauri.
The observations in which the parallax of the star
was recorded had been made by Henderson six
years previously, and for a different purpose, during
the course of his work at the Cape of Good Hope,
and his attention was only redirected to them with
a view to the investigation of parallax by the an-
nouncement of Bessel's success. The distance of
a Centauri, deduced from the parallax originally
announced by Henderson, is 180,000 times that of
the Sun, but the most recent measurements have
extended it to 270,000 times the distance of the Sun.
So far, no star has been found to lie nearer to the
solar system than a Centauri. A vague suggestion
of the unthinkable void that separates us from this,
The Life of a Star. 7
in all probability the nearest of the stars, may
perhaps be obtained from the fact, that upon such
a scale that the orbit of the earth should be repre-
sented by the circumference of a shilling, the star
would be removed to a distance of two miles.
Across such a distance as that of a Centauri, light,
travelling with a velocity of 187,000 miles in a
second of time, would speed onward for four and a
half years ; so that the star is seen, not as it is now,
but as it was four and a half years since, while
another equal period must pass before the rays now
leaving it will bring their record to the shores of the
Earth.
The discoveries of Bessel and Henderson im-
parted new life to the search after the parallaxes of
stars. So delicate, however, are the necessary
observations of direction, and so many and serious
are the sources of error, that, excepting a few
isolated successes, the record of the next forty
years is chiefly one of accumulation of experience ;
and when in 1881 Dr. Gill and Dr. Elkin com-
menced a series of observations at the Cape of
Good Hope, the parallaxes of not more than half
a dozen stars had been detected with certainty.
Since that date, however, parallax hunters have
been better rewarded, though up to the present
time it is doubtful whether success has been
achieved in more than fifty instances.
Of the stars the parallaxes of which have been
detected, Sirius is undoubtedly the most important.
As in the case of a Centauri, its parallax is indicated
with some degree of probability in observations
8 Recent Advances in Astronomy.
made by Henderson in 1832. For another half-
century, however, the numerous attacks made upon
it were chiefly remarkable for the discordance of
their results, discordance that ultimately vanished
in the frequently repeated observations that have
been made at the Cape since 1891. The most
recent estimate published by Gill in 1898 of a
parallax of -37 of a second of arc, agrees very
closely with his previous results, and indicates for
the star a distance of 556,000 times that of the Sun.
The mere statement of the distances of stars is
apt to be productive of weariness of the spirit; in
their absolute magnitudes they are entirely un-
thinkable, but in their relation to things familiar,
they may well produce a powerful impression of the
nothingness of the Earth so far as its physical
relations are concerned to the scheme of the
physical universe. From the days of the Psalmist
it has been customary to regard the heavens as
inspiring a sense of deep humility in man, but
whether Nature in her most sublime aspect would
appeal to one who had not already learnt the lesson
from communion with his fellows is doubtful.
The chief interest of the distances of the stars in
the present connection lies in the view to which they
necessarily lead regarding the nature of the stars
themselves. If it were removed to the distance of
Sirius, the Sun itself would fade into insignificance,
shining but as a star of the third magnitude, rather
less conspicuously than the brighter ones that form
the familiar W of Cassiopeia. Seventy-five such
stars would be necessary to supply light equal to
The Life of a Star. 9
that received from Sirius; hence, in the intensity of
its light radiations, Sirius exceeds the Sun 75 times.
Of the few stars in which parallaxes have so far
been detected, to appear with their actual luminosi-
ties at their estimated distances, many must far
exceed the Sun in light-giving power, while a few
must surpass even Sirius itself. Others, however,
and among them Bessel's star in the Swan, while
undoubtedly suns, would appear but as modest
specimens of their class if placed beside ours, and
it is scarcely possible so far to decide whether our
Sun reaches the average of splendour displayed by
the suns of space, or whether he surpasses it. It
will be seen in a later chapter that the sun-like
nature of the stars is further indicated in the analysis
of their light by the spectroscope.
When, as is the case in the overwhelming ma-
jority of instances, no parallax can be detected in a
star, its distance is of course indeterminate, but it
is possible to assign a minimum distance beyond
which it must be situated, if the smallest angle of
parallax that could escape detection is known. So
much depends upon the skill of the observer, upon
the position of the star in relation to others, and
even upon its colour, that it is not possible to give
any definite and general estimate of this maximum
parallax. According to a recent statement of Dr.
Gill, however, than whom undoubtedly there can
be no higher authority, under favourable conditions
a parallax of a fiftieth of a second of arc should not
escape detection, one that corresponds to a distance
of rather more than 10,000,000 of times that of the
io Recent Advances in Astronomy.
Sun. There is little doubt that but an insignificant
fraction of the stellar host lie within this limit.
To apostrophize upon the picture of the physical
universe revealed by these discoveries is an old
story. The concave vault of the Old Astronomy
has dissolved, and has revealed, beyond, a scheme
unthinkable in its vastness, and in its suns and
systems of suns radiant with energy. There are
few among us who have not experienced, as in our
more emotional moments we have endeavoured to
penetrate, however superficially, the inward mystery
of so majestic a scheme, and one in which man plays
apparently so humble a part, a sense of oppression.
We have been overwhelmed with the sense of in-
scrutable and immanent mystery; and we have
been ready to exclaim with the pilgrim of German
fable, "I will go no farther; for the spirit of man
acheth with this infinity. Insufferable is the glory
of God ! Let me lie down in the grave and hide me
from the persecution of the Infinite, for end, I see,
there is none!"
The stars, then, are suns; and the life of a star is
the life of a sun. Life is essentially a succession
of changes, a passage through varying conditions
of activity ; death is cessation of all activity. Are
there grounds for regarding our sun as undergoing
change? and if there are, what is the nature of that
change? Are there indications that in time the
activity of the Sun will cease? The Sun is clearly
a hot body continually throwing off an enormous
amount of heat into space by the process of radia-
tion. Unless, therefore, by some undiscovered and
The Life of a Star. n
entirely unsuspected process an equivalent amount
is supplied to it from some external source, it must
be becoming continually poorer in its store of heat. 1
In the remote past it must have contained far more,
and in the distant future it will contain far less heat
than it contains at the present time. Everything
goes to support the straightforward view that the
light of the Sun is the direct result of a vivid state
of incandescence of its surface consequent upon the
high temperature to which it is raised. As the Sun
cools, a time must come when, unless some catas-
trophe intervenes, its temperature will have fallen
so much that its beams will have lost their present
glory ; later, it will cease to glow ; and thereafter,
as a dark star, a sad memorial of its present splen-
dour, it will pursue its lifeless course through the
ages.
Attempts have been made to estimate the time
that must elapse before the period of this Sun-death,
but in our ignorance of the physical constitution of
the Sun, and more especially of that of its interior,
such estimates are affected by a very wide margin
of uncertainty. There is, however, no doubt that
the actual heat existing as such in the Sun forms
but an insignificant fraction of its total store of
radiation. Without doubt, the Sun is largely, if
not almost wholly, gaseous: and since all gases, as
also nearly all solids and liquids, expand with
iA rather attractive speculation of Julius Mayer, vigorously supported for
a time by Tyndall, sought to account for the maintenance of the solar
radiation by heat developed from the destruction of motion of meteorites con-
tinually falling into the Sun. It has, however, been shown that any heat that
the Sun may possibly gain in this way must be quite negligible in quantity.
i2 Recent Advances in Astronomy.
accession of heat and contract upon loss of it, the
sun must be shrinking. Further, the interior of the
Sun must be enormously compressed by the weight
of superincumbent matter; and as shrinkage takes
place it must become still more compressed. But
the act of compression of a gas produces heat;
hence heat is continually being generated in the
Sun by the compression of its substance. Each
step in the loss of heat, therefore, calls into exist-
ence other heat; and this may partly, wholly, or
even for a time more than compensate, for the loss.
Subjecting these principles to mathematical expres-
sion, Helmholtz has shown that the heat thus
evolved by compression of the Sun consequent
upon its shrinkage must be sufficient in amount
to maintain it as a self-luminous body for many
millions of years. It is unnecessary for our present
purpose to attempt to arrive at a more definite
estimate of the future of the life of the Sun.
Returning to the present condition of its system,
and from it projecting our thought backward into
the past, we see the Sun richer and richer in heat
during receding ages; becoming more perfectly
gaseous the ultimate effect of accession of heat
being to convert all things into the gaseous state
while ever increasing in volume ; the planets one by
one disappear in its expanding bulk; and there
appears as the first stage of Sun-life a diffused body
of gas, extending beyond the present limits of the
planetary system, and containing latent in itself a
store of energy that is through coming ages to
maintain the vitality of worlds.
The Life of a Star. 13
The Sun, and, by the same process of reasoning,
the stars, would thus appear to have originated in
extended volumes of tenuous gas, and to be fated
in the end to be degraded into cold inert masses.
These conclusions being accepted, it would appear
probable that both of these conditions would be at
the present time represented among celestial bodies,
for even upon the extreme assumption that all of
them were created at the same time and in the
same stage of development, it would follow that,
since they differ enormously in mass, they would
cool and therefore pass through their life stages
at different rates. It becomes, therefore, of great
interest to inquire whether there exist in celestial
space extensive bodies of gas, and whether there
exist dark stars. The answer is clear: astronomical
observation has revealed both.
There can be little doubt that the earliest stage of
star-life is represented in, at any rate many of, the
nebulas. The nebulas appear as faint clouds of
light, and are distributed in thousands over the
face of the heavens. The greater number are ex-
cessively faint, their very detection demanding the
aid of the highest optical power; while two only,
and those just hovering upon the verge of vision,
are visible to the eye upon the darkest and clearest
nights. These are the glorious objects in the
constellations of Andromeda and Orion, the one in
Orion being the more impressive of the two.
The Great Nebula of Orion is situated near the
centre of a line of faint stars that trail southward
from the middle of a line formed by the three
i 4 Recent Advances in Astronomy.
bright ones that constitute the "belt" of the
familiar winter constellation Orion. Visible to
the naked eye under favourable conditions as a
faint mist
A single misty star,
Which is the second in a line of stars
That form a sword beneath a belt of three ;
its cloudy nature clearly revealed in a hand tele-
scope or a good field-glass ; when viewed through
a telescope of large light-grasping power it becomes
one of the most impressive of natural objects,
though the vast extension of the heavens into
which its wreaths are thrown, and the abundance
of delicate detail permeating the whole, have only
become revealed in recent records of the photo-
graphic plate.
The Nebula of Orion appears through a fine
telescope as a faint green haze, suggesting a light
cloud floating in celestial space, in form not very
unlike that of the profile of a fish's mouth. The
whole is composed of clouds of light of different
degrees of brightness, some of extreme fantastic,
and not a few of highly suggestive forms. It is in
the perception of these that the photographic plate
has demonstrated, as powerfully as in any of its
applications, its great superiority over the eye in its
capacity of appreciating the faintest shades of light.
Structure is revealed throughout the whole nebula
by the manner in which streams of luminous matter
are directed from a brilliant and nearly central
region in close proximity to the mouth-like bay of
dark sky, the importance of this region being
The Life of a Star. 15
emphasized by the occurrence in it of a remarkable
group of stars "the trapezium of Orion" and in
the symmetrical arrangement of many of the cloud-
forms with reference to it. Many stars are scattered
over the picture; that those of the trapezium are
actually involved in the glowing wreaths of the
nebula itself, and do not owe their appearance in it
to the effect of optical projection, either by their
lying by chance in the line of sight towards the
nebula, or by being visible through its transparent
substance while actually far beyond, is rendered
overwhelmingly probable from their position with
reference to the cloud-forms, as well as by certain
relations that have been shown to exist between
their analysed light and that of the immediately
surrounding nebula, in the spectroscopic researches
of Sir William Huggins.
The diffuse character of the outlines of the nebula
renders it impossible to apply to it such delicate
measurements of direction as are necessary for the
determination of the parallax. For this reason its
distance cannot be directly investigated. The stars
of the trapezium have, however, shown no parallax;
from this it becomes possible to assign roughly a
minimum limit beyond which they, and therefore
in all probability the nebula, must lie. Such
distance can scarcely be less than a million times
that of the Sun. To appear of its vast extent, even
at this, the most modest estimate, its glowing
clouds must extend over such abysmal depths, that
the whole of the Solar System if plunged into it would
become contemptible in its utter insignificance.
16 Recent Advances in Astronomy.
The Nebula of Orion is a noble example of an
"irregular nebula". That of Andromeda, in its
regular ellipticity of outline, in the uniformity in
the central condensation of its light, and in the
system of elliptical rings by which it is enveloped,
forms so strong a contrast with it that it is difficult
to regard the two as objects belonging to the same
class. Other nebulae display a spiral structure;
others again appear as fairly sharply denned plan-
etary discs ; while the majority are to all appearance
nothing more than minute structureless clouds of
flocculent light.
In the early period of their discovery, a discovery
that followed naturally upon Galileo's first applica-
tion of the telescope to astronomical observation in
1609, nebulas were regarded as diffusions of a lucid
medium shining by its own inherent lustre. In
1780, the year that marked the commencement of
Sir William Herschel's classical researches upon
them, less than 150 were known; but as the result
of those researches, which extended over a period
of twenty-one years, their number had been in-
creased to close upon 2500. By their extended
distribution in space, as well as by the detailed
structure revealed in many of them by Herschel's
observations, the nebulas had acquired a new im-
portance in the system of the Universe.
From a not altogether satisfactory deduction from
the universality of gravitation, an extension of
natural law that his own discovery of the mutual
revolution of the components of double stars went
far to establish, Herschel was led, in the earlier
The Life of a Star. 17
period of his researches, to reject the generally
received view regarding the nature of nebulae, and
to substitute for it one according to which they were
clusters of stars, the component stars being too
faint, by reason, it was supposed, of excessive dis-
tance, for their individuality to be recognized.
While maintaining this view with regard to the
constitution of some nebulae, Herschel, however,
subsequently reverted to the former hypothesis to
account for many of them, these including the
Nebula of Orion, regarding them as " extensions
of a shining fluid of a nature unknown to us ". He
further framed a first consistent scheme of stellar
evolution, in suggesting that individual stars and
clusters of stars were formed by the condensation
of this nebula substance by the power of gravita-
tion.
During the first half of the present century
scientific opinion entirely reverted to the earlier
of Herschel's views. Changes in the outlines of
certain nebulae, and the absence of structure in
others of the "planetary" class, both of which
Herschel, thinking that he had established by
observation, had advanced in support of his later
views, failed to receive confirmation in their ex-
amination by later astronomers. As with increased
telescopic power many objects classed as nebulae
were one by one resolved into collections of stars,
the conviction became increasingly strong, that,
with sufficiently refined means, all would ultimately
succumb: and when at length, in 1850, the Great
Nebula of Orion was thought, from its appearance
(M620) B
i8 Recent Advances in Astronomy.
in the gigantic telescope of Lord Rosse, to show
indications of breaking into clouds of stars, the
riddle of the nebulae appeared to be approaching its
last solution. As clusters of stars the nebulae found
ready place in the speculations of many astrono-
mers, whose minds, in consequence of the perfection
displayed in the relations between the Sun and
planets, had become powerfully impressed with the
conception of a system as an essential unit in the
construction of the universe. The planets, with
their attendant satellites, formed systems, fair
images of the grander Solar System, in which they
were included. Each star was regarded as a sun,
the centre of a planetary system of its own. Visible
isolated stars formed with our Sun a larger but
essentially similar system or "galaxy", in which it
was even conjectured that all members might re-
volve round a central orb ; while nebulas were other
systems of suns, external galaxies, awfully remote
from our galaxy and from each other; oases of
active energy scattered through space. The de-
molition of this stupendous conception by later
researches has been advanced as supplying the
only instance in which astronomical discovery has
failed to reveal in the actual a more majestic scheme
than had previously been idealized in the boldest
imagination.
While, however, the colossal reflector of the
Earl of Rosse was engaged, it was fondly believed,
in finally establishing the nebulae as clusters of
faint stars, the researches of Angstrom, Bunsen,
Kirchhoff, and others were placing upon a firm
The Life of a Star. 19
foundation the principles of a new science that was
shortly to enter the arena, with the result of utterly
confounding general expectation. The develop-
ment of the science of Spectrum Analysis forms the
subject of a later chapter of the present work.
Here it must be sufficient to record that as early
as 1672 Sir Isaac Newton had shown that, upon
passing a ray of sunlight through a glass prism, it
became separated into its constituent colours, by
reason of the fact that all rays are deflected or
"refracted" on traversing the prism, but that rays
of different colours are refracted to different degrees;
that after the lapse of a century and a half the study
of the analysis of light was resumed and the instru-
mental means greatly improved by Fraunhofer of
Munich ; and that, by the labours of Kirchhoff and
Bunsen, the spectroscope assumed its place as a
powerful instrument of research about the year
1860.
The spectroscope is essentially an instrument
whereby light consisting of a mixture of colours
is, after entering the instrument by a narrow slit,
resolved into its constituent colours by a prism, or
occasionally by an equivalent "diffraction grating".
The separated colours are in either case spread out
into a tinted band or "spectrum". About the
middle of the present century observations with the
spectroscope had indicated that there was a remark-
able difference between light emitted by a glowing
gas and that radiated from an incandescent solid
or liquid body. With light emanating from an
incandescent solid or liquid, such as that emitted
20 Recent Advances in Astronomy.
by a glowing mass of white-hot metal, or by a gas
flame in which the greater part of the luminosity
is due to incandescent clouds of soot deposited in
the flame from the decomposition of the gas under
the intense heat of combustion, and, with a limita-
tion to be noticed subsequently, that from the Sun
and from the great majority of the stars, the spec-
trum consists of a continuous band in which all the
colours of the rainbow are represented, each passing
into the next by insensible gradations, while red
and violet occupy the extreme positions. In the
light from a glowing gas, however, at any rate
when the density of the gas is not excessive, this is
not the case, the light being now resolved into a
series of clearly-defined and separate colours, which
appear in the spectroscope as bright lines of
coloured light separated by dark intervals; the
lines are, in fact, images of the slit by which the
light enters the instrument, a separate image being
formed by each of the colours present. The light
from the flame of a spirit-lamp which has acquired
a strong yellow tint by sprinkling a trace of com-
mon salt upon the wick, is, for instance, resolved
into two closely coincident shades of yellow, indi-
cated in the spectroscope by the appearance of a
pair of closely adjacent yellow lines; and the peach-
coloured glow emitted by hydrogen gas when
rendered luminous by a discharge of electricity
through it, gives rises to the appearance of several
coloured lines, of which a crimson and an emerald-
green appeal most strongly to the eye.
In the year 1864 Sir William Huggins first
The Life of a Star. 21
applied the spectroscope to the study of the nebulae,
the particular one selected being a small but com-
paratively bright object in the constellation of the
Dragon. The light from the nebula was condensed
upon the slit of the spectroscope by the object-glass,
8 inches in diameter, of an astronomical teie-
scope; and at the first glance, the examination of
the spectrum showed it to be characteristic of the
light emitted from a glowing gas, since it consisted,
not of a continuous band, but of three separated
lines, all of them being of a green colour. The
luminous matter of the nebula consisted, therefore,
not of a host of stars, but of incandescent gas ; and
the more matured views of Sir William Herschel
were established upon a sound scientific basis.
During the four years following this observation
Huggins subjected the light from seventy other
nebulas to analysis; and of them about one-third,
including the Great Nebula in Orion, proved to be
gaseous. The remaining two-thirds yielded " con-
tinuous " spectra, spectra in which all shades of
colour were represented, and might, therefore, so
far as spectroscopic evidence was concerned, con-
sist of systems of stars, of gas possessing compara-
tively high density, or of gas in an incipient stage
of condensation. The structure of some of these as
revealed by the photographic plate lends strong
support to the last hypothesis ; in the Great Nebula
in Andromeda, for instance, it is scarcely possible not
to recognize the process of condensation as actually in
progress. Nearly one-half of the nebulas owe their
luminosity to the presence in them of glowing gas.
22 Recent Advances in Astronomy.
It is difficult not to see in the gaseous nebulae the
stuff of which future stars will be made. Granting
that their substance is subject to the law of gravita-
tion, it appears certain that in coming ages their
glowing matter must, under its influence, be drawn
towards centres of condensation ; the smaller and
more symmetrical of the nebulae possibly developing
into single stars, but such majestic collections of
cloudy structures as are revealed in Orion being
more probably the origin of hosts of separate
suns.
Turning from these impressive representations of
the birth of suns, it now becomes our task to seek
among the heavenly bodies for the more sombre but
scarcely less impressive record of their death; to
search among their resplendent brethren for evidence
of the existence of spent and dark suns. A dark star
may conceivably become known to us in either of
two ways : it may in its wanderings through space
interpose itself between the Earth and a bright star,
thus producing a total or a partial eclipse of the
latter ; or it may approach sufficiently near a visible
star to affect it sensibly by its gravitational influence,
in which case it may be possible to deduce the ex-
istence of the dark star from the disturbance apparent
in the movement of the bright one. There can be
no doubt that the existence of dark stars has been
revealed in both of these ways, and both methods of
research are admirably illustrated in the discovery
of the notorious dark companion of Algol.
From the extremity of the shallower and left arm
of the familiar W of Cassiopeia, and setting off in
The Life of a Star. 23
a direction making sensibly a right angle with the
limb, a gracefully curved line is naturally traced in
the heavens by the stars of Perseus and terminated
in the resplendent orb of Capella. A straight line
diverging to the left of this stream and proceeding
slightly forwards from the brightest and nearly
central star in Perseus, is directed to Algol, the best-
known of the variable stars.
The variable character of the light of Algol is said
to have been first observed by Montanari in 1669,
though, owing to the great difficulty in measuring
the intensity of starlight, it has only been possible
in recent years to trace the exact law of its variation
with any approach to scientific accuracy. Normally,
Algol appears as one of the conspicuously brilliant
stars of the heavens, its brightness being sensibly
the same as that of the Pole Star. At intervals ot
time that appear to be subject to a very slow varia-
tion, and which are at present represented by 2 days
10 hours 48 minutes and 52 seconds, its light com-
mences to fade, and continues to do so for 4^ hours,
by which time it has decreased to two-fifths of its
normal brightness. This minimum value it retains
for 20 minutes, after which it resumes its normal
lustre in a manner which is nearly, though not
exactly, the reversed image of its fading.
In 1782 Goodricke, impressed with the regularity
displayed in the repeated variations of the star's
light, suggested as the cause of it the existence of a
dark companion revolving round Algol in an orbit
presented edgeways to the Earth, so that at each
revolution the bright star would suffer partial eclipse
24 Recent Advances in Astronomy.
by the interposition of its companion between it and
the Earth. The explanation was obviously sufficient
to account for the mere fact of periodic variation, and
its truth appeared more probable, when, a century
later, the spectroscope showed the variation of the
star's light to be unaccompanied by any change in
its quality. Such change would indicate change in
the star itself, and is frequently a conspicuous feature
in the variation of other and less regularly variable
stars. The probability of the truth of the eclipse
theory of Algol was still further increased when in
1888 Professor E. C. Pickering of Harvard, by the
application of the "meridian photometer", an in-
strument by the invention of which it became possible
to measure the intensity of the light of a star with a
degree of accuracy previously unattainable, found,
from the examination of the light of Algol at repeated
intervals during the progress of its variation, the law
or method of its variation to be essentially such as
would result from the interposition of a dark sphere
between the Earth and a luminous one.
Closely following upon Pickering's researches,
and by the application of a principle suggested by
him, the final demonstration of the existence of
Algol's dark companion was effected by Vogel at
Potsdam, from observations made between the years
1888 and 1891. Assuming the existence of a star
revolving round Algol, it would appear probable
that the force necessary to constrain it to continually
follow its curved path would be found in gravitational
attraction exercised upon it by Algol. By such a
force, the attraction of the Earth, the Moon is main-
The Life of a Star. 25
tained in its nearly circular path around it; by such
forces, the attractions exercised upon them by the
Sun, the planets follow without deviation their de-
termined orbits. If, however, Algol attracts its
companion, it follows from the necessary equality
between action and reaction as expressed in Newton's
Third Law of Motion, that the companion must
attract Algol with an equal and opposite force, and
it is conceivable that motion of Algol caused by the
attraction of the companion might be capable of
detection.
The character of the motion of two mutually at-
tracting bodies was first determined by Newton. It
was shown by him to follow from the laws of motion,
combined with the fact that the attraction of gravi-
tation varies inversely with the square of the distance
separating the attracting masses, that the pair must
describe similar conic sections having a common
focus, which is continually occupied by the centre
of mass of the pair. 1 Which of the three possible
forms of conic section will be assumed by the orbits
depends upon the initial circumstances of the motion,
but the greatest interest is attached to the ellipse,
which, being the only conic section forming a closed
curve, must be the orbit in every case in which the
motion is repeated.
The mutual revolution of the Earth and Moon
supplies an interesting illustration of the nature of
the motion under consideration. The Moon is
J The term "centre of mass" corresponds to the point more generally
known in elementary mechanics as "centre of gravity". For obvious
reasons the term " centre of gravity" would be quite inappropriate in cases
similar to that under consideration.
26 Recent Advances in Astronomy.
maintained in its elliptical and nearly circular orbit
by the gravitational attraction of the Earth. 1 The
Moon must therefore attract the Earth with a force
equal to this; and the Earth, being in no way
anchored in space, must move under the influence
of the Moon's attraction. The fact of its motion is
beyond doubt, both from theoretical considerations
and from practical observation ; and the nature of it
is expressed by the statement that the Earth and
Moon continually describe similar elliptical and
nearly circular orbits about their centre of mass, this
point being in a common focus and nearly in the
centre of each orbit. The general statement that
the Moon describes an elliptical orbit round the
Earth is, therefore, though not inexact, incomplete.
It would be equally true, and not inexact, to regard
the Earth as describing an elliptical orbit round the
Moon. Since, however, the mass of the Earth is
eighty times that of the Moon, the centre of mass of
the pair is eighty times nearer to the centre of the
Earth than to the centre of the Moon, lying in con-
sequence well within the Earth itself; so that the
actual orbit described by the Moon is far larger than
that described by the Earth. It is the common
centre of mass of the Earth and Moon that describes
an elliptical orbit yearly about the Sun.
With the assistance of the diagram given in fig. 2
there will be no difficulty in forming a definite picture
of the system of Algol and its companion, and of
1 The reader may be reminded that the circle is merely a particular form
of an ellipse, that in which the greatest and least lines drawn through the
centre, or the major and minor axes, are of equal length. The focus of a
circle and its centre coincide.
The Life of a Star.
27
Fig. 2. The System of Algol.
their relative movements. The point o is the centre
of mass of the pair, and since it is represented as
one-half as far from Algol as from the companion,
the companion is regarded as possessing one-half
the mass of Algol.
The orbits are re-
presented as circles,
though, in accord-
ance with the law
of gravitation, they
might be any va-
riety of similar
ellipses. Whatever
the relative masses
of the pair, and
whatever the degree of ellipticity of the orbits, the
general principle, however, remains unaffected. The
Solar System is imagined as lying far away upon
the right; and, from the fact that no parallax has
been detected in Algol, it follows that upon the scale
according to which the orbit of Algol is represented,
the distance of the Solar System must be, at the
least estimate, 5 miles. When at A, Algol will be
eclipsed by the interposition between it and the
Earth of its dark companion at c. From these
positions the star and its companion will proceed in
their orbital revolutions, moving in the directions
indicated by the arrows, their relative speeds being
determined by the condition that the pair must at
every instant lie upon opposite sides of the centre of
mass, the position of which remains unaffected by
their motion. It will be clear, therefore, that if the
28 Recent Advances in Astronomy.
hypothesis of the dark star's existence is sound, to
an observer upon the Earth provided with sufficiently
delicate means of observation, Algol should appear
to swing to and fro about the point o, attaining its
greatest displacement upon either side of it when at
A' and A". If, however, the orbit of Algol were even
to equal that of the Earth round the sun in magni-
tude, the consequent displacement in its position
would be so slight as to escape detection by the
most refined observational means existing, 1 and it
has, in fact, never been detected.
During the orbital revolution of Algol there is,
however, a relative displacement of another kind
between it and the Earth. In executing one half
of its orbit the star must continually approach the
Earth, while during the other half it must recede
from it. Assuming the orbits to be circles, the
star should approach the Earth in moving from
the position A in which it is eclipsed, the approach
becoming direct, and therefore most rapid at A',
a quarter period later; while at A" there should
exist an equally rapid and direct motion of recession.
It is in the detection of these alternate movements
of approach and recession that Vogel has achieved
one of the most remarkable triumphs of observa-
tional astronomy.
The immediate principle, to the successful appli-
cation of which Vogel's demonstration of the
motion of Algol is due, will be more fully explained
in a later chapter. It follows as a necessary con-
i This conclusion is directly involved in the statement that the parallax of
Algol is inappreciable by the most refined observational means existing.
The Life of a Star. 29
sequence of the wave theory of light that a source
of light approaching the observer should crowd
together and thus shorten its light-waves in front
of it, and in consequence alter the nature of the
light, raising its colour in the spectral series, that
is, causing it to approach the violet in hue, and
increasing its refrangibility. A movement of re-
cession should correspondingly draw out, and thus
lengthen, the light -waves travelling behind the
source and towards the observer, lowering the
colour towards the red of the spectrum, and de-
creasing the refrangibility. In 1842 Doppler had
suggested that the apparent colours of certain stars
might thus be affected by their movement, a rapidly
approaching star acquiring a bluish, and a rapidly
receding one a ruddy tinge. The suggestion,
however, failed, for various reasons; among them,
the fatal one, that, owing to the high speed of light,
such transcendent velocities as would be necessary
to produce such a change in the colour of a star
that should be appreciable to the eye would trans-
form the whole aspect of the heavens in a few years.
The true direction in which to search for a record
in the light of a star of indications of its approach
or recession was indicated by Fizeau in 1848, and
lies in the careful measurement of the positions of
the dark lines with which the spectra of the Sun
and of the greater number of the stars are ruled
throughout, the suggestiveness of which was at that
time beginning to be recognized. These dark lines
simply indicate colours absent in sunlight and star-
light, and the absent colours are affected by the
30 Recent Advances in Astronomy.
motion of the source precisely as are those actually
present. Consequently the approach of a star
should raise the colours absent in its light towards
the violet, and the dark lines in its spectrum should
therefore be displaced toward the violet end of the
spectrum; the reverse occurring in the case of a
receding star. In 1868 Sir William Huggins
succeeded in detecting slight displacements in the
spectral lines of certain stars, and, assigning the
displacements to this cause, estimated from them
the velocities of the stars in the direction of the
line of sight. It was this method that Picker-
ing suggested should be brought to bear upon
the problem of Algol and its hypothetical com-
panion.
In photographs of the spectrum of Algol, taken
at intervals during the years 1888 to 1891, the
movement of the star, and precisely such movement
as was demanded by the eclipse theory, was estab-
lished beyond doubt. When under eclipse, as
well as later by an interval equal to one-half of that
between successive eclipses, at which time the star
should be between its companion and the Earth,
the spectrum of its light should be normal, since
at these instants its motion should be directly across
the line of sight, and should neither be towards nor
from the observer. During the half-period preced-
ing eclipse the motion of the star should be from
the observer, and the spectral lines should be there-
fore displaced towards the red as the result of the
drawing out of the light-waves, while after eclipse
the motion of recession should be replaced by one
The Life of a Star. 31
of approach, and the spectral lines should be shifted
towards the violet. Every one of these predictions
was confirmed in Vogel's photographs. The maxi-
mum displacement of the lines, which occurred, as
they should have, at quarter-periods before and after
eclipse, indicated velocities of recession of 24*4,
and approach of 28-6 miles per second respectively,
the difference between the two values being natu-
rally explained upon the assumption that the speed
of Algol in its orbit is 26*5 miles per second, the
mean of the two, and that the system of Algol and
the companion is approaching the Solar System
with a speed of 2-1 miles per second. Knowing the
orbital speed of Algol, as well as its period of re-
volution, the interval between successive eclipses,
it is a simple matter to calculate the circumference,
and from it the radius, of its orbit. The final result
is almost exactly a million miles, from which it
follows that the oscillation of Algol across the line
of sight is far too small to be capable of detection.
The dark companion of Algol suggests the picture
of the death-stage of a sun. In nine other stars,
variation in light, in nature similar to that exhibited
by Algol, points strongly to a similar cause. In
another star, Spica, the existence of an invisible
companion is indicated by the displacement of
spectral lines, though no eclipse results, probably
from the plane of the orbits making a sufficiently
large angle with the line of sight for the dark star
to clear the bright one at each revolution.
In the few cases in which the existence of dark stars
has been revealed, their detection has been due to
32 Recent Advances in Astronomy.
the fact of their close association with bright stars.
Only by an inconceivably remote chance would it
be possible to become aware of the existence of an
isolated dark star by either of the methods that
have been so successfully applied to the companion
of Algol. Such knowledge of stellar distances as
we possess renders it probable that the suns of
space are separated from their nearest neighbours
by depths so vast that were there dark stars scat-
tered at random among them exceeding the bright
ones by many times in number, the probability of
one of them approaching so near to a visible star
as to sensibly affect it by gravitation would be
excessively remote, 1 while, since it is not possible
to continually examine more than an insignificant
minority of the visible stars, either as regards
position upon the face of the sky, by change of
which motion across the line of sight would be
apparent, or spectroscopically, by which motion in
the line of sight might be revealed, it is probable
that millions of near approaches between isolated
dark stars and brilliant ones would occur before the
effect of one would be detected.
The probability of becoming aware of the exist-
ence of a dark star by its drifting by chance across
the line of sight directed from the Earth toward a
more distant brilliant one appears equally remote.
It is impossible to contemplate even the most
crowded regions of the heavens through a telescope
of fine quality, and of large light-grasping power,
1 With the exception possibly of the more crowded regions of the Milky
Way.
The Life of a Star. 33
without recognizing that among the myriads of
bright points scattered over the field of view there
is ample room for the existence of dark stars far
exceeding them in number. The brighter only
among the stars appear in the telescope as discs
of sensible dimensions ; but the reader is probably
aware that such " spurious" discs result from imper-
fections in the eye, and from the inherent principles
of telescopic construction. Were the telescope
and the eye alike perfect, such is the stupendous
remoteness of the stars, that, although suns, the
nearest of them, even if far exceeding our Sun in
magnitude, would appear under the highest magni-
fying power that has so far been applied to them, as
mere specks of light devoid of sensible dimension.
Although the apparent dimensions of stars are
far beyond the possibility of detection with the
most perfect optical means, it is, however, possible
to make a rough estimate of the extent of sky
covered by individual stars or by the whole collec-
tion. The possibility of effecting this is based
upon the fact that the apparent brightness of any
surface is independent of its remoteness. If, for
instance, the Sun were removed to three times its
present distance, the light received from it would,
according to the law of inverse squares, be reduced
to one-ninth of its present value; but since its
apparent size or the area in the sky covered by
it would similarly be reduced to one -ninth, the
apparent brightness of its surface would remain
unchanged. If, therefore, there were a number
of sun-like bodies in space, each of the same in-
(M520) C
34 Recent Advances in Astronomy.
trinsic brilliancy as the Sun, but differing both in
size and distance from the Earth, it would follow
that, since the apparent surface brightness of all
would be the same, the amount of light received
from any one of them would be in direct proportion
to its apparent size, or to the sky-surface covered
by it.
The total amount of light received from all the
stars above 9^ magnitude (visibility to the naked
eye terminates at the 6th magnitude) has been esti-
mated by Mr. Plummer. The result, slightly
modified in accordance with more recent measure-
ments of the brightness of Sirius, far the most
important star of the whole, is given by Miss Clerke
as one-eightieth of that of the Full Moon. The ratio
of the light of the Sun to that of the Full Moon
has been estimated by Zollner as 619,000 to i, so
that the Sun exceeds the total of the stars above
gtf magnitude in their illumination of the Earth
by 80 times 619,000, or nearly 50 million times.
If, therefore, the assumption be made that the
surfaces of the stars are of the same intrinsic
brilliancy as the Sun, it follows that the stars cover
a portion of the sky equal to one 5o-millionth of
that covered by the Sun ; and since the Sun covers
but one 2io,oooth of the total of the sky, it follows
that the stars would cover rather less than one 10-
billionth.
It was necessary to limit the above estimate to
the 324,000 stars above the g% magnitude as there
is no means of determining the light received from
the fainter ones. These 324,000 stars, however, far
The Life of a Star. 35
more than include those among which there could
be any hope of detecting an eclipse by an isolated
dark star.
Should dark stars far exceeding the bright ones
in number exist in celestial space, an eclipse of one
of the latter would therefore be a phenomenon of
rare occurrence ; while, should an eclipse occur, so
few are the stars the brightness of which is sub-
jected to continual scrutiny that the probability of
its passing unnoticed is overwhelming.
The possibility of an unseen system of stars per-
meating the seen is beyond doubt. The system of
the seen is indeed sufficient to satisfy the highest
ambition of imagination, but he would be bold who
should assert that it may not well form but an
insignificant fraction of a still more surpassingly
transcendent whole.
The question as to the nature of the changes now
taking place in the Sun is one of very great interest,
and its study involves physical considerations of
high importance and not free from grave difficulty.
The delicate and mottled tracery visible under
the most favourable conditions for telescopic obser-
vation over the entire surface of the Sun, is strongly
suggestive of the view that its bright surface
generally known as the photosphere consists of
an accumulation of incandescent clouds. Such a
cloudy structure of the photosphere is in harmony
with the general results of solar observation, more
especially, perhaps, with the nature and the rapidity
of the changes frequently characteristic of sun spots,
which, according to this view, are either depressions
36 Recent Advances in Astronomy.
or actual gaps in the photosphere. The spectro-
scope indicates as existing above the level of the
photosphere a solar atmosphere, as constituents of
which the vapours of hydrogen, calcium, iron, and
other metals are conspicuous, and in which are
traceable with greater difficulty those of a few of
the non- metallic elements. It is not difficult to
imagine the process of formation of the clouds of
the photosphere from the precipitation as fog of the
more readily condensable of these vapours by their
cooling consequent upon their being carried into
the upper regions of the atmosphere.
The question as to the physical conditions exist-
ing in the interior of the Sun is attended with
graver difficulty, and is of the first importance in
the problem under consideration. Herschel im-
agined as existing beneath the clouds of the photo-
sphere, a solid globe ; and even advanced the view,
so preposterous to modern students of physical
science, that it might, from the protection of a
second and intervening cloud shell cool and im-
pervious to heat radiation, be protected from the
intense glare of the photosphere above to such an
extent as to be a cool and habitable world. When
the necessity for the interior heat of the Sun being
at least as high as that of its exterior became recog-
nized, the solid globe was generally replaced by an
ocean of molten matter.
It is, however, scarcely possible to regard as
existing in the interior of the Sun, matter in either
the solid or in the liquid condition. The tempera-
ture above the photosphere is such that iron, car-
The Life of a Star. 37
bon, and other among 1 the most refractory of ele-
ments known to terrestrial chemistry are found in it
in the gaseous state; and the temperature of these
external regions must be far lower than that of the
interior. It was for a time regarded as barely pos-
sible that the enormous pressure that must exist at
great depths in the interior of the Sun might be
effective in maintaining matter in the solid or liquid
condition in spite of the high temperature, since it
is a familiar fact in laboratory experience, that lique-
faction of a gas is in every case assisted by pressure,
and may in many instances apparently be effected by
it alone. Since, however, it became apparent from
the classical researches of Dr. Andrews in 1869,
that there exists for every element a critical tempera-
ture, above which it is impossible for it under any
conditions of pressure to assume the liquid state, it
has generally been regarded that a liquid interior
to the Sun is next to an impossibility. The Sun is,
in all probability, essentially an enormous bubble,
enveloped in incandescent cloud, from which, by
the mechanism of radiation, its energy is transmitted
into external space.
From the fact that the degradation of a star from
its earliest nebular to its dark state is the direct
consequence of the radiation of its heat into space;
and as, in ordinary experience, loss of heat is ac-
companied by fall in temperature, it has frequently
been assumed that the life of a star must be the
record of continual fall in temperature ; and that in
the nebulae would be found the highest temperatures
represented in celestial bodies. To the "fiery mist"
429055
38 Recent Advances in Astronomy.
from which Laplace, in 1796, had imagined the
development of the system of the Sun and planets,
a temperature was assigned far higher than that of
the Sun at present; and the same view was extended
to the nebulas, when the demonstration of their
gaseous nature had indicated them as fulfilling in
the Cosmos the functions of embryonic stars.
Recent considerations based upon the experimen-
tally ascertained properties of gases and upon the
principle of conservation of energy have, however,
shown that this simple view cannot be maintained.
Attention has already been directed to the fact that
each step in the radiation of heat from the Sun brings
about a shrinkage of its bulk, or, more exactly,
enables the gravitation of its parts to draw them
closer together, and that by this act of compression
other heat is developed. In a very remarkable
paper, published in 1870, Mr. Homer Lane has
shown that if the Sun were entirely gaseous, and if
the gases composing it were under such physical
conditions that the laws of " perfect gases" should
be applicable to them, the heat developed by shrink-
age must not merely equal but must so far exceed
that radiated to effect it, that the temperature of the
whole must actually rise in consequence, and must
continue to do so for so long as a perfectly gaseous
condition is maintained.
A "perfect gas" is defined as one in which, for
so long as its temperature is unchanged, any increase
in pressure brings about a proportionate decrease
in volume. This condition, known as "Boyle's
Law ", is very closely fulfilled by hydrogen, oxygen,
The Life of a Star. 39
and nitrogen, as well as by gases in general when
under conditions far removed from those under
which they assume the liquid condition, so long as
their density is not rendered excessive by intense
pressure. Under extreme pressure, however, de-
crease in volume becomes increasingly less than
that demanded in Boyle's Law, and it is probable
that for every gas at a given temperature there is a
limiting volume beyond which it cannot be com-
pressed by any pressure however great.
The statement of Lane's theorem that it is pos-
sible under certain conditions for a body to rise in
temperature as the result of its loss of heat appears
at first so contrary to common experience that there
is generally great difficulty in thoroughly accepting
it. That emission of heat is not inseparably asso-
ciated with fall of temperature, will, however, be
clear from the consideration of such instances as are
supplied by the condensation of a vapour and the
solidification of a liquid. The passage of steam
into water at its boiling-point is unaccompanied by
any fall in temperature, though the amount of heat
given out is more than five times that necessary to
raise the temperature of the water from its freezing-
point to its boiling-point. Similarly, the freezing of
water is unaccompanied by any fall in temperature,
though here again a large amount of heat is emitted
by the solidifying water four-fifths of that required
to raise the water from its freezing- to its boiling-
point. In a mass of gas subject to no external force,
development of heat results from its compression
under forces due to the gravitation of its parts; it is
4 o Recent Advances in Astronomy.
loss of heat, not fall in temperature, that enables the
gravitational forces to become effective in producing
compression.
In the hope of assisting the reader towards form-
ing a clear picture of one of the most remarkable of
natural processes, a confessedly incomplete demon-
stration of Lane's theorem is given in the following
paragraph, which may be omitted if the mechanical
and geometrical principles involved should not
appear sufficiently familiar. An essentially similar
demonstration, by which indeed the one given here
was suggested, is given in Newcomb's Astronomy.
Let a globe of a " perfect" gas be imagined,
temperature being uniform throughout it, and let
the whole be at rest, free from internal currents, and
subject only to its own gravitation. All portions of
the globe are attracted toward the centre, and a
pressure is produced thereby that continually in-
creases toward the centre. According to Boyle's
Law the density must similarly increase toward the
centre. Let the whole globe be imagined as con-
sisting of a number of concentric spherical shells,
each enveloping those within it in the manner sug-
gested by the coats of an onion, and let attention be
directed to one of these shells. The total pressure
of the gas comprising the shell is due to the weight
that is, the gravitation toward the centre of the
portion of the gas outside of it, and this pressure is
distributed over the outer surface of the shell. Now
imagine the globe to lose heat by radiation, and to
shrink in consequence until its radius has become
reduced by one-half. If the process occurs so
The Life of a Star. 41
gradually that the temperature changes uniformly
throughout the whole, all portions will shrink
equally, the radius of the shell will be reduced to
one-half, and therefore, by elementary geometry,
its surface will be one-fourth and its volume one-
eighth of their former values. The distance of
every part of the globe from the centre will be
halved and the attraction of each portion to the
centre will, since gravitation is inversely propor-
tional to the square of the distance, therefore be
increased fourfold. The whole weight of the portion
of the globe that lies beyond the shell will therefore
be increased fourfold, but, as this weight is now
distributed over one-fourth of the former surface, the
intensity of the pressure will be increased to sixteen
times its former value. Such an increase in the
intensity of the pressure would, if the temperature
of the shell had remained unchanged, compress the
gas in it to one-sixteenth of its former value. It
has, however, been shown that the gas occupies
one-eighth of its former volume, double the volume
that it should occupy had the temperature remained
unchanged. Such an excess in volume can only
be due to increased temperature, and its tempera-
ture must consequently have risen.
It is scarcely necessary to add that a shrinkage of
the radius to one-half is assumed only for the sake
of simplicity; the same result a necessary rise in
temperature would follow from the assumption of
any given contraction.
If, then, the Sun behaves as a perfect gas, its
temperature must be increasing as the indirect
42 Recent Advances in Astronomy.
result of the torrent of heat continually radiated
into external space. There is good reason to re-
gard it as probable that the Sun is in the main
gaseous, but it would be rash to assume that under
the extreme conditions of pressure and temperature
existing in its interior, the laws of perfect gases are
fulfilled by it even approximately. The properties
of gases become markedly modified even at such
moderately high temperatures and pressures as it is
possible to produce in the laboratory, and in such a
manner as to suggest that could the matter of the
interior of the Sun be subjected to examination,
although it would prove to be neither solid nor
liquid, it would be difficult to trace in it the gaseous
characteristics with which we are familiar. It is
therefore impossible to decide the interesting point
whether the Sun is at present rising or falling in
temperature, though there can be little doubt that
in the remote past, when far more tenuous, its tem-
perature must have been lower than it is at the
present time.
Whatever the present trend of the temperature
of the Sun, it is, to say the least, unnecessary to
assume a high temperature for the nebula from
which it has been derived. Imagining a nebula
from which a single star is to be evolved as a com-
paratively cool diffuse extension of gas, of so low a
temperature and of so great a tenuity that it should
obey the laws of perfect gases, not necessarily suffi-
ciently hot for the whole of its constituents to exist
in it in the gaseous condition, but possibly embrac-
ing them in its volume as discrete solid or liquid
The Life of a Star. 43
particles, it becomes possible to take a rough fore-
cast of its future career. Under the influence of
the gravitational attraction upon each other of all
its parts, it would tend to acquire a spherical form.
Heat would pass into space by radiation ; gravita-
tion would in consequence be enabled to draw its
parts closer together; the temperature would rise,
and portions of its solid or liquid ingredients would
become gas. 1 The process would continue, and
after a time the nebula would become in the main
gaseous. At some period excessive local cooling
in the outermost parts would cause condensation
there and a photospheric cloud shell would be
formed. The nebula has now become a sun. For
a time its temperature continues to rise and its
radiation becomes more and more effective. At
length, however, possibly owing to excessive con-
densation in the photosphere, possibly to tempera-
ture and density increasing in the interior to such
an extent that the gaseous laws are widely trans-
gressed, the rise in temperature ceases and is soon
replaced by a fall. The Sun has passed the zenith
of its career and is now descending towards extinc-
tion ; a few more ages and its radiant activity has
ceased to be.
The question whether any evidence is supplied
by stars as to the course they have run from their
x No doubt heat would also be developed from collisions between the
non-gaseous constituents of the nebula, since these would be in motion
under the influence of gravitation. It might even be that the first evidence
of luminosity in the whole might be due to the generation of heat by these
collisions, the gas itself being generally below the temperature of incandes-
cence, as is suggested in Sir Norman Lockyer's Meteoritic Hvpothesis.
44 Recent Advances in Astronomy.
nebulous condition whether among those visible it
is possible to recognize individuals in the early
period of their career, others in the meridian of
their glory, and others again upon the descending
path towards extinction is among the most fascinat-
ing of the speculations of modern astronomy. It is
generally regarded that such evidence is indicated
in the spectroscopic analysis of their light, but it
must be confessed that this branch of scientific
inquiry can hardly as yet be regarded as having
passed beyond the speculative stage. From it we
may hope, perhaps, in the future, to be able to
decide whether our Sun is increasing in splendour,
or whether he has passed the period of his greatest
glory. Here it may be permissible to add that, in
the judgment of the writer, the evidence, though
not free from serious difficulty in its interpretation,
appears to indicate the former as the more probably
true hypothesis, and that in the remote future it is
not inconceivable that radiations of the Sun should
rival even those of Sirius at the present time.
Should this be so, the maximum of vitality of a
star must be thrown far forward in its life history,
and the duration of its decay must be correspond-
ingly brief.
The physical universe is inexpressibly glorious;
and it is scarcely possible that the contemplation of
the decay of its activity should be unaccompanied
by a touch of sadness. One is, therefore, led to
inquire, whether among the processes of nature no
means are indicated by which its lost energy may
be restored to a dead star. So far as the working
The Life of a Star. 45
of nature is revealed in the laws of physical science,
the only way in which a star can re-assume its
nebulous condition is by a collision between it and
another, by which encounter the whole or part of
the total energy of motion of the pair would be
transformed into heat. The establishment of the
equivalence between heat and motion, one of the
noblest achievements of modern science, is now a
familiar fact to everyone. By the destruction of
motion heat is generated; the amount of heat is
directly related to the masses and velocities of the
moving matter and can be readily calculated from
them ; while, in its turn, the heat itself may under
suitable conditions disappear, and in so doing re-
generate motion identical in amount with the quan-
tity that passed out of existence in the act of heat
creation. 1
That many stars are moving relatively to each
other is a matter of ready demonstration by observa-
tions of their positions upon the sky, with the instru-
ments of refinement now in use, at intervals of a few
years. Their movement may be, and commonly is,
so apparently insignificant, that centuries must
elapse before their displacement would be apparent
to the unaided eye; but, upon allowing for their
excessive remoteness, speeds are revealed, many
i To the term motion, a somewhat vague one as used generally, science
applies a definite meaning, the product of mass into velocity. The function
of a moving body that is in direct proportion to the heat developed in the
alteration of its speed is, however, not this quantity, but the product of its
mass into the square of its velocity, a quantity to which the term vis viva
was formerly applied. One-half of the vis viva, which is of course also
proportional to the heat equivalent, is known in modern mechanics as
kinetic energy and is of great importance.
46 Recent Advances in Astronomy.
comparable with, and some far greater than, those
of the planets in their orbits. Sirius drifts over the
face of the sky with such speed that in 1400 years
its position will be removed from its present one by
a distance that would just be covered by the diameter
of the Full Moon. From the known distance of the
star it is a simple calculation that to do this it must
travel athwart the direction of vision with a speed
of over ten miles per second, more than one-half of
that of the Earth in its orbit; and this takes no
account of any velocity the star may possess in the
direction of the line of vision, a displacement in
which direction would obviously not affect its
position upon the face of the heavens. The parallax
of Arcturus is inappreciable, from which it appears
improbable that its distance can be less than
4,000,000 times that of the Sun ; thus remote, the
drift of the star, by which it would be carried across
the diameter of the Full Moon in 700 years,
must represent a velocity of at least 130 miles per
second across the line of sight; the actual speed
in this direction being greater than this, in direct
proportion as the actual distance of the star exceeds
the minimum limit that is here assigned to it.
From similar considerations it appears, that in the
case of a remarkable star in the Great Bear invisible
to the naked eye, and known as Groombridge 1830,
from the number assigned to it in Groombridge's
catalogue, the speed by which the star would be
carried in 257 years over such a portion of the
heavens as would be covered by the Moon, the
most rapid displacement known, must at the dis-
The Life of a Star. 47
tance of the star of 2,300,000 times that of the
Sun, indicate a continual rush across the line of
sight of 227 miles per second.
The velocity of the Sun relatively to the stars, or,
more definitely, the velocity of the Sun relatively to
the mean positions of the stars, a quantity com-
monly alluded to as "the velocity of the Sun in
space ", an expression almost humorously devoid
of meaning, can be estimated from an accumulation
of such results as have been here illustrated. The
problem was first attacked by Sir William Herschel,
and has ever since been a favourite matter of re-
search of astronomers, who have been enabled to
introduce increasing refinements as more and more
data have become available. All the methods that
have been applied consist essentially of the deter-
mination of the average velocities of the stars, that
is, the determination of the velocity of the average
position of the stars relatively to the Sun, that of
the Sun relatively to the mean position of the stars
being equal and opposite to this. The outcome of
such investigations seems to indicate that the Sun
is travelling in a line directed very nearly towards
the brilliant star Vega, and that its velocity in this
direction is probably between 12 and 18 miles per
second. There is no doubt that the result as
regards direction is far more definite and accurate
than that as regards speed.
In the host of the "fixed stars" is found abun-
dance of motion, and that upon the most stupendous
scale. A century ago it was fondly hoped that the
movements of the stars might turn out to be of the
48 Recent Advances in Astronomy.
orderly and permanent character revealed in the
Solar System, and search was made for a colossal
Sun, that should by its gravitational attraction
control the whole. Sirius was suggested by Kant,
other stars took its place in succession, and in 1846
Madler, abolishing the conception of a central Sun,
imagined that every member of the stellar host
might describe an orbit about a centre, placed by
him in the Pleiades, the controlling power being,
not the overpowering attraction of one, but the com-
bined influence of all. As the motions of the stars
became more closely followed, it became clear that
the hope of revealed order was not destined to be
realized. System remains unrevealed in their move-
ments, and the stars appear to rush in random
directions through space.
The problem before us then is, whether in their
undirected career stars may not from time to time
come into collision. Were the Earth in its orbital
speed to meet in direct impact another planet, equal
to it in mass and travelling with an equal speed
in the opposite direction, and were the planets
to escape being shattered into fragments by the
impact, heat would be developed from the destruc-
tion of their motion sufficient in quantity to convert
both into a cloud of gas, 1 and it is conceivable that
i The collision between two solid planets might result in the shattering of
considerable portions of them into fragments, and in the fragments being
projected into space with high velocities. The motion retained by these
fragments would, of course, escape being converted into heat. In the case
of stars that had not cooled so far as to reach the solid condition, such
shattering would be less probable. See a paper by Lord Kelvin, Popular
Lectures and Addresses, vol. i. p. 366.
The Life of a Star. 49
a like result might arise from collision between
stars. From the insignificant dimensions of the
visible stars in comparison with the celestial spaces
in which they have their being, the chance against
a collision, even in geological ages, is perhaps
excessively remote; but in indefinitely prolonged
time collision appears certain. It must be remem-
bered, in addition, that the stars that are seen may
well be but a small fraction of the whole system;
and with each addition of dark suns the probability
of collision becomes more than proportionately
greater.
Regarding, however, the rejuvenescence of a star
by collision as possible, the last catastrophe is but
projected forward by a finite time. At each collision
the coalescence of a pair of cosmic masses will
reduce the existing number by one; while energy
of heat is gained at the expense of energy of motion.
As ason succeeds ason, and as new nebulas follow
those from the ruins of which they were formed into
extinction, the Universe becomes poorer in active
energy; and there appears, so far as physical
science has interpreted the processes of nature, no
escape from the picture of an accumulation of inert
matter as the last memorial of a glorious Universe
of Suns.
(M520)
50 Recent Advances in Astronomy.
Appendix to Chapter I.
The Measurement of Stellar Distances.
Bessel's discovery of stellar parallax, a discovery
that directly demonstrated the fact of the Earth's
annual motion round the Sun, has been generally
regarded as the first direct proof of the truth of
the Copernican System of Astronomy; though with-
out doubt a very strong case for priority in this
respect might be made out for the detection by
Bradley, rather more than a century previously, of
the aberration of light. In any case, however,
Bessel's achievement removed the last and a very
serious objection to the Copernican Hypothesis
however firmly established, and has rendered it in
every respect unassailable. The discovery itself
must take high rank among the greatest triumphs
of observation. The mere detection of so minute
an angle as even the relatively large parallax of
61 Cygni still necessitates instrumental means of
extreme refinement, as well as very great observa-
tional skill. Two lines inclined at an angle of a
second of arc would approach by no more than
i inch in a distance of 3^ miles, and the inclina-
tion, not only detected by Bessel, but measured
with considerable accuracy, was but a fraction
of this. If the smallness of the angles con-
cerned were the only difficulty in observations of
stellar parallax, its detection would be no mean
feat; but the observations are affected by numerous
The Measurement of Stellar Distances. 51
sources of error, the elimination of which involves
the utmost perseverance. The necessary observa-
tions must, of course, be made at widely separated
times of the year, and difference of temperature not
unfrequently causes change in the form and the
position of the observing telescope, that would, if
not taken into account, completely conceal the
insignificant angle of parallax by simply over-
whelming it. From a principle similar to that by
which the rain-drops of a falling shower appear to
slant towards a moving passenger to an extent
dependent upon the rapidity of his motion, light
rays arriving from a star appear to an observer
upon the moving Earth as he is carried by it in
its orbital rush across their streams to slant from
the direction of the Earth's motion, and the star
appears, in consequence, to be displaced toward
that point in the heavens to which the Earth's
motion is at the time directed. The phenomenon
is known as " aberration of light"; it was indeed
discovered by Bradley in 1725 during an unsuc-
cessful attempt to detect the parallax of a star, and
the displacement in the apparent position of a star
due to it varies from nothing to 20 seconds of arc
according to the direction of the Earth's motion
with respect to that of the star. In estimating the
true direction of a star from its apparent place in
the sky it is obviously necessary to take the most
careful account of the aberration of light.
The apparent position of a star is also seriously
affected by refraction. In accordance with the
general fact that a ray of light is deflected, or
52 Recent Advances in Astronomy.
refracted, in passing from a medium into another
differing from it in density, the refraction being to-
wards the perpendicular to the separating surface as
the ray passes from a rarer into a denser medium,
the rays from a star, after following a straight course
in external space, are deflected downwards on enter-
ing the atmosphere, and as the air continually in-
creases in density as the surface of the Earth is
approached, the deflection continually increases, so
that the ray reaches an observer after executing a
curve in the atmosphere, and the apparent direction
of the star, determined by the direction of the ray
on entering the eye, is sensibly different from its
true direction. Unlike the interference caused by
aberration, which may be corrected from an exact
knowledge of the speed of the Earth relatively to
that of light, the error due to refraction is incapable
of exact determination, since the curvature of the
ray is dependent upon the density, temperature,
and degree of moisture of the air, not only in the
observatory, but throughout the whole of its atmos-
pheric path, which is quite beyond the reach of
observation.
With a view of minimizing, and avoiding as far
as possible, these and certain other sources of error,
Bessel adopted in his observations upon 61 Cygni
a method originally proposed by Galileo, and
known as that of " relative parallaxes". In it no
attempt was made to determine the absolute direc-
tion of the star with exactitude, but its direction was
determined with a very high degree of accuracy
with reference to a neighbouring " reference star",
The Measurement of Stellar Distances. 53
and the observations were repeated at different
times of the year. The important assumption was
then made, and its justification will be examined
presently, that the reference star was so extremely
remote that the direction in which it was seen was
not appreciably af-
fected by the Earth's ,
movement in other / / /
words, that it pos- / I'' /
sessed no parallax ;' // /
that could be detected / /' / /
its apparent proxi- / /
mity to the star under / /
observation being
merely the result of
its lying by chance
nearly in the same
direction. The prin-
ciple underlying the
application of the ob-
Servations will be clear Fig. 3. Relative Parallax.
from the diagram of
fig. 3. Let the two parallel lines PC and QD represent
the lines of sight from the earth in its two positions P
and Q to the reference star, assumed to be so remote
that there is no inclination between them capable of
detection. Then, the Earth being at P, let the direc-
tion of the star under examination be observed with
reference to the reference star, by measuring the
angle between them. Setting off this angle in the
figure as CPA, the direction of the star is determined
by the straight line PA. Six months later, the
54 Recent Advances in Astronomy.
Earth being at Q, a similar observation may be
made ; if the angle separating the stars be observed,
and set off as DQB, the direction of the star is now
indicated by the straight line QB. The inclination
of PA and QB determine the position of the star at
their meeting point at s. It need scarcely be re-
marked that in actual practice the position and
distance of the star are determined by trigonometrical
calculations based upon the observed angles, and
not from graphical construction such as has been
here introduced to illustrate the principle involved.
The instrument that has so far been found best
adapted to the measurement of the required angles
is known as the heliometer, from the fact of its
having been originally designed to determine the
angular measure of the Sun's diameter; and it was
with such an instrument, constructed by the cele-
brated optician Fraunhofer of Munich, that Bessel's
observations were made. The heliometer is a
telescope, the object-glass of which is cut into two
along a diameter. With the two halves in their
normal positions each may be regarded as giving
a separate image of an object towards which the
instrument is directed, but the pair of images
coincide, and under these conditions the heliometer
is equivalent to an ordinary telescope. If, however,
one of the halves is displaced by sliding it along
the line dividing the pair, the image formed by
it is equally displaced, and the observer at the
common eye-piece sees all objects in the field of
view doubled, as if viewed through a crystal of
Iceland-spar. In practice one half of the object-
The Measurement of Stellar Distances. 55
glass is displaced until the image of the star under
observation for parallax formed by it coincides with
the image of the reference star formed by the other.
The amount of displacement between the two halves
of the object-glass then indicates the angle between
the directions of the stars.
The advantage, as well as the one grave dis-
advantage, inherent in the method of relative
parallaxes will be clear upon consideration. The
angles CPA and DQB are not appreciably affected by
aberration, since, the pair of stars under observation
lying in nearly the same direction in space, their
rays will traverse closely coincident paths, and the
apparent change in their direction caused by the
Earth's rushing across the streams of light rays will
be therefore nearly the same for each. For a similar
reason, error due to refraction is practically eli-
minated, since the rays from the two stars traverse
nearly the same column of atmosphere, and are
therefore refracted by it almost equally.
It is important to notice that it is not necessary
to know the absolute directions of the stars with
great accuracy. So long as PC and QD, the lines of
sight to the reference star, may be regarded as
parallel, a displacement of the pair of them to and
fro even through several degrees makes quite an
insignificant change in the estimated distance of the
star at s. It would be instructive to verify this by
repeating the construction for directions of these
lines slightly different to those in the figure, being
careful to represent them as parallel in every case,
and to retain the exact values of the angles CPA
56 Recent Advances in Astronomy.
and DQB, which are those determined by the helio-
meter.
The one serious and obvious objection to the
method of relative parallaxes lies in the necessity
for assuming that the reference star is so remote
that its parallax is inappreciable. It will be readily
seen, by modifying the construction of the figure,
that if the lines of sight to the reference star are
inclined to each other, the distance of the star s will
be underestimated. The justification for the method
may perhaps be stated in some such way as the
following. The enormous majority of the stars
show no parallax relatively to each other that can
be detected. This must arise, either from all such
stars being so immensely remote that their paral-
laxes are represented by angles so small as to be
beyond the power of appreciation even by the helio-
meter; or, if the stars are so near the Earth that
these angles are appreciable, from their all being
equally remote, so that all experience equal apparent
displacements when regarded from different points
of the Earth's orbit. There can be no hesitation in
regarding the first alternative as overwhelmingly
probable. There is, therefore, a great probability
that any given star selected is sufficiently remote
for its parallax to be ignored in its use as a refer-
ence star; and if concordant results are obtained
from the employment of two reference stars, which
should always be the case for the work to inspire
confidence, the probability of the soundness of the
assumption becomes overwhelming.
Before the application by Bessel of the method of
The Measurement of Stellar Distances. 57
relative parallaxes, the attempt had generally been
made to determine the absolute direction of the star
under observation for parallax by recording its
position upon the face of the heavens without the
assistance of reference stars. The position of the
star was determined by the observation of its "right
ascension" and " declination", celestial quantities
quite analogous to longitude and latitude upon the
surface of the Earth. Such a method is known as
that of "absolute parallaxes", and it is preferable to
the relative method in that it does not involve the
aid of reference stars. Owing, however, to observa-
tions by the absolute method being affected to the
fullest extent by refraction, aberration, and other
troubles, as well as from its involving the measure-
ment of large angles, which are far more difficult
of determination within the same limits of absolute
error than small ones, the possibility of its success-
ful application, even to the mere detection of paral-
lax, in any given case, is so extremely remote that
it has now been universally rejected. It is true,
Henderson's discovery of the large parallax of a
Centauri was effected by the absolute method, but
success could be scarcely anticipated with paral-
laxes of much smaller value.
Bessel's success was therefore largely due to the
adoption by him of the method of relative parallaxes,
as well as to the fact of his being in command of a
fine heliometer. It was also largely due to the
judicious selection of a star for examination. Pre-
vious to Bessel's measurements there was reason for
regarding is as probable that 61 Cygni was one of
58 Recent Advances in Astronomy.
the nearest of the stars, and that it therefore pos-
sessed a relatively large parallax. In appearance
one of the most insignificant and unattractive of the
stars, attention had been for some time directed to
61 Cygni by reason of its rapid drift across the face
of the sky. So great is this " proper motion" in its
case that in 350 years the star would traverse a line
in the heavens equal to that covered by the diameter
of the Full Moon, a rapidity of movement exceeded,
so far as is known, only by two other stars. Since,
other conditions remaining the same, the nearer a
moving star the more rapidly would it appear to
drift across the sky, it is evidently probable that
the more rapidly drifting stars are as a class nearer
than the others; hence in the search after the
parallaxes of stars special attention has been
directed to them, and 61 Cygni specially attracted
the attention of Bessel. Another criterion of pro-
bable nearness is supplied by brightness, it being
a priori probable that the brighter stars are nearer
the Earth than fainter, and for this reason special
attention has also been devoted to them. It is inter-
esting to notice that in the result rapidity of motion
has proved a far more favourable omen of success
in the search after parallax than great brilliancy.
The Milky Way and Star Distribution. 59
Chapter II.
The Milky Way and the Distribution of Stars.
Among the many and profound problems sug-
gested to the mind by the contemplation of the
heavens upon a clear, moonless night, there is no
one more mysterious, and few have proved more
baffling, than that presented by the dimly-luminous
arch of the Milky Way. Variously regarded in
classical mythology as the milk that flowed from
the sacred breast of Juno; as the last vestige of
the ruin that overwhelmed Phaeton in his bold but
fatal attempt to direct the fiery steeds of the Sun's
chariot; and as the road along which the gods
repaired to High Olympus; the fair shimmer of
the Milky Way has through succeeding ages been
associated with poetic fancy and romantic imagina-
tion. In modern German the popular term "Jacob-
strasse" recalls not unfitly the sublime vision of the
patriarch of Israel ; while to the Indian of North
America the Milky Way is still the path of departed
souls, and the brighter stars that stud its stream are
the camp-fires that mark the halting-places of his
fathers upon their weary march.
From a very early date there have been recorded
speculations of a more or less scientific nature re-
garding the Milky Way. Pythagoras is recorded
to have formed the shrewd conjecture that its faint
shimmer was due to the accumulated light of multi-
tudes of faint stars. Anaxagoras maintained the
60 Recent Advances in Astronomy.
view that the appearance might be caused by the
projection into space of the shadow of the Earth ;
while Aristotle regarded it as a mist formed by the
exhalation of terrestrial vapours. That it was a
ring of nebulous matter in external space encircling
the Earth appeared the probably correct solution
to both Tycho Brahe and John Kepler toward the
close of the sixteenth century ; but a few years later
its true character was revealed in Galileo's telescope,
and the speculation of Pythagoras was confirmed in
the discovery that its haze is indeed the combined
shimmer of hosts of stars, each one too faint by
itself to be distinguished by the unaided eye.
Seen under the most favourable conditions from
these latitudes, the Milky Way appears as a semi-
circular arch of light spanning the starlit sky.
The appearance suggests that what is seen is the
visible half of a complete zone encircling the
heavens, the other half being at the time of obser-
vation in the celestial hemisphere that is hidden
by the solid Earth under foot. On constructing a
complete map of the Celestial Sphere, and tracing
the course of the Milky Way upon it, it is seen that
this view is roughly correct; but the entire stream
departs from a simple zone-like character in several
respects. For about two-thirds of its circuit of the
heavens, the Milky Way, though irregular, appears
as an unbroken stream. In the constellation of the
Swan, however, it bifurcates, the divided branches,
after following appreciably parallel tracts for about
one hundred degrees, reuniting in the constellation
of the Centaur. The division in the Swan is excel-
The Milky Way and Star Distribution. 61
lently situated for observation from these latitudes,
from which, however, the reunion in the Centaur
is, owing to its proximity to the South Pole of the
heavens, permanently invisible. The luminosity of
the more northerly of the branches of the divided
stream fades at a short distance from the bifurcation
in the Swan, and, indeed, ceases for some distance,
while it is very remarkable that this fading is
accompanied by an increased brilliance in the
southern branch. That the branches, though separ-
ated, are not physically independent is indicated
by the existence of imperfect bridges of luminous
matter between them, while in several points the
more southern and brighter branch throws off com-
paratively brilliant projections toward its companion,
which projections, however, terminate before reach-
ing it.
A few degrees from the permanent reunion of its
divided streams, and upon entering the constellation
of the Southern Cross, the course of the Milky Way
expands into a brilliant and well-defined cloud of
stars, while, in the centre of the cloud, closely bor-
dering upon the four bright stars of the Cross, is
the dark pear-shaped lake popularly known as the
" Coal Sack ". The " Coal Sack " is one of a great
number of similar irregularities in the Milky Way,
though in no other is the passage from extreme
richness in stars to almost total vacuity so sudden.
The appearance of a dark void, unthinkable in ex-
tent, in the midst of a cloud resplendent with the
light of tens of thousands of suns, is indeed one of
the most impressive features presented by the system
62 Recent Advances in Astronomy.
of the stars, and it is scarcely remarkable that no
explanation of it has been advanced that it is not
easy to refute upon the most elementary grounds.
A third and no less remarkable feature of the
Milky Way is presented in the same region of the
heavens. Following the course of the Milky Way
past the Coal Sack, its stream contracts, becoming
almost at once reduced to little more than a narrow
neck of light. Beyond this, however, it as rapidly
widens, and a few degrees farther on, in the con-
stellation of Argo, it is broken clear across by a
dark chasm. In Sir John Herschel's beautiful
drawing of the Milky Way in the Southern Hemi-
sphere the stream upon either side of the gap is
shown as extending finger-like projections of faint
light towards the opposite side, as if in vain en-
deavours to bridge across some invisible barrier,
while in Gould's more recent drawing, executed at
Cordova, multitudes of faint stars are represented as
scattered over the break.
There is but little direct evidence as to the distance
of the Milky Way. It is conceivable that the dis-
tance of a portion of the Milky Way might be re-
vealed in observations for parallax made upon a
star, apparently lying in its stream while actually
situated far beyond. In such a case, if a number of
Milky- Way stars showed the same relative parallax
with reference to the selected star, it would follow
that the members of the group were at the same
distance from the Earth, and if the parallax of the
selected star were inappreciable, which would be
probable if other selected stars, similarly examined
The Milky Way and Star Distribution. 63
with proper precautions, gave the same result, the
distance of the Milky- Way stars would be deter-
mined. No such effect as this has, however, been
observed, and there can be little doubt that the
Milky Way is more remote than at least many of
the stars whose parallaxes have been determined.
There is, however, no known reason why the method
should not be successfully applied in the future.
At a first casual glance the stream of the Milky
Way appears to be of a uniformly diffused lumi-
nosity throughout. Upon more careful inspection,
however, the first suggestion of uniformity dis-
appears, and a vast amount of varied and intricate
structure becomes revealed throughout the entire
system. Some of this detail is readily apparent to
the naked eye upon a clear moonless night; but, to
appreciate it in its full beauty, the keenest sight,
supplemented by the most favourable conditions, is
essential. Such conditions involve, of course, a
moonless night; a highly transparent atmosphere;
a time when, as is admirably the case in the autumn
and winter evenings, the Milky Way extends high
into the vault of heaven, so that the greater part of
it escapes the effects of light absorption by the
atmosphere, always so appreciable when viewing
celestial objects low down towards the horizon ; and
a locality well removed from sources of artificial
light, and the consequent glare that they produce in
the sky.
As the eye, under such conditions, becomes more
and more accustomed to its faint light, the uniform
luminosity at first suggested by the Milky Way
64 Recent Advances in Astronomy.
becomes replaced by details of structure steadily
emerging from its haze, until the whole stream
spanning the sky assumes an entirely new signifi-
cation. Consisting in some parts of clouds of faint
stars, separated by connected dark or dusky rifts;
in others, of wisps of starry matter, sometimes inter-
lacing in inextricable maze over the body of the
stream itself, and frequently projected as delicate
streamers far into the neighbouring sky; the whole
appearance of the Milky Way has suggested, not
inaptly, the image of the knotted and gnarled trunk
of an old forest tree. The interpretation of the
scheme thus dimly unfolded may be beyond our
power, but in surveying it, the observer becomes
powerfully impressed with the conviction, that rather
than being a fortuitous concourse of suns, the Milky
Way is a system possessing a complicated and
varied structure.
The telescopic appearance of many regions of the
Milky Way is of extreme beauty, and structure is
revealed of a more minute character; that appreci-
able to the unaided eye being not unfrequently lost
owing to the limited area of the sky that it is possible
to embrace in one view. Appearing in some regions
as a collection of individual stars scattered apparently
at random over the dark background of the sky ; in
others as clouds of innumerable stars, which as his
telescope moved, suggested to Sir John Herschel
the image of a drifting scud ; while not unfrequently
its hosts of stars appear as if involved in extensive
nebulosity; the Milky Way, when viewed through
a fine instrument of large light-grasping power,
The Milky Way and Star Distribution. 65
presents a picture, as its "clusters and bee-like
swarms of stars" drift in silent procession across the
field of view, that is in the highest degree impres-
sive. To the true star-gazer, the whole picture
possesses an inexpressible and quiet charm, and
suggests thoughts to which few would find it easy
to give expression.
During the past twenty years the invention of the
gelatine dry plate, and the continual improvements
effected in its preparation, have placed a new method
of research, and one of tremendous power, at the
disposal of the astronomer. It is now a matter of
common knowledge that, in its power of recording
very faint light, the photographic plate far surpasses
eye observation, for the simple reason that, to be
appreciated by the eye at all, an object must be of
a certain brightness ; but that the effect of light upon
the photographic plate being cumulative, a percep-
tible image may be produced by light far below the
limit of visibility to the eye, if its action upon the
plate be allowed to continue for a sufficiently long
time. It is consequently in the representations of
extremely faint objects, such as the nebulas and the
streams of the Milky Way, that photography has
achieved its greatest triumphs in this direction.
In its application to astronomy, the photographic
plate is usually placed within a telescope from which
all lenses, with the exception of the object-glass,
have been removed; and at a distance behind the
object-glass equal to its focal length. Under these
conditions a sharply-defined image of the celestial
object towards which the telescope is directed is
( M 520 ) B
66 Recent Advances in Astronomy.
formed by the object-glass upon the plate. In some
of the most beautiful studies of the Milky Way,
however, a portrait lens and a camera closely re-
sembling the usual form have been substituted for
the telescope. Larger pictures are obtained by the
former method, but the latter embraces a larger field
of view and is the more sensitive. It is scarcely
necessary to add that, in either method, the entire
instrument must be mounted appropriately and
driven by clockwork so as to accurately follow the
celestial object in its apparent diurnal revolution
round the Earth.
Of the applications of the photographic plate to
the study of the minute detail of the Milky Way it
will only be necessary to refer to the beautiful work
of Mr. Barnard at the Lick Observatory. From
the glorious situation of the observatory upon
Mount Hamilton, under conditions nearly perfect,
so far as atmospheric transparency is concerned;
with 11,000 feet, and that the most troublesome
portion, of the atmosphere below, Mr. Barnard,
using a portrait lens of only 6 inches in aper-
ture, and exposing the plates for periods varying
from one to twelve hours, has obtained a series of
pictures of the Milky Way of the utmost beauty
and delicacy. It is scarcely too much to say that
the pictures as far surpass, in the amount of detail
revealed, the view presented to the eye through the
largest telescope, as does the latter the result of
naked-eye observation; though it must be con-
fessed that the point-like images and the brilliancy
of the star pictures, both of which add so greatly
The Milky Way and Star Distribution. 67
to the beauty of the telescopic view, are lost in the
photograph.
It is impossible to examine the exquisite photo-
graphs obtained by Mr. Barnard, even superficially,
without being impressed with the sense that over
enormous regions of the Milky Way, of a space so
vast that in comparison with them the whole of the
Solar System would shrink to utter insignificance,
influences have been at work, concerning the very
nature of which it is only possible to form the
vaguest conjecture. The most striking features are
perhaps the dark lanes or rifts that so frequently
appear to intersect clouds of stars. These rifts
seldom occur in isolation ; they more generally form
branching systems, many branches frequently
radiating from a common trunk or other apparent
vacuity. In some cases the rifts are dark, with
sharply-defined edges, and are nearly, or quite,
devoid of stars : in others, they are uniformly hazy,
as if viewed through an interposed star cloud:
while, again, they appear of dusky and less regular
forms that have suggested the view of dark clouds
of cosmic dust lying between the Earth and a more
distant background of brilliant stars. Another very
suggestive feature in the minute structure of the
Milky Way is the frequent occurrence in it of stars
arranged in lines. The lines of stars are frequently
simple, but they often assume curved and branch-
ing forms, and a very general characteristic is
shown in their tendency to arrangement in direc-
tions roughly parallel to dark rifts in the same
region. A remarkable case is supplied in the
68 Recent Advances in Astronomy.
constellation of Sagittarius, where Mr. Barnard's
photograph shows a group of upwards of thirty
small stars arranged in the form of a forked twig,
the end of the twig remote from the fork being
sharply curved round into a hook. In the imme-
diate neighbourhood of this group, and generally
parallel to it, are several well-marked, dark, and
dusky rifts. In many cases physical relationship
between stars thus arranged is emphasized by the
occurrence of streams and wreaths of nebulous
matter connecting and involving the stars. Star
streams also frequently assume the form of closed
oval curves, the included space being commonly
darker than the exterior.
The question whether the Milky Way is an
isolated structure in space, or whether it is related
in any recognizable matter to the system of the
stars, is one that has given rise to much study and
speculation during the last hundred years. To-
wards the close of the last century Sir William
Herschel noticed that, although the stars were dis-
tributed over the face of the sky with great irregu-
larity, there was upon the whole a decided tendency
to increased density in their distribution towards
the region of the Milky Way. Confining his
attention, by reason of the extended nature of the
problem, to a zone of moderate width intersecting
the Milky Way at right angles, and directing his
reflecting telescope of i8 7 / IO -inch aperture towards
every part, in succession, of more than one-half of
it, upwards of three thousand observations were
made of the number of stars visible at any one time
The Milky Way and Star Distribution. 69
in the field of view. The area of sky embraced in
any one view was nearly one-quarter of that covered
by the Full Moon, and, taking the average density
of star distribution for equal distances from the
Milky Way, it appeared that fewest stars appeared
at a distance of 90 degrees from it, and that the
number continually increased as its stream was
approached. In equal steps of 15 degrees each
from the poles of the Milky Way to its middle
plane the average numbers of stars visible in the
field of view were 4, 5, 8, 14, 24, and 53.
In recent years the crowding of stars towards the
zone of the Milky Way has been examined in a
more detailed manner. Herschel was content to
merely count the total number visible at any one
time through his telescope, ignoring any distinction
as regards brightness between the stars themselves.
In 1870 Mr. Proctor showed, by counting the num-
ber of lucid stars, i.e. those visible to the naked
eye, that a similar crowding was recognizable with
respect to them ; and still later Mr. Gore has shown
that a similar crowding can be traced in stars of
each individual magnitude, taken separately. Fur-
ther, a remarkable law of crowding becomes ap-
parent in treating the problem in this manner.
With the brightest of the stars the crowding to the
Milky Way is very marked, so much so that, as
Mr. Proctor has remarked, upon a moonlit night,
when the Milky Way is itself invisible, it is still
possible to trace its course, at any rate in the
Southern Hemisphere, by the track marked by the
brilliant stars. With descending steps in brilliancy
70 Recent Advances in Astronomy.
the crowding of the stars becomes, however, less
emphasized ; for stars just upon the limit of vision
it is scarcely recognizable, while for telescopic stars
just below that limit it practically vanishes. For
still fainter stars, however, the crowding is again
apparent, and continues to become more and more
pronounced until the faintest of the telescopic stars
are reached.
It is easy as well as interesting to trace this
remarkable law of distribution in the positions of
the brightest stars. Of the ten most brilliant stars
of the northern hemisphere, three Capella, Altair,
and Alpha Cygni are situated very near to the
central line of the Milky Way, though its entire
stream does not occupy more than one-seventh of
the hemisphere. Four others Vega, Procyon,
Betelgeux, and Aldebaran are placed upon its im-
mediate border, and all have been thought to be
involved in its faint extensions. A zone embracing
four-fifths of the sky, with the Milky Way in its
middle plane, contains, in addition to these seven,
the eighth, Pollux, that is, exactly twice as many
stars as it should if the distribution had been uni-
form. Two only of the ten, Regulus and Arcturus,
are far removed from the Milky Way, and it is
suggestive that of this pair, Arcturus is notorious
by reason of its very high proper motion, or drift,
across the face of the sky ; one that is only exceeded
in magnitude by eighteen other stars. It is quite
conceivable that an enormous speed might enable
a star to resist a tendency to distribution to which
less rapidly moving bodies would conform.
The Milky Way and Star Distribution. 71
From the remarkable law of star distribution
traced by Herschel and later observers, it appears
scarcely possible to regard the Milky Way as an
independent and isolated formation ; while a definite
relation between it and other celestial objects is still
further emphasized by a study of the distribution of
the nebulas in space. In the course of his researches
upon these celestial clouds Sir William Herschel
became aware of a curious antipathy displayed by
them towards the brighter stars. He continually
found groups of nebulas in spaces of the heavens
comparatively destitute of stars, and separated from
the richer regions around them by dark spaces. So
firmly did he become impressed with the reality of
this relationship of avoidance, that, during the
process of " sky sweeping", as his great telescope
was directed toward different regions of the heavens,
he was accustomed to warn his assistant to prepare
to record nebulas, for, by the thinning out of stars,
he anticipated that he was approaching nebulous
ground. He observed that the crowded stream of
the Milky Way was almost destitute of nebulas,
but that toward its poles, where stars were most
sparsely distributed, nebulas appeared in greatest
number. The avoidance of the Milky Way dis-
played by nebulas was still further emphasized in
the observations of Sir John Herschel, who extended
his father's researches into the Southern Hemi-
sphere.
The avoidance of the zone of the Milky Way
displayed by nebulas, and their condensation toward
its poles, may be very strikingly shown by con-
72 Recent Advances in Astronomy.
structing a map of the heavens, upon which the
positions of the nebulae and that of the stream of
the Milky Way are recorded. Such maps have
been frequently made, the most recent being by the
late Mr. Sydney Waters, who in 1893 marked, upon
the method known as that of " equal surface pro-
jection ", the positions of the 7840 nebulae and star
clusters recorded in Dr. Dreyer's New General
Catalogue of 1888. In view of the fact that at a
not very remote date the belief was generally enter-
tained that nebulae were clusters of stars, it is in-
teresting to notice the relationship displayed by star
clusters towards the Milky Way. Their condensa-
tion toward it is even more pronounced than that of
the stars, very few being found beyond the limits
of the Milky Way, while for a considerable part of
its course the centre of its stream is occupied by a
continuous line of them.
The application of the spectroscope to the study
of the nebulae has brought to light a curious modi-
fication of their law of distribution. We have seen
in the previous chapter that many of the nebulae, by
yielding a broken spectrum, are thereby demon-
strated, at any rate so far as their luminous con-
stituents are concerned, to be in a gaseous condition,
but that the light from others yields a continuous
spectrum, which is at present incapable of exact
interpretation. On examining the manner in which
these two classes of nebulae are distributed in the
heavens, it appears that the "gaseous" nebulae share
the tendency towards condensation displayed by
stars in crowding towards the Milky Way, the
The Milky Way and Star Distribution. 73
greater number of nebulas actually appearing in it
being gaseous. Upon deducting these from the
total number, and mapping the positions of the re-
mainder, the avoidance of the Milky Way displayed
by them is consequently more pronounced than
before.
In following the steps by which structure has been
traced throughout the entire stream of the Milky
Way, and system revealed in the distribution of the
stars and nebulae with reference to it and to each
other, we have so far followed a sure course, and
indeed have had occasion to do little more than
record the results of direct observation. That stars
and nebulae are not scattered at random throughout
space, that there is law in their manner of distribu-
tion upon the face of the sky, and that with this law
the stream of the Milky Way is in some manner
intimately associated, cannot be doubted. So much
is known ; and it is scarcely possible to refrain from
the attempt to gain some closer insight into the
nature and meaning of the entire system. Here,
however, our steps at once follow a less certain
track, and rapidly lead toward a region of specula-
tion in which not a few sound thinkers have become
sadly bewildered.
In 1784 Sir William Herschel, adopting in prin-
ciple a suggestion advanced by Thomas Wright of
Durham thirty-five years earlier, attempted to ac-
count for the appearance of the Milky Way, and the
condensation of stars toward it, as a perspective
effect. According to this view, all stars, including
those that appear to form the Milky Way, were
74 Recent Advances in Astronomy.
assumed to be, upon a broad average, uniformly
distributed in a stratum or layer, the thickness of
which was small in comparison with its dimen-
sions in its own plane; while the Sun was situ-
ated not far from the centre of the stratum. It
is clear that according to such a law of distribution
the line of sight from the Solar System directed at
right angles to the stratum would soon emerge into
external space, and that but few stars would appear
in this direction; but that in all directions parallel
to the faces of the stratum stars would appear to be
crowded, since the line of vision would be, in all
these directions, for a long distance involved among
stars. All round the Solar System, in the middle
plane of the stratum, stars would, therefore, appear
to be crowded, and by such perspective effect the
appearance of the Milky Way was imagined to be
produced. In passing from a direction along to
one at right angles to the stratum, the length of the
line of sight included in it would continually de-
crease, and fewer and fewer stars would therefore be
seen toward the poles of the Milky Way. It was
further suggested that the bifurcation of the Milky
Way was an optical effect, due to the projection from
the principal stratum of a secondary one, making
a small angle with it, and leaving it nearly in the
direction of a straight line passing from the Sun.
If the stars were distributed with perfect uniformity
throughout space, and if the most distant and the
faintest were visible through a given telescope, it
would of course be possible, by counting the num-
bers visible in equal areas of the heavens, to compare
The Milky "Way and Star Distribution. 75
the extensions of the system of the stars in the cor-
responding directions. For a time Herschel believed
that these conditions were fulfilled with sufficient
approach to accuracy to justify the application of
the method; and, with a view of " fathoming the
Universe " in this manner, he carried out a laborious
series of " star-gages ", his telescope being directed
successively to upwards of 3000 selected regions of
the heavens, and the number of stars in each field of
view counted. The results were published in 1785.
It follows from simple geometry, that if the stars
are distributed uniformly, and if the telescope em-
ployed is sufficiently powerful to reveal all of them,
the extension of the system in different directions is
proportional to the cube root of the number of stars
Fig. 4. Sir William Herschel's earlier view regarding the Form of
the Stellar Universe.
appearing in equal areas of the heavens in those
directions; and upon this principle Herschel con-
structed the familiar hypothetical section of the
sidereal system which is reproduced in fig. 4. In
this the Solar System occupies the position indicated
by the point s, the straight line SA is directed rather
to the north of Sirius, and the great length of it in-
volved among stars gives rise to an optical crowding
to which the appearance of the Milky Way is as-
cribed; the cleft upon the opposite side accounts
76 Recent Advances in Astronomy.
upon the same principle for the apparent division of
the Milky Way into two branches, the lines SB and
sc, by fathoming the starry extensions shown in the
section, giving rise to the bifurcation in the neigh-
bourhood of Altair.
To the reader, acquainted with the intricate struc-
ture of the Milky Way revealed by more recent
observation, it will be at once apparent how com-
pletely at variance with nature was Herschel's as-
sumption that its appearance could be due to any
such merely optical effect; but it is difficult to
imagine how, with the knowledge of star distribution
that was the result of his own work, Herschel could
have, even for a time, regarded his fundamental
assumption of approximately uniform distribution
as valid. To account for the Coal Sack and the
other less-pronounced vacuities of the Milky Way
upon the hypothesis of optical effect, vast cone-like
tunnels must be imagined as converging upon the
Solar System from the external void; while every
appearance of exceptional richness must be re-
garded as arising from immense columns of stars
projecting from the system afar into external space,
and similarly radiating from the Sun. As a
centre of such converging tunnels of vacuity and
radiating projections of starry cones, the Sun
assumes a unique place in the Universe entirely
without warrant. Upon the optical hypothesis,
again, except upon similarly extravagant assump-
tions, the appearance of crowding of stars towards
and within the Milky Way should take place
gradually. From the external sky to the centre of
The Milky Way and Star Distribution. 77
its stream, density of star distribution should in-
crease continually and by such perfect gradation
that it should be impossible to define the limits of
the Milky Way. Such an effect is, however, directly
contradicted by observation. Throughout its entire
course the boundary of the Milky Way is generally
marked with fair definition, while in some regions
the transition from its star-crowded depths to the
external sky is so abrupt that one half of the field of
view of the telescope may be dark while the other
half is crowded with its stars. Such features point
definitely to the Milky Way being a real and definite
structure, separate from, though doubtless in some
way closely related to, external celestial bodies.
That this conclusion is unavoidable was clearly
recognized in his later life by Herschel himself, and
was fully acknowledged by him, although, unfor-
tunately, his earlier hypothesis was never formally
withdrawn.
Closely bearing upon the problem of the Milky
Way and the relation of external objects to it, is
the question whether the stars are distributed over
a finite region, or whether they are scattered with-
out limit through infinite space. In other words,
is the visible universe finite or infinite in extent?
Upon the random suppositions that stars are distri-
buted uniformly and without limit through infinite
space, and that all are of the same magnitude
and of the same intrinsic brilliancy as the Sun,
a very remarkable conclusion is reached, which is
often quoted, and to which it is worth while to
direct attention. With the Earth as centre, let an
78 Recent Advances in Astronomy.
infinite number of concentric spherical surfaces be
imagined in space having radii proportional to the
natural numbers i, 2, 3, 4, &c., and let a certain
number of stars be imagined to be distributed
over the first surface. The second surface, having
twice the radius of the first, will have four times
the area, and will therefore contain, upon the
hypothesis of uniform distribution, four times as
many stars. Since, however, the apparent disc
of each star would, at its doubled distance, be
diminished in area to one-fourth, so far as regards
their combined areas, the increased number of stars
would exactly counterbalance the effect of their
reduced discs, and the stars upon the second surface
would cover the same extent of sky as those upon
the first. By the same reasoning this would also
be true of those upon the third, fourth, and every
succeeding surface; so that, if the stars extend
through space without limit, upon including a
sufficient number of surfaces the whole face of the
sky would at length be entirely covered by stars.
Further, as the apparent brightness of an object
does not diminish with its distance, unless its light
undergoes absorption during its passage from the
body to the eye, the whole vault of heaven would
be both by day and by night resplendent with a
dazzling brilliancy equal to that of the Sun itself
except for a few black dots marking the positions
of the interposed planets, and for a group of small
dusky objects, spots upon the Sun, by which alone
its position in the heavens would be apparent.
The actual appearance of the heavens is so strik-
The Milky Way and Star Distribution. 79
ingly different from this that it is clear that the as-
sumptions lying at the base of the deduction are
very wide of the mark.
With reference to this reasoning it must be
noticed that the apparent brightness of a surface is
independent of its distance, when, and only when,
there is no absorption of light in its passage from
the surface to the eye. If there is a general absorp-
tion of light the argument fails. Less and less
light would be received from increasingly distant
spheres, and the combined light from all those
beyond a certain distance, a distance depending upon
the intensity of the absorption, would be a negli-
gible quantity. 1 Absorption of light in interstellar
space might result from imperfect transparency of
1 The demonstration of this statement is comparatively simple. Suppose,
as a concrete case, that in its passage from the nearest of the spherical sur-
faces imagined in the argument to the eye, ^ of the light that would if
there were no absorption reach the eye, is actually absorbed. Then if L
represents the whole light that the eye would receive from the stars upon the
first surface if there were no absorption, the light actually received from
them would be ^ L. Consider next the light from the stars upon the second
surface. Since the distance separating the surfaces is equal to that from
the first surface to the eye ; of the light that would reach the first surface
from the second on its way to the eye if there were no absorption % would
escape absorption, while in its farther passage to the eye % of this would
escape, the light arriving being therefore % of % L, or (%) 2 x L, since L
is the same as before, the light received from each sphere upon the assump-
tion of no absorption being the same. By similar reasoning the light
received from stars on the third surface will be ( % )s L, and so on, the total
being
an infinite series of terms, the value of which, by the law of geometrical
progression, continually approaches without limit, but can never exceed,
2 L. That is, for this special value of absorption, the whole light from an
infinite distribution of stars would be only double the amount that would be
received from the stars on the nearest surface if there were no absorption.
8o Recent Advances in Astronomy.
the ether of space by which all radiation is trans-
mitted, or from interposed dark matter in the form
of dark stars, clouds of cosmic dust, or swarms of
meteorites. It is perhaps too much to regard im-
perfect transparency of the ether as inconceivable ;
but should absorption in the ether occur, energy
in some other form must appear equal in amount
to that of the radiation absorbed, and no trace of
any such developed energy has been detected. It
has, however, been shown in a former chapter that
the existence of dark matter in interstellar space
is possible if not probable, both in the form of dark
stars and as clouds of cosmic dust in the earliest
stage of a nebula, while meteor swarms are an
obvious fact. Light absorption might arise from
all or any one of these causes, cosmic dust clouds
appearing the most probably effective, so that the
impossibility of an infinite distribution of stars can-
not be demonstrated. The distribution is, however,
clearly not uniform.
In spite of the serious objections to the view that
the density of star distribution upon the face of the
heavens can be taken as an indication of the exten-
sion of the sidereal system, Sir John Herschel,
during his residence at the Cape between 1834 an< ^
1838, extended his father's method of star-gaging,
and generally maintained the view that the appear-
ance of the Milky Way is, at any rate in part, due
to optical crowding. The comparatively abrupt
limits of its stream led him, however, to modify
Sir William Herschel's original theory, in assum-
ing the stars to be distributed within the volume
The Milky Way and Star Distribution. 81
of a flat ring, indefinitely extended in all directions,
the Solar System being imagined as situated near
the centre of the hollow of the ring. By splitting
the ring in its medial plane nearly to its centre,
and slightly deflecting outward the divided portions,
the appearance of the great fissure in the Milky
Way was explained. The same fundamental con-
ception underlies the modification suggested by
Wilhelm Struve in 1847. According to this scheme
the whole of the stars were distributed in parallel
layers or strata, the strata being more and more
densely crowded towards a central plane very nearly
passing through the position of the Sun. Accord-
ing to this view there was a real condensation of
stars towards the central plane, which was ap-
parently increased by the effect of optical crowding,
the appearance of the Milky Way being the com-
bined effect of the two causes. It was suggested,
in addition, that absorption of light in space might
reduce the last effect to within narrow limits.
To these modifications of Sir William Herschel's
first hypothesis the objections urged against it,
though not so entirely overwhelming, are neverthe-
less fatal. Both are equally inconsistent with the
appearance of the Coal Sack and other vacuities,
with the dark rifts, and with the minute detail
revealed by later researches. According to each
of them, the transition from greater to less density
of distribution of stars beyond the limits of the
Milky Way should be far more gradual and regular
than it is. Star-crowding should become less and
less pronounced in passing from the limits of the
(M 520) F
82 Recent Advances in Astronomy.
Milky Way towards its poles; but, in actual obser-
vation, while this is found to be the case so far as
the average of the stars is concerned, the law is
very far from being maintained in isolated regions.
Several regions of the sky in close proximity to the
poles of the Milky Way are exceptionally rich in
stars, while others, closely bordering upon its
stream, are among the poorest in the heavens.
The views of the two Herschels and Struve are
alike untenable, and, since the publication of the
last, they have failed to find support save in the
pages of certain popular works on astronomy.
The older "disc" and "strata" theories having
been found wanting, the Milky Way has come to
be generally regarded as a real formation, and
attempts have been made to construct in imagination
a stream of stars that should give rise to the appear-
ance actually presented. It is obvious that, in
its visible aspect, the Milky Way appears as the
projection of all of its parts upon the background
of the sky. Of the possible depth of the formation
the eye takes no cognisance. Its many apparently
confused and interlacing wreaths may be, in actual
fact, entirely separate, some being projected inward
toward the observer, while others may be thrown
backward into the more remote spaces behind.
In 1869 Mr. Proctor attempted to show that the
chief features of the Milky Way, and more particu-
larly the appearance of the three most pronounced
irregularities, the Coal Sack, the bifurcation, and
the great Break in Argo, might arise from the con-
volutions of a single stream of stars. One of the
The Milky Way and Star Distribution. 83
diagrams accompanying his suggestion is repro-
duced, with a slight modification in the arrangement
of the lines of vision, in fig. 5. In this, the single
stream of the Milky Way is represented as being
somewhat fantas-
tically curved, the
ends being folded
back upon the
course of the main
stream. The Solar
System occupies
the position indi-
cated by the point
s, and the line of
sight s A, escap-
ing into external
space between the
refolded portions of the stream, accounts for the
appearance of the great Break. Close to the Break,
in the direction SB, the two portions of the stream
are optically superposed, and give rise to the bril-
liant neck in Argo, while a little farther on one or
both portions diverge from the median plane for a
short distance, producing the supposed purely opti-
cal effect of the Coal Sack in the direction s c. After
a farther short distance through which the two por-
tions continue superposed, and through which the
stream consequently appears single, apparent divi-
sion again occurs owing to a second divergence
from the median plane, and continues until the
branches reunite along SE in the constellation of the
Swan. Of these apparent branches, one, the more
Fig. 5. Mr. Proctor's suggested explanation
of the Appearance of the Milky Way.
84 Recent Advances in Astronomy.
northerly, fades and becomes for a time invisible by
reason of excessive distance, while the other in-
creases greatly in brilliancy owing to its nearness.
From SE to SL the stream appears as it really is,
single; but beyond SL, and extending to the great
Break, the series of lakes or vacuities, which are
a characteristic feature of the Milky Way in this
region, are accounted for by repeated temporary
deviations of the two portions from the median
plane, as in the direction SK.
Mr. Proctor's view of the formation of the Milky
Way displays the ingenuity that would have been
anticipated from so sagacious and original a thinker,
but it is open to objections of so serious a nature as
to render its acceptance impossible. It has been
urged against it, that, since the brightness of an
object does not diminish with increased distance,
the fading away and ultimate disappearance of the
northern of the two branches bifurcating in the
Swan cannot be due to excessive remoteness. It
has, however, been seen that the apparent bright-
ness of an object is only independent of its distance
if its light undergoes no absorption in space; and
since such absorption has been shown to be possible,
if not probable, a fatal attack upon Mr. Proctor's
scheme cannot be maintained upon this ground.
It is curious that Mr. Proctor himself makes no
reference to this point, though he was beyond all
doubt thoroughly familiar with the elementary law
in question. A more serious objection to the sug-
gested explanation lies in the fact that it is scarcely
possible to regard the divided branches of the Milky
The Milky Way and Star Distribution. 85
Way as physically independent. For a consider-
able distance from the division of the main stream
in the Swan, the southern branch continues to cast
off incipient streamers and bright projections toward
its companion, and the same indication of intimate
physical connection between the two is shown in
the near neighbourhood of the reunion of the
branches in the Centaur. It might seem probable
that the varying apparent breadth of the Milky
Way might serve as an indication of the proximity
or remoteness of different parts of its stream ; but
it is found that upon the whole the narrowest por-
tions, which would otherwise appear to be the most
remote, are the brightest, and it would be indeed
difficult to regard enhanced brilliancy as generally
associated with increased distance.
The failure of these and other attempts to explain
the appearance of the Milky Way, and of the irregu-
larities displayed by it, as being primarily due to
perspective effect or optical projection, point to the
probability of the more simple and direct view, that
the Milky Way is a definite and complicated struc-
ture, and that its bifurcation, its vacuities, its gaps,
and its other irregularities, are definite physical
facts. To this view astronomers have now become
reconciled. Adopting it, the sense of the overwhelm-
ing mystery of the whole undoubtedly becomes
greater, while it must be confessed, that something
is gained in the rejection of schemes in which a
rather painful suggestion of artificiality somewhat
conflicts with the majesty of the problem toward the
partial solution of which they have been directed.
86 Recent Advances in Astronomy.
The crowding towards the zone of the Milky Way
displayed by the naked -eye stars, and more par-
ticularly by the brightest of them, suggests that their
distribution in space has been controlled either by
the Milky Way itself or by the influences to which
it owes its being. Not only, however, is a general
relation thus indicated between the external stars
and the Milky Way, but it has been maintained,
notably by Mr. Proctor and Mr. Cowper Ranyard,
that there is evidence in many instances of a more
direct and intimate relationship between separate
stars and groups of stars and the Milky Way. The
evidence consists in the arrangement of bright stars,
both individually and in groups, with reference to
structures in the Milky Way. While acknowledging
that in some instances it is difficult to avoid a feel-
ing of doubt that the arrangements in question
may not be the results of chance distribution, it is
scarcely possible not to recognize in others very
strong evidence of intimate physical connection.
It must be sufficient here to illustrate the general
nature of the arguments adduced by reference to a
few selected cases, referring the reader for a more
complete discussion of this very intricate and deli-
cate subject to more exhaustive works. 1
The constellation of the Swan lies in the very
heart of the Milky Way, in a region particularly
interesting from the evidence of structure apparent
in it both to the naked eye and in photographic
1 The subject is developed in considerable detail in the chapter on ' ' The
Stars " in The Old and New Astronomy, by Proctor and Ranyard ; and has
formed the subject of a number of articles that have appeared in Knowledge
during the past fifteen years.
The Milky Way and Star Distribution. 87
examination. Upon a photographic plate exposed
for thirteen hours in the autumn of 1891 by Dr.
Wolf of Heidelberg, accumulations of stars are
shown of a richness unimaginable in the finest
telescopic view, while throughout vast regions the
stars appear to be involved in a faintly luminous
cloud. This enveloping nebulosity is extensively
furrowed by dark lanes and rifts, the borders
of which are, in the manner so generally charac-
teristic of them, emphasized by lines of faint
stars. Conspicuous in Dr. Wolfs photograph are
the images of the bright stars <* and 7 Cygni, the
two that form the head and centre of the familiar
cross of the Swan. Upon the photograph, both of
these stars appear to be nebulous, their blurred
images passing by insensible gradation into the
surrounding cloud. It is true that a similar appear-
ance is recognized to some extent in all photo-
graphic pictures of bright stars; an extension of
the photographic image being caused by the dis-
persion of light from the point-like and brilliantly
illuminated image of the star formed upon the
sensitive plate during exposure into the sensitive
silver compound around it, but here the want of
definition of the images appears to be far more than
could possibly be ascribed to this cause, and the
unsymmetrical form of the haze by which 7 Cygni
is enveloped makes it scarcely possible to accept
such explanation of the appearance in its case. In
addition, the position of a Cygni at the base of a
remarkable shrub-like dark formation in the bright
nebulosity, and that of 7 Cygni as the centre of
88 Recent Advances in Astronomy.
several diverging nebulous structures, point strongly
to a definite physical connection between both stars
and the apparently surrounding masses. It appears,
therefore, probable that the stars are actually bathed
in the depths of the Milky Way in which they
appear, and do not owe their appearance in it to the
chance effect of optical projection upon it.
The arrangement of many of the stars in the con-
stellation Orion is very remarkable. The proba-
bility against three such brilliant stars as those
forming the belt falling in a straight line and
appearing in such close proximity as the result of
chance distribution is overwhelming. The close
proximity of the Great Nebula of Orion, as well as
the arrangement of many of its contained structures
with reference to the direction assumed by the belt,
is moreover suggestive ; as is also the fact that the
belt is situated upon the immediate border of the
Milky Way, to which it is very closely parallel. A
group of five faint naked-eye stars lies immediately
below the belt; they are arranged in a line parallel
to it, and it is easy to imagine the line continued
upwards and towards the west by a farther stream
of five stars separated from the first by a short
break and deflected northward towards the Milky
Way. A little farther to the west a second stream
of naked-eye stars, at least thirteen in number,
leaves the constellation in a westward direction,
becoming in like manner deflected to the north and
actually penetrating the Milky Way. Both of these
star streams are represented in a beautiful drawing
of the Milky Way as seen by the naked eye, re-
The Milky Way and Star Distribution. 89
cently executed by Dr. Boeddicker, as involved in
its cloudy extensions. To the last point it may,
however, be urged with considerable force, that the
appearance of a faint nebulous stream connecting
the members of a line of faint stars may be an
optical illusion, and the failure of the photographic
plate to verify the existence of the cloudy stream
indicated by Dr. Boeddicker as involving the line
of stars immediately below the belt, shows this to
have been the case in this instance. From the
whole of the evidence it will, however, probably be
conceded that the configuration of the stars of the
belt, and the symmetrical arrangement of so many
conspicuous stars in its neighbourhood in lines
having the same general trend, demonstrates the
reality of a close physical relation between the
whole, while their peculiar relation to the course
of the Milky Way renders probable an intimate
relation between them and it.
Mr. Proctor has directed special attention to the
close association of lucid stars with the course of
the Milky Way in the neighbourhood of the Coal
Sack. Although by no means devoid of telescopic
stars, the dark area of the Coal Sack does not in-
clude one visible to the naked eye. If the naked-
eye stars had been distributed over the heavens
with uniformity, the number assigned to the Coal
Sack should have been, at the least estimate, seven;
and their avoidance of it is the more remarkable in
that in the regions immediately surrounding it they
are particularly richly represented. It becomes,
therefore, scarcely possible not to regard the bright
90 Recent Advances in Astronomy.
stars as intimately associated with the Milky Way
in this region, and as being situated at nearly the
same distance. The brilliant expansion of the
Milky Way in which the Coal Sack is situated
contains the five bright stars of the Southern Cross;
and of these, the most brilliant the first-magnitude
star a Crucis is actually upon its border. From
the appearance of this star in a photograph taken
by Mr. H. C. Russell at Sydney in iSgo, 1 Mr.
Cowper Ranyard has maintained that there is
strong evidence of its close association with groups
of faint stars that appear upon the plate in its
neighbourhood. The great star appears to be the
centre of several diverging streams of small ones,
while other groups of small stars are arranged con-
centrically in circles round it.
The apparent luminosity of a Crucis cannot
exceed that of the smaller stars by less than three
million times, so that if it be regarded as probable
that all are equally remote, this proportion is also
that of their actual luminosities. No data exist
from which it is possible to form an estimate of the
distance either of a Crucis or the faint stars, from
which it would be possible to compare the actual
light-giving powers of the stars with that of the
Sun ; but if the great star be regarded as being not
very different from the Sun in light-giving power,
the small ones need scarcely be more than self-
luminous planets; while, if the small ones are re-
garded as the equivalents of the Sun, the large star
1 The photograph is produced in Knowledge for June, 1891, and in The
Old and New Astronomy.
The Milky Way and Star Distribution. 91
must be of such luminosity that, for its surface
brilliancy to be no more than equal to that of the
Sun, it must be capable of enclosing within itself
the entire orbit of Saturn. Before accepting so
astounding a view as the last, it is well to consider
whether adequate grounds exist for regarding the
former as improbable.
From the accumulation of a considerable body of
evidence, the general nature of which has been
sketched in the preceding paragraphs, an intimate
association appears probable between the naked-eye
stars and the system of the Milky Way. The stars
immediately below the naked-eye stars in brightness
appear to be influenced by its course to a far less
degree. It has been seen that the tendency to
crowd towards the zone of the Milky Way practi-
cally vanishes with stars just below the limit of
vision; and any direct relation displayed by these
faint stars towards the configuration of the streams
of the Milky Way is difficult of detection. Stars of
the ninth magnitude are, for instance, distributed
with apparent uniformity over the space included
between the divided streams between the Swan and
the Centaur, scarcely if at all less richly than over
the branches themselves; and faint telescopic stars
are scattered with approximate evenness over the
darkness of the Coal Sack. Thus, in their relation
to the Milky Way the naked-eye stars appear to be
differentiated from those immediately below them
in brightness. Again, since the apparent brilliancy
of a star is directly affected by its remoteness, it
appears probable that the brightest among the stars
92 Recent Advances in Astronomy.
are upon the whole those that are nearest. Since
individual stars, as has been seen, vary enormously
in magnitude, exceptions to such a generalization
would of course be expected; they are, in fact,
directly illustrated in the near proximity of so in-
conspicuous an object as 61 Cygni, and in the un-
fathomable remoteness of Arcturus ; but it would be
anticipated that, with an increasing number of stars,
such irregularities would gradually disappear in
a general average. The intimate association of
the brighter, and, therefore, in all probability the
nearer, stars with the Milky Way, suggests the
view that the Milky Way itself is a comparatively
near neighbour in space.
By another investigation, proceeding upon en-
tirely independent lines, we are also led to regard
it as probable that the brighter of the stars are
differentiated from the rest in a special manner.
From the time of Ptolemy it has been the custom
to classify stars in ''magnitudes" in the order of
their increasing faintness. To very bright stars,
such as Aldebaran and Altair, a position in the first
magnitude is assigned; visibility to the naked eye
terminates under favourable conditions at about the
sixth magnitude; the penetrating power of the
largest telescopes probably reaches stars of the
fifteenth magnitude, while the power of the photo-
graphic plate possibly extends to some four or five
magnitudes beyond. Since, however, the measure-
ment of the brightness of a star is a matter of con-
siderable delicacy, and, in fact, has only become
satisfactorily possible in recent years, magnitudes
The Milky Way and Star Distribution. 93
assigned to stars by different astronomers have been
largely a matter of individual judgment, and it is
not surprising that the scales that have been adopted
are very conflicting. Since the middle of the pre-
sent century, however, at the original suggestion of
Pogson, it has become the custom to apply to the
term magnitude a more definite meaning than it
had previously received. It had been noticed by
Sir John Herschel, that, according to all generally
accepted scales, stars of the first exceeded those of
the sixth magnitude in luminosity very nearly 100
times, and Pogson suggested that a ratio in light-
giving power of 100 to i should be regarded as the
definition of a difference of five magnitudes, the four
intermediate magnitudes being interposed in such
a manner that stars of any one magnitude should
bear a constant ratio in their luminosity to those of
the magnitude following. To satisfy this condition,
it follows that any star must exceed in brightness
another of one magnitude fainter by 2*512 . . times,
this "light-ratio" being the fifth root of ico. 1 In
the result, the estimations of magnitude by the
older astronomers are found to conform very closely
to the absolute scale so far as the naked-eye stars
are concerned, but to deviate considerably from it
for fainter ones.
From the absolute definition of star magnitude it
is possible to calculate the relative numbers in which
1 Thus a first-magnitude star is equivalent to 2-512 second-magnitude
stars, a second-magnitude star to 2-512 third-magnitude stars, and so on.
Hence a first-magnitude star is equivalent to 2-512 x 2*512 or (2'5i2) 2 third-
magnitude stars, (2'5i2) 3 fourth-magnitude stars, and (2'5i2) 8 or 100 sixth-
magnitude stars.
94 Recent Advances in Astronomy.
stars of different magnitudes should appear in the
heavens upon the assumption of their uniform
distribution in space. From the result of the
calculation, the details of which may be left as an
exercise in geometry to the reader and do not
present any serious difficulty, it appears that if a
number of stars, either of the same or different
degrees of intrinsic brilliancy, were scattered in
space, subject only to the condition that those of
each degreeof luminosity were distributed uniformly,
there should appear nearly four more accurately
3-981 times as many stars of any given magni-
tude as of the next exceeding it in brightness. For
every star of the first magnitude there should
appear, for instance, nearly four stars of the second,
sixteen of the third, and sixty-four of the fourth.
It cannot fail to be interesting to compare these
numbers with those actually observed.
Data for such a comparison are supplied in star
catalogues. Of these the most extensive that has so
far been constructed is known as the Bonn Durch-
musterung. It was compiled under the supervision
of Argelander between 1859 and 1862, and in it are
recorded the positions and magnitudes of 324,198
stars, all those of the northern hemisphere down
to the 9-5 magnitude. The more recent catalogue
constructed at Harvard by Professor E. C. Pickering,
giving the magnitudes of stars as measured by the
meridian photometer, is undoubtedly the more
accurate in this respect, but, since it includes only
those brighter than the 6-5 magnitude, it is not so
well adapted to the present purpose. The same
The Milky Way and Star Distribution. 95
general result, however, appears from the adoption
of the data of either the Bonn or the Harvard cata-
logues, as well as from Dr. Gould's catalogue of
stars visible from the southern hemisphere.
The result of the comparison is expressed in the
following table. In the second column are given
the numbers of stars between the limits of magni-
tude indicated in the first. These numbers are given
by Littrow as the result of his examination of the
Bonn Durchmusterung. The third column contains
the numbers of stars that should have appeared
upon the hypothesis of uniform distribution ; and the
last, numbers obtained by dividing the figures in
the second column by those in the third, that is, the
numerical proportion of stars actually observed to
those that should have been observed upon the
uniform -distribution hypothesis, a quantity that
may be conveniently described as the "apparent
crowding ".
COMPARISON BETWEEN THE OBSERVED NUMBERS OF STARS OF
DIFFERENT MAGNITUDES WITH THE THEORETICAL NUMBERS
UPON THE HYPOTHESIS OF UNIFORM DISTRIBUTION IN SPACE.
Limiting
magnitudes.
Numbers of
stars actually
observed.
Theoretical
numbers.
Apparent
crowding.
I to 2
IO
4
2 '5
2 to 3
37
15
2-46
3 to 4
130
58
2-24
4 to 5
312
234
i "33
5 to 6
1,001
i -08
6 to 7
3,705
1-18
7 to 8
13,822
"94
19,698
19,698
96 Recent Advances in Astronomy.
The general result of the comparison, as ex-
pressed in the figures of the last column, is very
remarkable. There appear to be many more stars
of the first five magnitudes than there should be
according to the hypothesis of their uniform distri-
bution in space. Rejecting the stars tabulated
between the limits of the first and second magni-
tudes as being too few from which to draw any
reliable deduction, and also as comprising several
stars, such as Arcturus and Vega, which should in
strictness be excluded, as being too bright to be
regarded even as first-magnitude stars, there is
apparent in the record a crowding of the brighter
stars, that diminishes with increasing faintness and
becomes insignificant beyond the limit of the fifth
magnitude that is, near the limit of visibility to
the unaided eye. It has been stated that a similar
appearance of crowding is recognizable in the more
exact record of star magnitudes contained in the
Harvard catalogue, as well as in Dr. Gould's
catalogue of stars of the southern hemisphere.
An apparent crowding of the brighter stars
would be quite consistent with their uniform dis-
tribution if light experienced absorption in space,
since such absorption would affect the light from
distant stars to a greater extent than that from
nearer ones. This explanation, however, scarcely
appears to be applicable here, since from the fifth
magnitude to the eighth, stars appear in numbers
not very different from those estimated upon the
assumption of their uniform distribution. Were
there appreciable light absorption within the limits
The Milky Way and Star Distribution. 97
of space in which stars as far as those of the eighth
magnitude are distributed, its effects should be as
conspicuous in the falling off of numbers in succes-
sive magnitudes among the fainter as in brighter
magnitudes.
The most simple view to take with reference to
the apparent crowding of the brighter stars is that
it results from a real crowding of stars in the neigh-
bourhood of the Sun. There is nothing inherently
improbable in this view, since the study of the sky
reveals numerous analogous instances of the cluster-
ing of stars. Setting aside such extreme cases as
are presented in the Pleiades, the Hyades, and
other such strongly pronounced clusters, many rich
regions of the heavens, not even included in the
Milky Way, furnish instances in which the local
density of star distribution is far in excess of that
which could have resulted from chance distribution.
The suggestion of a clustering of stars in the
neighbourhood of the Sun acquires additional
interest from the indications already recognized,
that the nearer among the stars are differentiated
from the rest by an intimate association displayed
by them towards the stream of the Milky Way. It
appears scarcely possible not to recognize in the
complete testimony a suggestion that the Sun is a
member of a star-cluster, one in which the Milky
Way is involved as a stream of stars far smaller than
the more conspicuous members of the cluster, but
closely associated with the fundamental scheme of
its structure. According to such view the appear-
ance of uniformity in the distribution of the stars
(M520) G
98 Recent Advances in Astronomy.
from the fifth to the eighth magnitude, as well as
their comparative indifference to the zone of the
Milky Way, are alike due to their lying for the
most part beyond the region in which the clustering
tendency and the apparently attractive influence of
the Milky Way extends. The Milky Way, to-
gether with the cluster containing the Sun, may
conceivably constitute a true independent system,
while it is possible that similarly associated with
other star-clusters there may exist other streams of
star-dust, undistinguishable from their excessive ,
remoteness.
Before regarding this speculation as probable, it
is essential to imagine some possible explanation
of the crowding towards the Milky Way again
exhibited by the still fainter telescopic stars. It is
not inconceivable that this appearance may be due
to the escape of true Milky-Way stars from within
its stream into external space. It is not possible
here to develop this suggestion fully, but it would
appear probable, that, if a number of stars were
distributed at random and with random velocities
both as regards magnitude and direction through
a definite region in space, a condition of things
would result not unlike that imagined in the kinetic
theory of gases. Pairs of members of the swarm
would from time to time approach so closely as to
describe, under the influence of their mutual gravi-
tation, hyperbolic orbits round each other, the
common centre of mass of the pair marking the
position of a common focus. If, by chance, it
happened that the masses and velocities of the pair
The Milky Way and Star Distribution. 99
were so related that their centre of mass was at rest,
the direction only of the star motions would be
affected by their near approach, the velocity of each
being reduced upon separation to an extent equal
to its increase upon approach ; but if, as would
generally be the case, the centre of mass was in
motion, since the velocities of the stars would be
ultimately unaffected with reference to it, the actual
velocities would be changed, the star receding
after the " encounter" in the direction of motion of
the centre of mass having its velocity increased,
while the speed of the other would have become
less. Encounters between stars continually occur-
ring, all velocities, without limit of magnitude,
would be continually being produced in the cluster,
and from time to time a star would acquire sufficient
speed to carry it beyond the limits of the cluster,
while its speed might be so great as to place it
beyond the controlling influence of gravitation, in
which case it would leave the cluster never to
return. The system would slowly disintegrate, and
during the process the escaped members would be
found scattered in external space most densely
distributed in the immediate neighbourhood of the
original swarm.
Similar encounters must occasionally take place
between members of the main cluster in which the
Milky Way is involved, which, by similar reasoning,
it is scarcely possible to regard as a stable system.
The extreme velocity of such "runaway" stars as
1830 Groombridge may well be due to their having
experienced a number of favourable encounters with
ioo Recent Advances in Astronomy.
other stars, and in any case does not point to their
being, as has been suggested, temporary visitors to
the system of the visible stars from external regions
of space, ploughing their way through it by reason
of enormous initial speeds incapable of generation
by the gravitational attraction of the system. It is
conceivable that in the remote past the sun-cluster
may have been far richer than it is now, and the
firmament may have been more resplendent with
brilliant stars, but that from age to age its members
may have been gradually scattered, and the vault of
heaven may now be growing poorer.
It is scarcely necessary to remind the reader that
in this chapter no attempt has been made to explain
the function of the Milky Way, or its connection
with the stars. An attempt only has been made in
the latter pages to define its possible relation to the
stars, and it is not suggested that the attempt ex-
tends beyond the limits of speculation. Ignorance
of the distances of more than an insignificant
minority of its members appears at present an in-
superable obstacle towards extending the web of
exact knowledge far into the system of the stars;
but, in the absence of more exact methods, it is
impossible for the lover of the picture of infinite
grandeur and majesty, mapped out night by night
upon the fair face of the starlit sky, to refrain from
indulging in some conjecture, however vague and
in itself unsatisfactory, as to the meaning of so ex-
quisitely beautiful and mysterious a record.
The Recent Study of Mars.
Chapter III.
The Recent Study of Mars. ''. ' "-H
From the time that the telescope revealed to Sir
William Herschel the first clear picture of the planet
Mars, and led him to regard the details of the
delicately-tinted image presented to his view as
indicating the existence upon the surface of the
planet of physical conditions not very unlike those
familiar to the inhabitants of the Earth, a special
interest has attached to this, the only one of the
orbs of heaven that it is possible to contemplate
with any degree of confidence as a sister world. In
their distribution, general configuration, and colour,
the planetary markings have appeared during the
greater part of the present century to harmonize
well with their tempting interpretation as oceans,
continents, and polar regions bound in eternal
snow. The rotation of the planet and the haze in
which many of its features appeared to be en-
veloped indicate the regular succession of day and
night, each passing into the other by the insensible
gradations of morning and evening twilight; while
the tilt of the axis of rotation of the planet to the
plane of its orbit demonstrates the constant recur-
rence of seasons. In recognizing upon a planet
so many conditions essential to its well-being as a
world, it has been impossible to restrain imagina-
tion from supplementing actual discovery in regions
lying beyond the power of telescopic observation.
102 Recent Advances in Astronomy.
The lands and waters of Mars teemed with animal
and VegetabieVlife. In lands over which the cold
and, heat qf.winjter an4 summer and day and night
never ceased;, - seed-time and harvest were added;
while, passing their brief span of struggle and
passion, and fighting to maintain their mastery over
nature, were intelligent beings, towards whom, in
imagination, the right hand of fellowship was
longingly extended across a separating chasm of
nearly 50,000,000 miles.
For the greater part of the century that followed
Herschel's observations, although knowledge of
Martian detail steadily increased, little was added
to it materially to affect the nature of the picture
drawn by him. In the drawings of Beer and
Madler, Dawes, and other astronomers, as in the
exquisite pictures constructed by Green from his
study of the planet from Madeira during its
specially favourable appearance in 1877, the planet,
though shown in greater detail and perfection, was
essentially the Mars of Sir William Herschel, and his
view of Mars as "a miniature of the Earth" appeared
to derive additional confirmation. More recently,
however, interest in Mars has been reawakened
and maintained at the highest pitch by the alleged
appearance upon the surface of the planet of a
variety of detail of the most unexpected and per-
plexing kind. The " canal system" of Mars, the
first suspicion of which was suggested to Schiapar-
elli in 1877, if it has led to speculations that scarcely
add to the dignity of science, has renewed the youth
of Martian study, and has directed towards the
The Recent Study of Mars. 103
planet a keen scrutiny only rendered possible by
the construction of the great telescopes of modern
times. In the present chapter an attempt will be
made to trace the course of recent discovery upon
Mars, and to discuss, though necessarily imper-
fectly, certain views that have been suggested, and
some difficulties that have arisen, in the attempted
interpretation of the appearances.
The planet Mars lies next the Earth in order of
increasing distance from the Sun, the distance
of Mars being 141,500,000 miles, while that of the
Earth is rather less than 93,000,000. The respec-
tive distances are therefore in the proportion of
1-523 to i, or nearly of 3 to 2, a relation that
will be useful in the sequel. The diameter of
Mars is 4230 miles, that of the Earth being 7918
miles, from which it follows, from simple geometry,
that in volume the Earth exceeds Mars by 6-57
times. From disturbances produced by Mars in
the movements of other members of the Solar
System, by its gravitational attraction upon them,
it appears that its mass or quantity of contained
matter is less than that of the Earth in the pro-
portion of i to 9-34. Consequently, the Earth
being 9*34 times as massive while only 6*57 times
as bulky as Mars, the density or mass of a given
bulk of the Earth must exceed that of Mars in
the proportion of 9^34 to 6*57, or of 1-42 to i. It
also follows from these data that the intensity of
gravitation exercised by Mars upon bodies at its
surface must be less than that exercised by the
Earth upon bodies at its surface in the proportion
io4 Recent Advances in Astronomy.
very nearly of 2 to 5, that is, a body the weight of
which had been determined at the surface of the
Earth, would, if it were transferred to the surface of
Mars, weigh only two-fifths as much. Like the
Earth, Mars travels round the Sun in an orbit that
is, although nearly circular, slightly elliptical, the
planes of the two orbits very nearly coinciding.
The period occupied by the planet in completing
its orbit that is, the year of the planet is 686-9
days, the Earth's year being 365-26 days. The
longer period of Mars is due, partly to the greater
length of its orbit, and partly to the planet's speed
in its orbit being less than that of the Earth in its
orbit, a consequence of its greater distance from the
Sun.
Like all planets the orbits of which inclose that
of the Earth, Mars is seen to best advantage when
in " opposition " to the Sun that is, at the instant
at which the Earth, overtaking the planet in its
slower journey, passes directly between it and the
Sun. Under these conditions, not only is the dis-
tance separating the Earth from Mars less, and the
apparent size of the planet therefore greater, than
at other times; but the hemisphere of the planet
that is illuminated by the Sun is presented directly
towards the Earth, so that the disc appears " full ".
At other times, when the illuminated hemisphere
is not directly presented to the Earth, the planet
exhibits phases resembling those of the Moon when
not far from the full. The phases of Mars show
that, like the Earth and Moon, it is not inherently
luminous, but that it is rendered visible by sunlight
The Recent Study of Mars. 105
scattered from its surface. It is clear that, as is
the case with the Moon when full, a planet in
opposition to the Sun must rise in the east as the
Sun sets in the west, and, after ascending the
heavens during the evening hours and attaining
its greatest altitude in the south at midnight, must
descend towards the west in the early morning,
setting at sunrise. Hence, a further advantage of
a planet's being in opposition arises from its being
then visible through all the hours of the night.
If the orbits of the Earth and Mars were circles
lying in the same plane and having the same centre,
and if the Sun occupied the common centre, then,
at every opposition, no matter what position in its
orbit the Earth might happen to occupy, its distance
from Mars would be the same. Under such circum-
stances Mars would appear under the same con-
ditions at every opposition, and all would therefore
be equally favourable. These simple conditions do
not, however, exist. The planes of the orbits are
indeed so nearly coincident that their deviation from
perfect coincidence may for the present purpose be
ignored; but the forms of both orbits are ellipses,
deviating, especially in the case of the orbit of Mars,
appreciably from the circular form ; while, in accord-
ance with Kepler's first law of planetary motion, the
Sun is situated, not in the centre, but in a focus
common to each ellipse. The orbits of the Earth
and Mars, and the Sun's position relatively to them,
are represented to scale in the accompanying figure
(fig. 6), and it is interesting to notice that the ellip-
ticity of each orbit is indicated far more clearly in
io6 Recent Advances in Astronomy.
the displacement of the Sun from the centre than by
deviation from circularity in outline, which indeed,
even in the case of the more elliptical orbit of Mars,
is probably inappreciable to the most critical eye.
Ql899Jan.18.
O
901 Feb. 22.
1894 Oct. 20,
O
W88 April 11.
( Jl890May27.
Fig. 6. Oppositions of Mars.
7892 August 3.
From the eccentricities of the orbits of the Earth
and Mars, and from the position of the Sun rela-
tively to them, it follows that, in one direction
indicated by the line sx in the figure the distance
between the orbits, as measured along a straight
line radiating from the Sun, is least. If, therefore,
at the time that the Earth is crossing this line, Mars
also happens to lie upon it, an opposition will result
The Recent Study of Mars. 107
which will be the most favourable possible; the
planet then appearing brighter to the naked eye,
and presenting a larger disc when viewed through
the telescope, than at any other time; while other
oppositions will be more or less favourable accord-
ing as to whether the direction of the Earth and
Mars as viewed from the Sun is nearer or farther
from the line of most favourable opposition sx.
The Earth in its annual journey round the Sun
crosses the line sx upon the 26th of August in each
year; hence, the nearer to this date of occurrence,
the more favourable is an opposition of Mars.
Since the periods of revolution of the Earth and
Mars round the Sun are 365*26 and 686*9 days
respectively, it follows, from simple arithmetic,
that, upon the average, the Earth must overtake
Mars, and an opposition must therefore occur, at
intervals of 780 days, or nearly two years and two
months. This would be the constant interval be-
tween any two successive oppositions if the orbits
were circles with the Sun in their common centre,
and if, as would then necessarily be the case, the
speed of each planet were uniform. Owing, how-
ever, to the elliptical forms of the orbits, and to the
fact, expressed in Kepler's second law of planetary
motion, that the velocities of both the Earth and
planet vary with their distance from the Sun, the
intervals between successive oppositions are some-
times greater and at other times less than the aver-
age, the exact calculation for particular cases being
a very laborious matter. In the figure the positions
of the Earth and Mars are given for all oppositions
io8 Recent Advances in Astronomy.
occurring between 1886 and 1901. It will be seen
that oppositions are now (1898) becoming less and
less favourable, and that they will continue to
deteriorate until 1901, in which year an opposition
will occur under almost the most unfavourable con-
ditions possible. After 1901, however, improve-
ment will set in, culminating in fine oppositions in
1907 and 1909. The circular discs arranged outside
the orbit of Mars in the figure represent to scale the
relative apparent sizes of the disc of Mars as seen
from the Earth at the different oppositions. They
show in a striking manner the special advantages
attending oppositions that occur in the early autumn
months.
Viewed through a fine telescope and under favour-
able conditions, Mars, when in opposition, presents
a picture of singular beauty and charm. Markings,
some so distinct as to be clearly recognized at a
first glance, others less strongly pronounced, and
others again so faint as to tax the powers of the
keenest vision assisted by the finest optical power,
are distributed over the disc-like picture ; while the
beauty of the spectacle is enhanced by the presence
and variety of colour, and by exquisite gradations
of tint in different regions.
Upon continuing the study of the planet, it soon
becomes evident that change is in progress, not in
the form of the features themselves, but in their
positions relatively to the outline of the planet.
Details first seen near the centre of the disc have
drifted to the left; others, originally near the left
limb, have disappeared ; while others, previously
The Recent Study of Mars. 109
invisible, have appeared within the right-hand limb.
These changes clearly indicate, that, like the Earth,
Mars is in rotation. Further, it becomes apparent
that the period of rotation of the planet does not
differ very much from that of the Earth, for in little
more than twenty-four hours the picture presented
is again that originally seen. 1 Day and night, the
appearance of diurnal revolution of the heavens, as
well as all other celestial phenomena resulting from
the rotation of a planet, follow therefore upon Mars
with the same regularity, and at nearly the same
rate, as upon the Earth.
From the study of the planet's rotation it is a
simple matter to determine the position within it of
the axis about which the rotation takes place.
When this is done, it is found, that, as is the case
with the Earth, the axis of rotation of the planet
is inclined to the plane of its orbit round the Sun,
the inclination of the axis (24 50') curiously approxi-
mating in value to that of the inclination of the
Earth's axis to the plane of its orbit (23 27'). The
tilt of the axis of rotation gives rise to the phenomena
of seasons; hence upon Mars, spring, summer,
autumn, and winter follow with the unceasing
regularity familiar to inhabitants of the Earth.
A further point of similarity between the physical
conditions existing upon Mars and those upon the
Earth becomes apparent from the study of the
planet's rotation. As different features are carried
by the rotation towards the left-hand limb, they
disappear while still at an appreciable distance from
1 The period of rotation of Mars is 24 hours 37 minutes 23 seconds.
no Recent Advances in Astronomy.
it, melting into a luminous ring known as the
"limb-light", that appears to continually cling to
the outline of the planet, extending inwards for
some distance from the limb. In a similar manner,
features brought into view by rotation do not at
once appear as they are brought on to the disc, but
as gradually emerge from the limb-light upon their
side of the planet, only becoming distinctly visible
when the rotation has carried them a considerable
distance on to the disc.
The suggestion that Mars is enveloped in an
atmosphere similar in its physical properties to
that of the Earth offers so simple and sufficient an
explanation of the limb-light that it is scarcely
possible not to regard it as the true one, more
especially as the existence of an atmosphere upon
Mars is independently demonstrated from the
nature of changes continually in progress in the
visible features of the planet, to which attention will
shortly be directed. According to this view, the
appearance of the limb-light results from the
scattering of the Sun's rays in the atmosphere of
Mars. Simple considerations, such as, for instance,
the darkened tint of the sky when viewed from
high altitudes, indicate that the appearance of
the sky as a vault of deep-blue eternally extended
overhead is due to a scattering of the Sun's rays
in the atmosphere of the Earth. Tyndall has
shown by a series of experiments of extreme
beauty that this scattering is in all probability
effected by innumerable minute vesicles of water
floating in the atmosphere, and that the forma-
The Recent Study of Mars. in
tion of these vesicles is assisted by, or may
indeed be entirely dependent upon, the presence
of specks of dust, which form nuclei around which
condensation of the vapour of water present in
the atmosphere takes place. It is in harmony
alike with theory and experiment that the more
refrangible of the Sun's rays should experience
such scattering to a far greater degree than those
less refrangible ; so that, of the component colours
of a ray of sunlight penetrating a column of atmos-
phere, the more refrangible colours those near
the violet end of the spectrum should be scattered
in all directions around, while the less refrangible
the red and adjacent rays of the spectrum should
pass through more readily, a law illustrated in the
familiar fact of the great penetrative power of a red
light in a fog, and also supplying an explanation of
the transparent blue of the noonday sky and the
crimson colours of sunset. Assuming the existence
upon Mars of an atmosphere possessing similar
dispersive powers, the limb-light is simply and
naturally explained. Bathed in the Sun's rays
and containing floating matter capable of scatter-
ing them, the atmosphere of Mars would form a
luminous shell enshrouding the visible hemisphere
of the planet. The line of vision from the Earth
directed to the centre of the disc pierces this shell
perpendicularly ; the portion of its length included
in the shell is therefore the least possible, and the
illumination of the atmosphere is barely appreciable.
The line of sight to the limb, however, meets the
visible hemisphere tangentially, and, traversing the
ii2 Recent Advances in Astronomy.
air-shell very obliquely, its intercepted length is
great, and the illumination of the air very apparent.
From the edge towards the centre of the disc the
length of the line of vision involved in the atmos-
phere of the planet continually decreases, the
appearance of illumination therefore becomes less
and less, and the limb-light is the result.
The fading of the planetary features upon ap-
proaching the limb is further aided, first, by the
fact that as they approach the edge of the visible
hemisphere their actual illumination becomes less,
as does the terrestrial landscape towards sunset,
both from the increasing slant of the Sun's rays and
by the greater absorption exercised upon the rays
from the greater length of their atmospheric path; 1
and, secondly, from the greater length of the
Martian atmosphere through which they are viewed,
and the consequent increased absorption exercised
upon the rays in retraversing the atmosphere, after
reflection from the surface of the planet.
Explanations other than the one advanced here
have been suggested to explain the limb-light.
The appearance has been ascribed to the deposition
of hoar-frost, upon the approach of night, over
regions about to enter the dark hemisphere of the
planet; and to the lingering of the frost in the early
morning over those that have recently emerged
from it. This suggestion appears, however, to be
disproved by the observed symmetry of the limb-
light in the cold polar and warmer equatorial
regions of the planet; and by the fact that, in
x This only applies to Mars when in or near opposition.
The Recent Study of Mars. 113
observations made at times when Mars is not in
opposition, and when, consequently, its disc does
not appear to be full, the limb-light has been seen
to cling to the limb itself in preference to those
regions in which morning and evening are in-
dicated by proximity to the terminating line sepa-
rating the dark from the bright hemisphere.
The tenuity of the veil spread over the illuminated
hemisphere by the atmosphere is generally taken to
indicate that the surface density of the atmosphere
that is, the quantity of air accumulated over each
square mile of surface is less in the case of Mars
than in that of the Earth. It is probable, that if
the surface density of the Martian atmosphere were
equal to that of the Earth, its veiling effects would
be far more pronounced than they are; and that,
even in the centre of the disc, the surface markings
would be permanently concealed beneath a brilliant
haze. Recognizing that the appearance of the sky
is the result of the scattering of sunlight in the
Earth's atmosphere, it will be apparent that, to
an observer who should ascend above the highest
reaches of the air, the Earth would appear upon a
clear day to be veiled by the blue haze of the sky,
now lying between him and the landscape beneath.
From actual measurements of the brightness of
the sky carried out by Langley it has been con-
cluded that to such an observer all terrestrial
features except the most brilliant would be scarcely
visible, their fainter light being overwhelmed by the
more intense glare of the intervening atmosphere.
To a possible inhabitant of another planet, provided
(M520) H
ii4 Recent Advances in Astronomy.
with adequate instrumental means, the Earth would
appear as a dazzlingly brilliant, but probably a
nearly uniformly illuminated orb. The ice-bound
regions near the poles and the snow-clad summit of
a mountain, might, here and there, be clearly dis-
tinguishable in the general luminosity of its disc,
but it would scarcely be possible to trace upon it
any of the more familiar terrestrial features. Upon
Mars, however, surface markings are clearly re-
cognized unless fairly close to the limb, while those
in the centre of the disc appear to experience but
slightly the effects of atmospheric veiling. It is
therefore commonly assumed that in the density of
surface distribution of atmosphere, Mars is poorer
than the planet Earth.
It must be acknowledged that this reasoning,
though lending a strong probability to the view, is
not quite conclusive. It essentially rests upon the
assumption that the power of an atmosphere to
scatter light may be taken as a measure of its
density. It has been seen, however, that the
scattering of light is effected by solid and liquid
matter suspended in the air, and is not, therefore,
an inherent property of an atmosphere itself. Were
there no floating matter in the Earth's atmosphere,
there would be no scattering of light within it. The
blue sky, even in the immediate neighbourhood of
the noonday sun, would under such conditions be
replaced by a vault of intense black, in which, by
day as by night, the stars would shine with a lustre
unknown even on the clearest and darkest nights.
Absorption of light would still occur, but to so
The Recent Study of Mars 115
slight an extent such is the transparency of pure
air as to be barely appreciable ; while it is hardly
necessary to state that the absorbed light would be
entirely extinguished, and that no appearance of a
sky could result from it. That the atmosphere of
Mars contains floating matter in proportion to its
density may be true, but it is an assumption that it
is not possible to verify. It will be seen later that
there are indications of a scarcity of water on the
surface of Mars, and that there is a very strong
probability that its atmosphere is charged with the
vapour of water to a far less extent than is the
atmosphere of the Earth. It is probable that the
condensation of the vapour of water plays an im-
portant part in the dispersive action of the atmos-
phere on light, and that, therefore, under conditions
otherwise similar, a less moist atmosphere would
possess a feebler scattering power than another.
That Mars possesses a more tenuous atmosphere
than that of the Earth may be probable, but an
equal or even a greater density is not inconsistent
with the telescopic aspect of the planet. 1
Spectroscopic evidence bearing upon the question
of the Martian atmosphere is so curiously conflicting
that it is perhaps better to wait for further observa-
tions before taking it seriously into account.
Of the different features apparent upon the disc of
Mars, generally the most conspicuous are two white
patches, nearly always visible in the neighbourhood
1 The a priori arguments, based upon the relative volumes and masses of
Mars and the Earth, that are frequently adduced as evidence for a rare
atmosphere on the planet appear to possess little if any value.
n6 Recent Advances in Astronomy.
of the poles, though not arranged symmetrically
round them. So brilliant are they, that they have
been seen sparkling like twin stars at times when
the sky has been covered by haze to such an extent
that the outlines of the planet itself have been in-
visible. From their general appearance, as well as
from their situation in the immediate neighbourhood
of the poles, they have been regarded as accumula-
tions of snow and ice, similar in their nature and in
their mode of formation to the polar caps of the
Earth. This conclusion is strongly supported by
the nature of the changes apparent in both of them
during the progress of the Martian seasons. Upon
the approach and during the continuance of winter
in either hemisphere of Mars, as, in the orbital
movement of the planet, the hemisphere is turned
from the Sun, the white cap surrounding its pole
continually increases, its boundaries extending
farther and farther towards the equator; while later,
during spring and summer, as the hemisphere is
again turned towards the Sun, its white covering
dwindles in dimension, becoming generally reduced
to an insignificant oval patch. Upon a recent occa-
sion, indeed, when the planet was near its opposition
in 1894, tne south polar cap entirely vanished, the
substance composing it having been apparently
dissipated beneath the rays of the summer sun.
Other appearances, more rarely recognized in the
caps and in their immediate neighbourhood, lend
additional support to this view. On June 8th, 1894,
Mr. Lowell, while observing Mars from Arizona,
saw two points of light of dazzling brilliancy flash
The Recent Study of Mars. 117
out in the midst of the south polar cap. For a few
moments they sparkled in the surrounding white-
ness and then disappeared. It is difficult to resist
Mr. Lowell's interpretation that their appearance was
due to the glint of ice-slopes flashing the sunlight
towards the Earth, as, during the rotation of the
planet, the slopes were for a few moments placed at
the proper angle to the rays. Similar appearances
had been noticed by Mr. Green during the opposi-
tion of 1877, but in this case they were seen near to,
but not actually involved in, the polar cap.
Distributed round the planet in a rough zone ap-
preciably parallel to its equator, and extending over
considerably more than a half of its entire surface,
are a number of patches, generally of a soft rounded
outline, and of a colour that has suggested the
orange-yellow of a field of ripe corn. It is to these
that the planet owes the familiar ruddy tint that has
caused it to be associated in name with the god of
war. Bounding, and frequently deeply indenting,
these orange masses are regions of a gray-green
tint, and these complete the picture of the planet's
surface as seen with moderate optical power. En-
couraged by the close similarity in appearance and
behaviour between the polar caps of Mars and the
Earth, it has been the custom to pursue the analogy
further, and to see in the orange patches and in the
gray-green markings upon Mars the continents and
oceans of a miniature world.
That the orange masses upon Mars are indeed
land appears probable, from the similarity in their
appearance to that which it may be well supposed
n8 Recent Advances in Astronomy.
the great deserts of the Earth would present under
similar conditions of observation, as well as from
the permanent appearance upon them of delicate
markings revealed by higher optical power; though,
perhaps, as strong an argument as any lies in the
difficulty of suggesting any other explanation for
their appearance. That the gray-green markings
are the surfaces of Martian seas appears at a first
glance a scarcely less plausible suggestion. Their
colour is not unlike that of water; the existence of
extensive tracts of water upon Mars harmonizes well
with the view of the polar caps as accumulations of
snow and ice ; and their aqueous character was sup-
posed to have received its final confirmation in 1867,
from the announcement of Sir William Huggins,
that, from the spectroscopic examination of the light
from Mars, he had detected the existence of the
vapour of water as a constituent of its atmosphere.
Of late years, however, considerable doubt has been
thrown upon this rather attractive view. In 1877
Schiaparelli of Milan maintained that, if the gray-
green markings were the surfaces of water, they
should occasionally, when turned at the proper
angle to the directions of the Sun and Earth, unless
indeed their surfaces were continually in a state of
violent disturbance, reflect the Sun's rays in such a
manner that its image should appear as a bright
star sparkling upon them. It is not difficult, upon
the assumption that the water surface is clean and
still, to calculate the intensity of the solar image
that should be formed under the actual conditions,
and it appears that it should be so brilliant as to be
The Recent Study of Mars. 119
readily capable of recognition. No such appear-
ance has, however, ever been recorded upon the
disc of Mars.
A very interesting though not in itself a con-
clusive observation has recently been made by Pro-
fessor W. H. Pickering, in the examination of Mars
under the polariscope. It is well known that the
fraction of light that is regularly reflected from the
surface of any transparent substance exhibits the
phenomena of polarization it is capable of being
again reflected more or less effectively by a second
transparent surface, according to its direction of
incidence upon it; it is transmitted through certain
crystals such as tourmaline more or less readily,
according to the direction of the axis of the crystal;
and upon traversing many crystals, and in its
subsequent analysis by a second polarizing appa-
ratus, it is capable of developing the exquisite effects
of colour familiar to many observers with the micro-
scope. It is a simple matter to detect polarization
in light reflected from a plane glass or a water sur-
face, especially for certain angles of incidence, and
in the light of the sky, which is strongly polarized
as the result of its scattering by water vesicles
suspended in the atmosphere. In 1894 Pickering
examined the light from the gray-green markings
upon Mars with a specially-constructed polariscope,
but failed to detect in any of them any trace of
polarization.
There is no doubt that if polarization had been
evident in the light of the gray-green markings,
their liquid nature would have been demonstrated;
120 Recent Advances in Astronomy.
but the interpretation of the negative evidence is not
so definite. Polarization would be produced by
regular reflection by which is meant reflection as
from a mirror, the angle of incidence being equal to
that of reflection or it might conceivably result
from the scattering of light by fine particles sus-
pended in a body of water, in a manner analogous
to that by which the appearance of the sky is pro-
duced. If, however, the surface of water were not
clean, the impurities upon it would scatter light
incident upon it in all directions, and in light so
scattered no polarization should be apparent; there
would, in fact, be no water surface exposed to the
light, but a dirt surface concealing a water surface
beneath. Pickering's observations would appear to
indicate that if the aqueous view of the gray-green
markings is to be retained, it must be modified in
this direction. The same modification would also
account for the non-appearance of the image of the
Sun upon the surfaces of Martian seas.
Still more recent and direct observations appear
to involve a complete refutation of the aqueous
character of the gray-green markings on Mars.
Mr. Douglass at Arizona in 1894, an ^ M r - Barnard
at the Lick Observatory in 1896, have succeeded in
distinguishing over the entire surfaces of them a
considerable amount of delicate and permanent
detail, an intricate tracery clearly inconsistent with
the older view as to their nature. Mr. Barnard, in
particular, examining the planet with the superb
refractor of the Lick Observatory upon Mount
Hamilton, under atmospheric conditions that fre-
The Recent Study of Mars. 121
quently approximated to perfection, describes the
detail revealed in the regions of the so-called seas
as being so intricate, small, and abundant, that it
baffled all attempts to properly delineate it. He
suggests that, to those who have looked down upon
a mountainous country from a considerable eleva-
tion, some conception of the appearance presented
may be formed. From the appearance of the
country round Mount Hamilton as seen from the
observatory, it was possible to imagine that, as
viewed from a great altitude, this region, broken by
canon, slope, and ridge, would closely resemble
the surface of the Martian seas. During the obser-
vations the conviction seemed to force itself upon
the observer that he was actually looking down
from a great elevation upon just such a surface
as that above which the observatory was situated.
It appears, therefore, that if water exists at all
upon Mars in the liquid form, it must be sought
elsewhere than in the so-called seas; and it is
possible that, in an observation made in 1894 by
Mr. Lowell and Professor Pickering, its place upon
the surface of the planet was revealed for the first
time. During a careful study of Mars, when near
its opposition in that year, with the aid of a fine
refracting telescope of 18 inches of aperture, there
appeared a dark belt forming a fringe to the south
polar cap. The belt first appeared after rather
more than a Martian month following the spring
equinox of the planet. It was estimated as being
the darkest marking on the disc, and appeared to
be of a decidedly blue colour. As the polar cap
122 Recent Advances in Astronomy.
dwindled, the belt followed, clinging to its edge.
At midsummer upon Mars it was described as a
barely discernible thread drawn round the minute
white patch, which was all that then remained of
the enormous snow-fields of some months before.
Finally, when the cap vanished, the spot, where
its girdle, long since too small for detection, had
existed, had become one yellow stretch. 1
That the belt seen upon this occasion was water,
or at any rate liquid formed by the melting of the
polar cap, appears a plausible suggestion, and
appears more probable from the fact that Professor
Pickering, on subjecting it to examination with
the polariscope, was convinced that its light showed
marked evidence of polarization. The interpreta-
tion of the sequence of the observed phenomena
appears to be that the melting of the polar cap
gave rise to a fringing belt of liquid, which first
appeared as such, but was rapidly distributed over
the summer hemisphere in streams too fine for
detection.
A dark belt surrounding the north polar cap had
been seen as early as 1830 by Beer and Madler,
and other like appearances, which may have been
of the same nature, have been recorded by other
astronomers.
The atmosphere of Mars appears to be in striking
contrast with that of the Earth, in its almost entire
freedom from cloud or mist. From time to time
during the study of the planet, extensive regions
have appeared, sometimes for a considerable time,
1 Mars, by Percival Lowell
The Recent Study of Mars. 123
to be indistinct, the result, it has been generally
supposed, of accumulated cloud or mist; but as
more perfect optical means have been applied, and
as observations have been conducted from localities
specially selected for their atmospheric steadiness
and the consequent improvement in the definition
of the telescopic picture the appearances have
become less and less frequent. During the entire
course of a series of observations upon the planet,
continually maintained at Flagstaff in Arizona,
under the direction of Mr. Lowell, from May to
the end of November, in the year 1894, no case
of obscuration that could be ascribed to cloud or
mist was recorded by anyone of the three astrono-
mers engaged in the work, with the exception,
perhaps, of some minute white specks, limited in
position to the immediate neighbourhood of the
line of division between the bright and dark hemi-
spheres, and which may have been transient morning
and evening clouds. During the actual progress
of these observations, however, other astronomers,
observing Mars with less perfect optical means and
under less favoured atmospheric conditions, believed
that they recognized one of the most extensive for-
mations of cloud that has ever been recorded. To
Mr. Stanley Williams at Brighton, for instance, the
greater part of the Miraldi Sea, one of the largest,
darkest, most definite, and most characteristic of the
green regions, disappeared almost entirely from
view, apparently densely obscured by cloud or mist. 1
There is no doubt that changes in the tint of several
i Observatory, 1894, p. 391.
124 Recent Advances in Astronomy.
regions of the planet's surface are of frequent occur-
rence, and it is possible that such changes, which
may cause the disappearance of detail, especially
if accompanied by a general lightening of tint,
may have been interpreted as cloud and mist. In
the apparent absence of cloud, and, consequently,
of rain, upon the surface of the planet, it is probable
that the polar caps are formed by the continued
deposition, as hoar-frost, during the long Martian
winter, of the vapour of water or of some other
liquid present in the atmosphere.
When the planet was near its very favourable
opposition in 1877, Schiaparelli at Milan, while
observing with a telescope of rather over 8 inches
in aperture, detected certain faint dusky lines pro-
jecting from the gray-green regions well into the
interior of the orange continents. The streaks
appeared to be most conspicuously visible shortly
after the mid-winter of the hemisphere in which
they appeared. At the rather less favourable
opposition of 1879, the streaks first seen were traced,
accompanied by others; to Schiaparelli they ap-
peared to be more sharply defined than before;
while one of them appeared to be double, consisting
of a pair of parallel streaks separated by a distance
of between one and two hundred miles. As before,
as well as at succeeding oppositions, the streaks,
to which Schiaparelli had now given the most un-
fortunate name of "canals", appeared more clearly
during the latter part of the Martian winter and
the early spring. At succeeding and increasingly
unfavourable oppositions, the numbers, length,
The Recent Study of Mars. 125
and instances of duplication, of the canals were
steadily increased; in character they seemed to be
more rectilinear and more sharply defined ; and,
to Schiaparelli, they at length appeared to form
a reticulated network, extended over nearly the
whole of the orange continents. Until 1896 to
no other observer had the canals so much as ap-
peared, but in that year a few were recognized
by Perrotin and Thollon at Nice, by the aid of a
then newly - constructed telescope of 29 inches of
aperture. As oppositions again became more
favourable, however, they appeared to quite a
number of astronomers supplied with the most
ordinary instrumental means; and they have now
become entirely notorious.
According to the evidence of astronomers to
whom they have appeared, the canals are faint
lines that appear to become finer and straighter
as the eye becomes accustomed to their appearance.
Their width is estimated as being not less than
fifteen, or more than sixty, miles. They follow,
as closely as can be seen, the course of great circles
upon the surface of the planet, 1 and can be fre-
quently traced for upwards of 1500 miles. They
mutually intersect in a most remarkable manner,
several of them frequently passing through the
same point, from which, again, it is not uncommon
to find other canals originating, so that the entire
1 A great circle of a sphere is the circle that divides it into two equal parts.
It is the largest circle that can be drawn upon the surface, and its course
marks the shortest line that can be drawn between two points on the surface.
Great circles are illustrated in meridians of longitude. Parallels of latitude
are known as small circles.
126 Recent Advances in Astronomy.
surface of the planet appears as if involved in a
complicated network of delicate tracery. Before
the year 1894, canals had only been recognized
upon the orange continents; but as the planet
approached opposition in that year they appeared
to Mr. Douglass at Flagstaff to be distributed
scarcely less richly over the green of the so-called
seas. Points of intersection of canals are frequently
emphasized by the occurrence at them of round or
oval dusky spots, which have received the name of
" lakes". The system of canals and their associated
lakes varies in visibility with the Martian seasons,
being commonly invisible during winter, gradually
appearing in the early spring, and again disappear-
ing during the progress of late summer and autumn.
It has been suggested that the canals of Mars
are waterways, and that their emergence from
invisibility upon the approach of spring may be
due, either to the dissipation of their winter cover-
ing of ice and snow, or from their becoming
extensively flooded by large volumes of water dis-
charged into them by the melting of the polar
snows. Their light revealed no trace of polarization
when examined by Professor W. H. Pickering,
from which he has advanced the conjecture that
the streaks may be tracts of country lying upon
either side of water-courses, themselves too fine
for detection, and irrigated by them, rather than
water -courses themselves; and that their appear-
ance in the spring may be due to a general growth
of early vegetation over them as they become
fertilized by the flooding of the streams.
The Recent Study of Mars. 127
The apparent regularity of the canals, as well as
the difficulty of suggesting any other explanation
for them, have been at times regarded as indicating
artificiality. According to Mr. Lowell, who is a
strong advocate of this view, the canals and their
connected lakes, which, according to this view,
may be more suitably regarded as oases, are the
visible result of an extensive system of irrigation
carried out by intelligent beings on Mars. For the
inhabitants of Mars, as for man, water is a necessity
of life; and since water appears to be scarce on the
planet, being, indeed, apparently only to be ob-
tained from the melting of the polar snows, the
inhabitants have, with consummate engineering
skill, constructed an extensive network of channels,
extending from the polar regions over the entire
surface of the planet. By these channels, upon the
melting of the polar snow, the lower lands are well
supplied with water, the vegetation springing up
on them giving rise to the appearance of the gray-
green tracts ; while the irrigation of the higher and
desert districts is confined to the immediate neigh-
bourhood of the channels, and results in growth of
vegetation over belts of country irrigated by them
on either side, and the oases at their junctions.
From these follow the appearances of the " canals"
and " lakes".
So much for the outlines of a romance, the lead-
ing features of which have become, largely through
the co-operation of "our own correspondent" and
Mr. Lowell, familiar to the greater number of
readers of the daily press about the times of recent
128 Recent Advances in Astronomy.
oppositions of Mars. To those not practically ac-
quainted with the extreme delicacy involved in the
telescopic observation of detail so faint as just to
hover upon the verge of the visible and the unsee-
able, it must appear that, to the concurrent testi-
mony of so many laborious observations, conducted
by astronomers, some of established reputation, and
the greater number of unquestioned honesty of
purpose, there can be but one interpretation; and
that, upon the surface of Mars, features and a sys-
tem alike unique in the revelations of the Universe
have been firmly established. The examination of
more complete evidence, however, suggests grave
objections to the unhesitating acceptance of this
view, while to many thoughtful observers it has
appeared hard to escape from the conclusion that
the complicated meshes of the canal system upon
Mars must be regarded as little more than optical
illusions, faulty interpretations of the faintest shades
of tint, the exact nature of which has not so far
been established.
Although originally discovered, and the courses
of many of them traced, by the aid of a telescope of
scarcely more than 8 inches in aperture ; although
continually seen in England and elsewhere through
instruments of still less power; and although, by
such aid, the surface of Mars has been mapped by
harsh black lines in a manner that suggests the
transformation of a world into a gigantic shunting-
yard, the canals, at any rate in their generally
assumed characteristics, have consistently refrained
from appearing upon the picture of the planet
The Recent Study of Mars. 129
formed in many of the finest telescopes in the
world, directed by astronomers who, in other and
independent work, have earned the highest reputa-
tion for keenness of vision. Through the Wash-
ington refractor of 26 inches of aperture, the
instrument by which, in 1877, Professor Hall first
detected the moons of Mars, the canals have never
been traced. Dr. Keeler, of the Alleghany Obser-
vatory, made a special study of Mars when near its
opposition in 1892 with a refractor of 13 inches of
aperture. During the course of the observations
the definition of the planetary outlines was fre-
quently so excellent that the moons of Mars were
clearly visible in the field of view; but although
certain ill-defined shaded streaks were recognized
near the recorded positions of canals, no trace of
their hard rectilinear character, or of their marvel-
lously reticulated system, was detected. Near the
time of the opposition of 1894, Mr. Barnard, at the
Lick Observatory, frequently directed the great
telescope of 36 inches of aperture, the instrument
by which he had already discovered the fifth moon
of Jupiter, towards Mars. At times, when the
seeing was most perfect, although the gray-green
regions of the planet appeared richly covered by
delicate and intricate detail, the very suspicion of
which had never been suggested to other observers
to whom the canals had been so startlingly con-
spicuous, features were, indeed, recognized upon
the orange continents, but they were for the most
part irregular, and consisted only of delicate gra-
dations of light and shade. There was no appear-
(M520) I
130 Recent Advances in Astronomy.
ance of hard, sharp lines. A few short, hazy
streaks in the neighbourhood of the "Lake of the
Sun " appeared as nearly the sole representatives of
the Martian canals.
To explain the inconsistency apparent in these
and other similar observations, it is not for a
moment necessary to assume any want of good
faith on the part of astronomers to whom the
system of the Martian canals has appeared in all
its wonderful complexity. Experience has fully
shown, as eveiy observer with the telescope has
soon become painfully aware, to what a serious
extent the eye may be deceived in its interpretation
of details so faint as just to hover upon the verge
of vision, and how readily unconscious bias, the
result of even faintly preconceived ideas, may affect
the judgment. Illustrations are abundantly sup-
plied in the history of astronomical observation, and
it will be sufficient to give three, selected almost
entirely at random. In comparing recent photo-
graphs of the nebulas surrounding the star tj Argus
with the beautiful drawing of the same object made
by Sir John Herschel during his residence at the
Cape, differences of so startling a nature are found
as to administer a severe shock to those who would
put their trust in the eyes of man. The curiously
definite border assigned, even by so careful an
observer as Sir John Herschel, to a dark space in
the nebula, known as the "key-hole", when com-
pared with the perfect shading of light into dark-
ness shown in the photograph, is an indication of
the tendency of the eye to assign to excessively
The Recent Study of Mars. 131
faint details a sharpness and a regularity that they
do not possess. In inspecting sketches of the
delicate detail of the Corona of the Sun, made at
the same time and from the same place by different
observers, it is frequently difficult to believe that
the same object has been represented. Drawings
of the Milky Way, as seen by the naked eye, have
been recently executed by two independent obser-
vers, Dr. Boeddicker and M. Easton, each drawing
the result of long and arduous observation, but, in
comparing them, it is the exception rather than the
rule to find any approximation in agreement in
respect of the more delicate features.
Altogether it appears scarcely possible to avoid
the conclusion that the existence of the canal system
has not been established. There is no doubt, how-
ever, that in the course of a few years further light
will be forthcoming upon the problem. At present
the planet is becoming at each appearance less
favourably situated for observation; but upon the
return of favourable oppositions in 1907 and 1909,
its disc will be scanned with an attention thoroughly
aroused by the conflict of recent evidence, and with
the aid of more powerful instrumental means than
have hitherto been available. It might also be well
if each observer should, before attacking the main
problem, subject himself to a severe examination,
in sketching through his telescope a number of
illuminated distant discs, on which faint markings
had been traced, but of a nature unknown to him.
A personal tendency might be detected by the
subsequent comparison of the drawings with the
132 Recent Advances in Astronomy.
discs, which should serve as a valuable check upon
his subsequent observations of Mars. By the com-
parison of a number of such carefully corrected
records, it might be confidently anticipated that the
riddle of the canals of Mars would receive its final
solution.
It has frequently appeared a grave difficulty in
interpreting Martian phenomena that the apparently
mild climate, to which they have been generally
thought to point as existing upon the planet, is
inconsistent with its great distance from the Sun.
There can be little doubt that if the Earth were re-
moved to the distance of Mars, it would, by reason
of the diminished intensity of solar radiation, become
so much cooler, that nearly if not the whole of its
oceans would be eternally bound in ice. Yet upon
Mars the polar caps do not extend to lower latitudes
than do those of the Earth, while the polar ice on the
Earth is never reduced during the hottest summer
to the insignificant remnant by which it is generally
represented in summer upon Mars. From these
facts it has frequently been assumed that the climate
of Mars is actually warmer than that of the Earth.
Although it is not possible to make an exact esti-
mate of the fall of temperature that would result if
the Earth were removed to the distance of Mars from
the Sun, a simple illustration will indicate its very
serious extent. Taking the approximate ratio of 2
to 3 to indicate the relative distances of the Earth
and Mars from the Sun, it follows that, since the
intensity of radiation is inversely proportional to the
square of the distance of the radiating body, the
The Recent Study of Mars. 133
heating effect of the Sun's rays at every place upon
the Earth's surface would be reduced by the square
of %, that is, to 4/9 of its present value. The heat-
ing effect of rays further depends, however, upon
the angle at which they are incident upon the surface
that absorbs them. The greater the obliquity, the
less the heat developed upon equal areas, since the
more slanting the incidence the larger the area over
which the rays of a given columnar bundle would
be distributed. In passing from the equator to
either pole, for instance, the heating effect of the
Sun's rays continually decreases, as the surface
covered by bundles of rays of equal section increases.
Let us now suppose the Earth to be at an equinox,
and that it is regarded by an observer stationed upon
the Sun. Imagine two parallel zones, each a mile
in width, to be described entirely round the Earth
upon its surface, one at the equator, and the other
in latitude 63. It can be shown by an exercise in
elementary geometry that the second is, by reason
of its lesser circumference, 4 / 9 of the first in area, and
that, as seen from the Sun, it appears from this cause,
and also from its obliquity to the direction of vision,
to be (V 9 ) 2 or l6 / 8l as large as the equatorial one.
This fraction then is the proportion between the
angles subtended at the Sun by the zones, and it
therefore also represents that of the quantities of
heat received by them in equal times. The smaller
zone therefore receives ( 4 /g) 2 of the heat of the larger,
but, as its surface is only / 9 as great, the heat re-
ceived by a given area of the smaller zone is 4 / 9 of
that received by an equal area of the larger one.
134 Recent Advances in Astronomy.
But it has been shown that, if the distance of the
Earth from the Sun were increased to that of Mars,
the heating effect of the solar rays everywhere upon
its surface would be reduced by this amount. Hence
the equatorial heating effect upon the Earth, if it
were transferred to the position of Mars, would be
that at present found in a zone of 63 latitude. The
parallel of 63 north latitude skirts the south of Ice-
land, it passes through Finland, and it traverses
Northern Siberia, Alaska, the Hudson Bay Terri-
tory, and Greenland; and there is little doubt that
the temperatures of these regions are upon the whole
higher than would result from the direct radiation
that they receive, since the Arctic and adjacent
regions are warmed by currents of air from lower
latitudes. Were the Earth, therefore, transferred
to the distance of Mars, it might be confidently
anticipated that, even at its equator, its climate
would be of Arctic character.
The phenomena visible upon Mars do, however,
suggest, though they do not probably demonstrate
as conclusively as has frequently been assumed, that
the temperature of the planet is very different from
this, and attempts have been made to imagine some
means by which the climate of the planet may be
rendered less rigorous than its small allowance of
solar heat would suggest. The possibility, one
indeed that has never been seriously maintained,
that the interior of Mars may be hotter than the
Earth, and that its surface may be appreciably
warmed by the outward flow of heat from its in-
terior, may be briefly dismissed. It is probable
The Recent Study of Mars. 135
that the interior of Mars, like the interior of the
Earth, is hotter than the surface ; but from the pro-
bability indicated by the nebular hypothesis, that
Mars was developed in a highly heated condition
before rather than after the Earth, and from the
certainty that its cooling must, by reason of its
smaller size, have proceeded far more rapidly, Mars
should be the colder of the two planets. Since, in
addition, measurements have shown that the heat
conducted from the interior to the surface, even in
the case of the Earth, is entirely insignificant in
amount when compared with that received from the
Sun, and is, therefore, a negligible quantity in
directly affecting climate, it would appear impossible
that the climate of Mars should be sensibly affected
by the internal heat of the planet.
The only serious attempt that has been made to
account for the assumed mild climate of Mars is
based upon the property of selective absorption,
exercised in some degree by all, and in a very
marked manner by many, transparent substances.
Selective absorption is illustrated to a remarkable
degree in glass. If a plate of clear glass be held
between the Sun and a thermometer, the bulb of
which should be blackened upon the exterior to
prevent the reflection of rays from the metallic sur-
face of the enclosed mercury, it will be found that
the indication of the thermometer is scarcely affected,
the glass, transparent to light, being similarly trans-
parent to the greater part of the rays that, upon their
incidence on the blackened surface of the thermo-
meter, develop heat. If, however, the same glass is
136 Recent Advances in Astronomy.
interposed in the course of rays proceeding from a
red-hot fire to the thermometer, a fall in temperature
will be indicated, almost as great in amount as if an
opaque screen had been substituted for the glass.
Glass, therefore, is very transparent to the heat
radiation of the Sun, but is practically opaque to
that of a red-hot fire. It is to this last property that
the efficiency of a glass fire-screen is due. Speaking
generally, it is found that the higher the temperature
of a body the more transparent is glass to the heat-
ing effect of its radiation; and this, not from the
greater intensity of the radiation of a hotter body
for the experiment already described succeeds equally
well even if, as may well be the case, the glass is
placed so close to the fire that its radiation is, owing
to its close proximity, more intense than that of the
Sun on its arrival at the surface of the Earth but
by reason of some property impressed upon the
radiation by the source. The wave theory of light,
and of radiation in general, leaves little doubt that
the difference in question is one of rapidity of the
vibration of the ether of space as it transmits the
waves that constitute radiation, but it is unnecessary
here to go beyond the actual property of selective
absorption as demonstrated by experiment.
The familiar fact that upon a clear day the air in-
side a greenhouse may be raised by the Sun's rays
to a temperature far in excess of that outside, is
frequently advanced as an illustration of the effects
of selective absorption, though probably in this case
the prevention of circulation of the enclosed air is
partly responsible for the rise in temperature. The
The Recent Study of Mars. 137
rays of solar radiation traversing the glass with
readiness arrive at and are absorbed by the surfaces
of the plants and other objects exposed to them.
These, becoming heated in consequence, radiate
their acquired heat in rays of essentially different
character, which, being effectually absorbed by the
glass covering, cause it to be heated by them.
Consequently the interior, heated now both by
radiation from the Sun and from the warmed glass,
acquires a temperature which is frequently far in
excess of that which would result if the glass had
allowed a free path to the radiation from the objects
within.
Tyndall has shown that a closely similar selective
absorption may be exercised by many gases and
vapours. He was unable to detect the property
with certainty in dry air, but the presence of a very
small quantity of the vapour of water in the air
produced it in a marked degree. From the results
of experiments arranged with considerable care and
skill, he arrived at the conclusion that the vapour of
water present in the atmosphere exercises an im-
portant influence upon the meteorology of the Earth,
permitting the transmission through the air of the
solar rays, but largely arresting the heat upon its
return, by absorbing the radiation from the warmed
Earth. It appeared, indeed, that under conditions
in which the atmosphere contains an average amount
of water- vapour in England, as much as 10 per
cent of the Earth's total radiation should be arrested
within 10 feet of its surface.
Based upon the selective absorption of water-
138 Recent Advances in Astronomy.
vapour, the interesting speculation has been ad-
vanced that a mild climate upon Mars may result
from the distribution throughout its atmosphere of
water-vapour, in quantity so abundant that, by the
efficiency of its trapping effect upon the solar radia-
tion, it should more than atone for the great distance
of the planet from the Sun. There is, however,
considerable difficulty in imagining such a state of
saturation of the Martian atmosphere as probable,
or even possible. Although the physical conditions
at the surface of Mars do appear to be in some re-
spects favourable to the formation of water-vapour
in its atmosphere, in others they appear to be ex-
tremely unfavourable ; and though it is not possible
to estimate accurately the opposing conditions, there
can be little doubt of the unfavourable ones being
the far more effective of the two.
The conditions upon Mars favourable to the
existence of the vapour of water in its atmosphere,
consist in the low intensity of gravitation at the
surface of the planet, and the probable tenuity of
its atmosphere. The amount of water that is ca-
pable of existence in an atmosphere in the state of
vapour is entirely independent of the density of
the atmosphere; experiment showing that although
evaporation takes place more rapidly into rare air
than into dense, yet the amount of vapour ultimately
formed is the same, and is still the same even if
the space into which evaporation takes place was
initially a vacuum. In every case evaporation pro-
ceeds until the vapour immediately in contact with
the evaporating surface has acquired a definite
The Recent Study of Mars. 139
density, a density increasing with, and solely de-
pending upon, the temperature, after which evapora-
tion ceases. In the case of a planet, evaporation
of surface-water would therefore continue until an
atmosphere of the vapour of water had been formed
that should, independently of any other atmosphere
present, and therefore solely by its own weight,
produce such a density in the vapour at the surface
as would prevent further evaporation. Since the
intensity of gravitation at the surface of the Earth
is two and a half times that at the surface of Mars,
an extension of water vapour two and a half times
that necessary to prevent evaporation at the surface
of the Earth would be possible in the atmosphere of
Mars.
In the preceding argument it was necessary to
assume a very simple condition that evaporation
rhould steadily continue until the whole atmosphere
had become saturated. It is scarcely necessary to
add, that, in the atmosphere of the Earth, this is
very far from being the case. Owing to alterations
in temperature in extensive bodies of air due to
different meteorological changes condensation is
continually occurring, resulting in the formation
of cloud and mist, and the atmosphere of the Earth
as a whole is, at all times, very far from being
saturated. No doubt such condensation would also
occur on Mars, but it is probable, from the low
intensity of gravitation, that the meteorological
changes would be less violent, and that condensa-
tion would therefore be less copious. Also, under
conditions otherwise similar, evaporation would
140 Recent Advances in Astronomy.
take place more rapidly into the rarer atmosphere,
and the loss of vapour due to condensation would
be more quickly restored. Perhaps the most de-
finite form in which it is possible to express the
general conclusion is that if the intensity of gravi-
tation upon the surface of the earth were reduced,
and if, at the same time, the density of its atmos-
phere were diminished, there is little doubt that
the atmosphere as a whole would be more richly
charged with the vapour of water than it actu-
ally is.
In some respects, therefore, the physical condi-
tions existing on Mars appear to be favourable to
the formation of the vapour of water in its atmos-
phere. On the other hand, other conditions appear
to be so extremely unfavourable that it is difficult
to believe that these can be of much avail. Evapora-
tion is the direct result of the radiation of the Sun
acting upon the surface of water. Not only is
the intensity of the solar radiation upon Mars less
than one-half of its amount on the surface of the
Earth, but the water surface exposed to it appears
to be woefully restricted. The greater part of the
Earth's surface is occupied by water. A nearly
continuous tropical belt of ocean is exposed day
after day to the direct radiation of a vertical sun.
Upon Mars, on the contrary, the tropics are al-
most completely occupied by the orange continents.
Gray-green regions, the aqueous character of which
is more than doubtful, extend over the temperate
zones. It is only in the arctic regions the arctic
regions of a planet whose tropics receive heat
The Recent Study of Mars. 141
from the Sun that compares unfavourably in its
amount with that received by lands of ice and snow
upon the Earth that there is, in the polar snows,
any indication of water in either the solid or the
liquid state. The faith of a keen believer in the
habitability of Mars may see under such conditions
an atmosphere heavily laden with moisture, but to
us it appears that poor success has accompanied
the attempt to warm Mars by a cloak of vapour.
Further, it appears certain that the trapping effect
of the vapour of water has been much overestimated.
If we accept Tyndall's estimate, that under average
conditions in this country, 10 per cent of the heat
radiated by the Earth is absorbed within 10 feet of
its surface, it follows, as has been pointed out by
Lord Kelvin, that so high a rate of absorption can-
not continue; for if it did, 10 per cent of the heat
escaping absorption in the first 10 feet being ab-
sorbed in the next 10, and so on, 90 per cent, or
nearly the whole, would be absorbed in 200 feet, a
conclusion that is directly contradicted by the very
marked effect of cloud in checking the fall of tem-
perature by radiation from the Earth's surface. It
is probable that water vapour absorbs only a few
waves of definite lengths among the many compos-
ing terrestrial radiation, that the very rapid absorp-
tion of these gives rise to the strongly-marked effect
actually observed, but that the remaining waves,
bereft of their more susceptible companions, escape
without much further loss.
To reconcile the dissipation of the polar caps with
the intense cold that it appears necessary to regard
142 Recent Advances in Astronomy.
as prevailing over the Martian world, it has been
suggested by Mr. Cowper Ranyard and other
astronomers that the Martian snows may be the
solidified form of some liquid other than water, and
freezing at a lower temperature. The occurrence of
carbonic acid gas as a constituent, howbeit a minor
one, of the Earth's atmosphere, and the fact that
by extreme cold it becomes condensed as a white
powder, very closely resembling snow in appear-
ance and melting at a temperature of about 120
Fahrenheit degrees below the freezing-point of
water, has pointed to it as the origin of the polar
caps on Mars. There are, however, very serious
objections to this view. Under ordinary conditions
of pressure, carbonic acid is incapable of assuming
the liquid state, the solid upon being heated passing
directly into the condition of gas. Under consider-
able pressure, however, the heated solid does melt,
the resulting liquid boiling at a still higher tem-
perature and becoming a gas. The least pressure
necessary for this purpose is about five times that
of the atmosphere at the surface of the Earth.
Hence for liquid carbonic acid to exist on Mars,
in consequence of the low intensity of gravitation,
12^ times the mass of air must be accumulated
over each square mile as is accumulated over a
square mile of the Earth, an estimate that cannot
possibly be accepted. If, therefore, the polar snows
consist of solid carbonic acid, they must be formed
by a direct precipitation of the hoar-frost of carbonic
acid, and their disappearance must be a similarly
direct process of evaporation. This conclusion is
The Recent Study of Mars. 143
directly challenged by the appearance to Mr. Lowell
and Professor Pickering of the blue-black belt fring-
ing the disappearing cap of 1894, and the evidence
that it furnished as to its liquid nature.
The actual deposition and dissipation of the hoar-
frost of water is not inconsistent with a temperature
considerably below the freezing-point, since direct
evaporation takes place from ice at such low tem-
peratures. Were it not for the evidence of the
fringing belt, the gray-green regions might well be
ice-bound seas, from which evaporation would take
place under the cloudless Martian skies. The air
would thus become charged to a slight extent with
the vapour of water, which, distributed over the
planet by atmospheric circulation, would be ulti-
mately deposited as frost on the coldest polar
regions.
In common with the greater number of other
celestial objects to which it has become possible to
apply a detailed examination, Mars has passed
through a first stage in which it appeared a simple
and an easy thing to interpret the features revealed,
and has reached another, in which the first pleasing
views have been rudely shaken, as observation has
revealed difficulties at a far greater rate than it has
solved them. Could we but traverse the millions
of miles of planetary space that separate us from
our ruddy neighbour, and dwell for a time upon
its surface, we should look around us in vain for
evidence of that fair miniature of the world we had
left that formed the romantic picture of our fathers.
It is more likely that we should find in Mars a
144 Recent Advances in Astronomy.
succession of bleak arid deserts over which the rays
of the vertical Sun would seem to struggle in vain
to mitigate the blasting chill of the attenuated air.
We should find, in higher latitudes, a succession
of plains, clothed, perhaps, with elementary forms
of vegetation capable of withstanding the rigours
of a climate more than arctic in character. We
should possibly encounter animal life, but assuredly
in no familiar form. With the whole aspect of
nature it would be difficult to associate romance,
and we should be well content for the future to limit
our acquaintance with the planet to the softened
picture presented in the field of view of a telescope
mounted on the more genial Earth.
Chapter IV.
The Analysis of Sunlight.
In the year 1672 Sir Isaac Newton published,
among other discoveries in optics, the account of
an experiment, in principle closely agreeing with
one less perfectly arranged and interpreted by
John Kepler more than half a century before, that
was to form the foundation of a new branch of
physics ; one that, in its application to Astronomy
a century and a half later, was destined to renew
the youth of the oldest of the sciences, not unfre-
quently regarded then as approaching the termina-
tion of its active career and as having achieved its
last triumphs. Newton's experiment was that of
The Analysis of Sunlight. 145
the analysis of sunlight, and the science that owes
its origin to it is Spectrum Analysis.
In Newton's experiment, a circular hole was
bored in the shutter of an otherwise darkened room,
and through it, when the Sun was unclouded in
the sky beyond, a beam of sunlight penetrated the
room, and, following a straight course, formed an
oval spot of white light upon the opposite wall. A
prism a block of triangular section of glass was
then placed in the path of the beam and immediately
against the shutter, and, as the result, the white spot
disappeared, and was replaced by a luminous band
of coloured light considerably displaced from it in
position. The band displayed from one end to the
other a series of colours which closely corresponded
with those seen in the rainbow, red appearing
nearest the original position of the white spot, and
violet at the end farthest from it.
From the fact of the displacement of the luminous
image on the wall, the prism clearly exercised a
deflecting effect upon the beam of light; and New-
ton accounted for the appearance of colour by the
supposition that the light of the Sun was compound
in nature, being, in fact, a mixture of all colours of
the rainbow ; that the combined effect of the whole
upon the eye was to develop the sensation of white ;
but that the differently-coloured rays were deflected
in different degrees in traversing the prism; red
experiencing the least, violet the greatest, deflection,
while the colours appearing between these were
deflected to intermediate and different extents. The
complete series he described as red, orange, yellow,
(M520) K
146 Recent Advances in Astronomy.
green, blue, indigo, and violet; but it is probable
that to most eyes the blue and indigo would appear
as only different shades of the same colour. New-
ton supported his explanation by a number of simple
though ingeniously-arranged experiments, and no
doubt has since existed as to its soundness.
Fig. 7 represents, in simple diagrammatic form,
Fig. 7. Newton's Experiment
a vertical section of the arrangement of Newton's
experiment, and requires but little explanation, s
is the section of the shutter, A that of the circular
hole. The Sun being in the direction of AC, a
beam that is, a bundle of its rays arrives from
along CA, enters the hole, and in the absence of the
prism would traverse the room and form a white
spot upon the opposite wall at B. Upon interpos-
ing the prism, however, at P, the rays suffer deflec-
The Analysis of Sunlight. 147
tion, and are thrown upward, forming a coloured
band between the limits R and v, the least deviated
the red rays arriving at R, and the most deviated
the violet at v, while the original white spot
disappears.
The reader, if generally unacquainted with the
first principles of optics, will probably find it well
to trace the action of the prism on light in the more
thorough manner developed in the next few para-
graphs, but, should the above explanation appear
entirely satisfactory and complete, these may be
passed over.
It is a matter of common experience that light
behaves normally at any rate, to a very close
degree of approximation as if it travelled along
straight lines. Were it not so, a distant object
would not become hidden by the interposition of an
opaque screen in the straight line between it and
the eye. As a matter of fact, refined observations
of the phenomena included under the term "diffrac-
tion" indicate that the transmission of light is not
completely expressed in so simple a statement, but
no error will arise from its assumption in the pres-
ent instance. To light thus regarded as proceeding
along a straight line, the term " ray" is applied.
A ray of light continues its path in a straight line,
however, only for so long as the substance or
medium through which it is transmitted is of
absolute uniformity. It is a matter of common
knowledge that, in passing from one medium into
another, the course of light is deflected or " re-
fracted" at the separating surface. Thus, in fig. 8,
148
Recent Advances in Astronomy.
let PQ indicate the course of a ray in air incident
upon the surface of water at Q. The path of the
ray within the water will still be along a straight
line QR, but this will not be the continuation of its
former direction. It has been found that, in travel-
ling from a rarer into a denser medium, a ray of
light is commonly refracted towards the normal or
perpendicular to
/ T the separating sur-
face at the point
of incidence; and
that, conversely, in
its passage from a
denser into a rarer
one, it is deflected
from the normal.
Thus, the ray PQ,
if continued in its
original direction,
would proceed
along QS; but, on
entering the water,
it is actually deflected towards the normal NN' into the
direction QR. In travelling the reverse way, a ray,
following the course RQ while within water, would,
upon entering air, be deflected from the normal into
the direction QP. Only a ray incident normally upon
a separating surface penetrates it without experienc-
ing refraction. From the refraction of light follows
the familiar fact that an object in one medium, when
viewed from another, generally appears to be in a
direction different from that in which it really is-
Fig. 8. Refraction of Light.
The Analysis of Sunlight.
149
Thus, once more referring to the figure, one of the
rays proceeding from an object situated at R would
follow the paths RQ and QP in succession, and,
should it enter the eye at P, the object will appear
as if it were in the direction PS, since it is from this
direction that the ray arrives. By analogous reason-
ing, an object at P, as seen by an eye situated below
the surface of water at R, would seem to be in the
direction RT.
The amount of deflection experienced by a ray in
the act of refraction depends upon the angle at
which it is incident upon the refracting surface, as
well as upon the natures of the two media. The
exact law by which it is determined was discovered
by Snell about the year 1621, but its statement is
unnecessary for the purpose of the present study.
The fact of re-
fraction, as well
as Snell's law,
are simply ex-
plained by the
wave theory of
light.
In fig. 9
the paths are
traced of three
rays traversing
a transparent
prism of glass, the amount of refraction being in
each case calculated from Snell's law. The manner
in which the rays are always deflected towards the
normal upon entering the glass, and from the nor-
Fig. 9. Paths of Rays traced through a
Glass Prism.
150 Recent Advances in Astronomy.
mal on leaving it, should be carefully followed,
and the construction of similar diagrams for other
prisms of different vertical angles would form an
instructive exercise. It is found that in every case
the ray, by its passage through a prism, is deflected
from the refracting edge, or that at which the two
faces concerned in the refraction meet.
Experimenting with lights of different colours, it
is found that they experience refraction in different
degrees, red experiencing the least and violet the
greatest deviation. In this lies the complete ex-
planation of Newton's experiment. A very great
number of colours and shades of colour are present
in the light of the Sun. All of them coexist
throughout the whole of the beam that enters the
room. On traversing the prism, the violet rays in
the beam are deflected far from the refracting edge,
and by themselves would form a violet spot on the
wall, largely displaced from the position of the
original white one. The red rays would by them-
selves form a red spot, displaced to a less extent,
while each colour and shade of colour would form a
corresponding spot of coloured light between these
two extremes. The coloured band or spectrum
is therefore formed by a number of coloured spots,
one formed by each colour and shade of colour
present in the original light
This explanation of the formation of the spectrum,
although beyond doubt the correct one, is not free
from difficulty, for it is truly wonderful that the
very definite and distinct sensations of colour that
are produced by the rays separately should so
The Analysis of Sunlight. 151
entirely disappear in the white that results from the
excitement of all of them simultaneously. One of
the most direct evidences of the soundness of the
theory is that when, as may be effected by several
simple methods, the colours of the spectrum are
recombined, a perfect white, indistinguishable from
the original, appears as the result of the mixture.
The number of colours represented in the spec-
trum of sunlight, as apparent to a normal eye, is
generally regarded as seven, though it is probable
that most observers would suggest six. Since the
different colours owe their appearance to their being
refrangible in different degrees, the experiment may
at first suggest the view that there are in sunlight
seven, and only seven, different kinds of light, each
of a definite refrangibility. If this were the case,
the spectrum would be formed of seven coloured
patches. If the hole in the shutter were large, the
section of the beam, and consequently each coloured
patch, would be correspondingly large: and ad-
jacent, or even non-adjacent patches, might overlap ;
but, by making the hole small, and so restricting
the section of the beam, it should be possible to get
rid of this confusion, since the angular separation
effected by the prism would remain unchanged,
while each coloured patch would decrease in size.
Thus, let the row of seven black dots repeated in
the first three lines of fig. 10 represent by their
positions the amount of separation of the supposed
seven different kinds of light in sunlight. Then,
with a very small hole in the shutter, seven small,
separate, and differently-coloured spots would ap-
152 Recent Advances in Astronomy.
pear, as indicated in the first line; while, by increas-
ing the aperture, the size of the spots would
increase and overlapping would occur, as repre-
sented in the second and third lines, a spectrum
being formed, in which, as in the actual spectrum
of sunlight, each col-
our would pass into
the next by insensible
gradations.
That the perfect
gradation of tint in
the spectrum of sun-
light is not due to the
overlapping of only
seven differently-col-
oured patches, is, how-
Fig. 10. Formation of Pure and
Impure Spectra.
ever, shown by the
fact that it is quite im-
possible to reduce the
aperture to such dim-
ensions that the spec-
trum shall be resolved into separate patches of
colour. However small the aperture, and however
distant the screen, the removal of which to a greater
distance would have the same effect in tending to
separate the coloured patches as reducing the
aperture, the different patches pass the one into the
other by perfectly insensible gradations. It is
true that, with sunlight, breaks in the band might
ultimately appear, due, as will be seen later, to
another cause; but by substituting the light of a
candle- or lamp-flame for that of the Sun, the
The Analysis of Sunlight. 153
spectrum would continue unbroken from end to
end.
The impossibility of separating the coloured
patches indicates that there are, for all practical
purposes, an infinite number of colours in the light
of the Sun or that of a candle. The description of
the spectrum as consisting of seven colours indi-
cates that in the gradual transition through the
infinite series of rays from one extreme of refrangi-
bility to the other, seven fundamentally different
sensations are successively excited. The seven
colours of the spectrum have reference to a physio-
logical, not to a physical fact.
Instead of allowing the rays, after their separation
by the prism, to illuminate a wall or other screen
before being appreciated by the eye, they may be
received by the eye directly; and, with most
sources of light, this is the only means by which a
sufficiently brilliant result can be obtained. The
optical principles involved in this method of view-
ing the spectrum will be clear from the construction
of fig. n. Here A represents a small hole in a
screen, supposed to be placed in front of a candle-
flame, and E the eye of an observer, which should,
however, be placed in practice immediately behind
the prism. A ray of red light traversing a definite
point in the hole will follow some such course as
ABE, and will enter the eye. As the result, the eye
will picture the position of A at some point, such as
R, in the direction from which the ray arrived.
The red rays traversing all points of the hole will
behave in a similar manner, and the whole collec-
'54
Recent Advances in Astronomy.
tion will appear as originating from a red circle at
R. Such a reproduction of the appearance of the
hole, a ghost from which the rays appear to come,
is technically called its image. A violet ray
traversing the same point in the hole, and accom-
.,<
Fig. ii. The Principle of the Spectroscope.
panying the original red ray along AB, will
experience, in traversing the prism, a greater
deflection than the red ray, and will be thrown into
the direction ABC. It will, therefore, miss the
eye altogether, and will be ineffective; but some
other violet ray, such as AD, starting in a direction
still more removed than the red ray from the direc-
tion of the eye, will, by its greater refraction by the
prism, enter the eye, and will appear as if it came
from v. The red rays, therefore, will develop in
the eye an appearance of a red image of the hole at
R, and the violet rays will similarly develop the
The Analysis of Sunlight. 155
appearance of a violet image of the hole at v.
Every other colour present in the light will similarly
develop a corresponding image of the hole in its
own colour, in position intermediate between R and
v, and the entire series, which will of course over-
lap, as when projected upon a screen, will form the
spectrum of the light under examination. If the
screen in front of the candle were removed, the red
rays emitted by every point of the flame would give
rise to the appearance of a red flame at R; the
violet rays to that of a violet flame at v ; and the
other colours behaving similarly, there would
result the appearance of a very impure spectrum
formed by a great number of overlapping pictures
of the flame in all the colours present in the light.
In 1802, Wollaston effected a great improvement
in the method of obtaining the spectrum, by trans-
mitting the light to be examined through a fine slit
instead of through a round hole. The slit was ar-
ranged parallel to the edge of the prism. By this
arrangement, the overlapping of images was very
much reduced, or, in technical language, the purity
of the spectrum was very much increased. Every
colour present in the light would now be represented
in the spectrum by a narrow band, which would
become a fine line if the slit were sufficiently
narrow; and these would overlap to a far less
extent than the images of a round hole that should
allow of the passage of an equal amount of light.
If, for instance, under the conditions assumed in
fig. 10, a slit were substituted for the hole, the
appearance would be as represented in the fourth
156 Recent Advances in Astronomy.
line, and the confusion occurring in the two im-
mediately above it would be entirely avoided. It
is of the highest importance to continually bear in
mind that the appearance of the spectrum, whether
formed by projection upon a screen or viewed
directly by the eye, is due to a series of pictures
or images of the aperture by which the light is
admitted ; a separate image being formed by each
colour, or, more definitely, by each kind of light,
as determined by its degree of refrangibility, repre-
sented in the original light.
Admitting sunlight through such a fine slit, and
viewing the slit through a glass prism, Wollaston
perceived the spectrum to be crossed at right angles
to its length by four diffused dark lines. The lines
were not seen in the spectrum of a candle-flame or
with other artificial sources of illumination. A
first glimpse was thus obtained of a discovery that
was in a few years to revolutionize the study of the
Sun and stars.
In 1814 further refinements were introduced by
the celebrated instrument-maker, Fraunhofer of
Munich. Fraunhofer's remarkable skill as an
optician enabled him to construct prisms of finer
quality and with faces more truly plane than
had been found possible before, both points of
great importance in giving accurate definition to
the images produced ; while, instead of viewing the
spectrum directly, it was examined through a tele-
scope, which received the rays immediately after
their passage through the prism. As the result,
the dark lines faintly seen by Wollaston became
The Analysis of Sunlight. 157
far more distinct, and their number was increased
to 576. Some of them were also recognized in the
spectra of planets and of fixed stars.
A further refinement, and one by which the pris-
matic spectroscope practically acquired its present
form, was effected by Simms, another famous
instrument-maker, in 1839. The image formed by a
prism, or by the refraction of light at a single plane
surface, is not, excepting under special conditions,
sharply defined. This is due to the fact that, even
with light of a pure colour, and, therefore, of only
one degree of refrangibility, rays originating and
therefore diverging from a point, do not after
refraction diverge from any one point, a defect that
arises from the particular form of the law of refrac-
tion and that we shall make no attempt to explain
here. Since, owing to its dimensions, the eye
receives not one ray of light but a number of rays ;
and since the refracted rays do not diverge from a
point, the image appears slightly out of focus and
indistinct. Light is diffused beyond what should
otherwise be the sharp limits of the image, and
overlapping of neighbouring images in the spectrum
is unduly pronounced. If, however, the object is
so far distant from the prism that the rays falling
upon the face of the prism are sensibly parallel, all
are refracted to the same extent, the emergent rays
are still parallel, and indistinctness of the image
due to this cause does not occur. Fraunhofer, who
was the first to appreciate the importance of this
condition, had been careful to approximate as
close as possible to it, by placing the prism at a
158 Recent Advances in Astronomy.
considerable distance in some cases as much as
24 feet from the slit; but the method was in-
convenient, and involved a waste of light; since
the greater number of the rays diverging from the
slit missed the prism altogether. The improve-
ment effected by Simms consisted in introducing a
condensing lens in the path of the rays between the
slit and the prism, at a distance equal to its focal
length from the slit. The lens, known as the
"collimating lens", collected the conical bundle of
rays diverging from any point of the slit, and con-
densed them into a sheaf of parallel rays before
they fell on to the nearest face of the prism. With
a collimating lens, the slit need not, therefore, be
farther from the prism than the focal length of the
lens, which may be only a few inches. By these
means distortion of the image due to the divergence
of the incident rays was entirely obviated.
Reference must be made to one other condition
of spectral purity, familiar to Newton and to later
workers upon the analysis of light. It can be
shown to follow from the form of the law of refrac-
tion, that, other conditions being the same, the
definition of the image is least imperfect when the
path of the ray in the prism makes equal angles
with the refracting faces, a case illustrated by
the central of the three rays traced in fig. 9. In
practical work with the spectroscope, the prism
is always adjusted for this to be, as nearly as
possible, the case. The adjustment is made by
turning the prism to and fro until it is observed
that the spectral image is displaced from the direc-
The Analysis of Sunlight. 159
tion of the slit to the least possible extent; optical
theory having shown that this condition of " mini-
mum deviation" is coincident with the desired
symmetrical passage of the ray through the prism.
There should now be little difficulty in following
the theory of the modern prismatic spectroscope,
a diagrammatic section of which is given in fig. 12.
Fig. 12. The Spectroscope.
The slit is formed by bringing the carefully finished
edges of two metal plates a a, almost into contact.
By an adjusting screw, one of these can be moved
towards or away from the other, by which means
the slit, which is to be regarded as standing perpen-
dicularly to the paper at A, can be made wide or
narrow. The rays of light that enter the instrument
by any point of the slit travel down a metal
"collimating tube" B. usually from i to 3 feet in
length, at the end of which they fall upon the colli-
mating lens c, by which their paths are rendered
parallel. Since everyone of them falls upon the
first surface of the prism at the same angle, all
1 The telescope and the collimator are usually from two to three times
longer in comparison with their diameters than is represented in the figure,
in which their sectional dimensions are exaggerated for the sake of clear-
ness.
160 Recent Advances in Astronomy.
of the same colour are refracted equally, and their
courses on traversing the prism, and after leaving
it, are still parallel. Following now rays of one
colour only, represented by the unbroken lines in
the figure, they pass on to the telescope. The
object-glass o, a condensing lens similar to that
of the collimator, condenses then to a focus at v.
On passing this they diverge again, traverse the
eye-piece, and are received by the eye, to which,
as the result, the appearance is presented of a line
of light an image of the slit of a colour defined
by the nature of the rays, and in position by the
degree to which their courses have undergone de-
flection by the prism. Regarding, for instance, the
rays, the paths of which have been traced, as violet;
the red, represented by the dotted lines in the figure,
experiencing less deflection, would be focussed in
some such position as R, and other rays would be
condensed in positions intermediate between R and
v. As in the previous cases, a coloured line appears
in the field of view for each kind of light radiated
by the source. When the light has been sufficiently
powerful to admit of a greater extension of its spec-
trum without undue enfeeblement of its colours, two
or more prisms have sometimes been inserted be-
tween the collimator and telescope in such a manner
that the light traverses all of them in succession.
The separation effected is, of course, in direct pro-
portion to the number of prisms employed.
Recently a "diffraction grating" a surface of
polished silver ruled very closely by a diamond
with a number of parallel lines has been frequently
The Analysis of Sunlight. 161
substituted for the prism. The light from the colli-
mator falls upon the grating, and is thrown back,
separated in the act of reflection into its constituent
colours. A spectrum is thus formed, which is
examined through a telescope, arranged nearly
parallel to the collimator, and by the side of it. It
is not possible to give here the explanation of the
analysis of light by the diffraction grating, which
is, however, in perfect accord with the wave theory
of light. The separation of the colours is propor-
tional to the closeness of the lines, and gratings
recently constructed by Professor Rowland of
Baltimore contain as many as 40,000 lines to the
inch. The diffraction spectrum possesses certain
advantages and disadvantages as compared with
that formed by a prism.
In 1802 Wollaston detected four shaded lines
intersecting the spectrum of sunlight at right angles
to its length. Bearing in mind that the spectrum
is, in reality, a series of pictures of the slit, one
being formed by each kind of light present in the
mixture submitted to analysis, it is clear that the
shaded bands must indicate colours absent from,
or feebly represented in, the light; as missing
colours in sunlight. Three of the four lines
Wollaston regarded as forming the natural divisions
between different shades of colour at best a not
very satisfactory hypothesis, and one shortly to be
disproved.
By the refinements introduced by Fraunhofer in
1814, the number of dark lines in the solar spectrum
was increased to close upon six hundred, and it
(M520) L
1 62 Recent Advances in Astronomy.
may be added that in a fine modern spectroscope
upwards of ten thousand are visible. By rotating
the observing telescope until the more conspicuous
of them were brought in succession to the centre
of the field of view, Fraunhofer was able, from the
reading of a graduated circle attached to the tele-
scope, to measure the positions of the lines in the
spectrum, or, more definitely, the degree of re-
frangibility of each missing colour. From these
measurements he constructed the first map of the
solar spectrum, denoting the more conspicuous
lines by the capital letters from A to H. The dark
lines are still known as the Fraunhofer lines, and
are identified by Fraunhofer's nomenclature.
The lines indicated by A and B upon Fraunhofer's
map are situated in the deep-red of the spectrum,
and are, under ordinary conditions, both strongly
marked, c is a sharp well-defined line in the
region of the spectrum where the red is passing
into orange; D, a close pair of lines in the yellow;
E, a condensed group in the bright-green; F, a
sharply-defined line in the greenish-blue; G, a
group in the deep-blue ; and H, a rather wide pair
of broad diffused lines, or rather bands, that lie
in the extreme violet. This pair have assumed
great importance in recent researches, and are now
generally known as H and K.
For the Fraunhofer lines to appear in the spec-
trum, the slit must be very narrow. If it is opened
beyond a certain degree of fineness, the images
of the slit formed by the colours lying in the spec-
trum on either side of the position of the colour,
The Analysis of Sunlight. 163
to the absence of which the dark line is due, expand
across its place and mask the effect of its absence.
Observations of the very impure spectra obtained
by viewing flames directly through a prism had
already been made by different observers with
but little result, but now Fraunhofer subjected
their light to examination in his more refined
spectroscope. With no flames were any dark lines
observed in the spectra, but bright lines frequently
made their appearance, shining out conspicuously
upon the background of a continuous spectrum.
The bright lines clearly pointed to the presence,
in the light under examination, of strongly devel-
oped pure colours that were not decomposed by
the prism. In all flames that were examined, and
especially strongly represented in the blue base of
a candle-flame, were two nearly coincident shades
of yellow, revealed by the appearance in the spec-
trum of two closely adjacent yellow lines. Fraun-
hofer remarked with astonishment that these yellow
lines coincided exactly in their positions in the
spectrum with the two components of the double
dark line that he had already distinguished by the
letter D in the spectrum of the Sun. The two
shades of yellow, so abnormally abundant in the
light of a candle, appeared, therefore, to be absent
from, or, at any rate, but feebly represented in, the
light of the Sun.
Later, in 1823, Fraunhofer examined the spectra
of the brighter stars. In all of them he recognized
dark lines, but although, in a few instances, the
spectra appeared to be similar to that of the Sun
164 Recent Advances in Astronomy.
in the distribution and in the relative intensity
of the lines, in the greater number they were
essentially different. The spectrum of Sirius, in
particular, displayed only three dark lines, but all
were far broader and more strongly marked than
any recognized in the spectrum of the Sun.
For these as well as other reasons, Fraunhofer
strongly maintained that the dark lines denoted
colours initially absent in the radiation of the Sun
and stars themselves, and that they did not originate,
as had been suggested, either in the atmosphere
of the Earth, or by some optical effect in the
spectroscope, similar to that by which dark diffrac-
tion lines appear when an illuminated slit is viewed
from some distance through a second slit, a pheno-
menon that was at that time attracting considerable
attention.
The origin of the bright lines in the spectra of
flames: the source of the dark lines in the spectra of
the Sun and stars: and the remarkable coincidence
established by Fraunhofer between the two yellow
rays emitted by flames and the D pair absent in the
light of the Sun, aroused the highest interest and
evoked the keenest inquiry. For many years the
exact study of the bright lines in flame spectra
made but little progress. It was found that, upon
saturating the wick of a flame with different
chemicals, with the exception of the yellow pair
that was always present, different sets of bright
lines appeared in the spectrum. It appeared pro-
bable that the colours represented by the bright
lines were emitted by the glowing vapours of the
The Analysis of Sunlight. 165
substances introduced into a flame, and that the
continuous spectrum upon which they appeared was
due to the normal radiations of the flame itself, for,
with the scarcely luminous flame of burning alcohol,
the continuous spectrum nearly disappeared, and
the bright lines that flashed out upon the introduc-
tion of various chemicals seemed as if separated
by intervals of almost complete darkness. The
demonstration of the now familiar fact that each
glowing vapour emits definite colours, indicated by
bright lines occupying definite positions in the
spectrum, and that from the appearance of its
spectral lines the presence of an element may be
inferred with certainty, was, however, only estab-
lished by the classical researches of Bunsen and
Kirchhoff in 1859.
But, in the meantime, facts of great interest were
brought to light in connection with the dark
Fraunhofer lines. In 1832 Sir David Brewster
noticed that as the Sun approached the horizon
many of the Fraunhofer lines became intensified.
Several groups of lines towards the red end of the
spectrum, for instance, while delicately defined
when the Sun is high, appear at sunrise and sunset
as massive black columns standing in front of the
deep-red of the spectrum. Since, when at a low
altitude, the rays of the Sun penetrate the atmos-
phere obliquely, and their path included in the air
is therefore very great, Brewster suggested that
those lines that were affected in this manner were
caused by absorption by the Earth's atmosphere of
the colours corresponding to them ; their intensifica-
166 Recent Advances in Astronomy.
tion with the low sun being due to increased absorp-
tion by reason of the greater atmospheric path of
the light. The truth of this view has since been
abundantly confirmed, and the lines that thus
originate are known as telluric lines. The great
majority of the Fraunhofer lines appeared, however,
to be independent of the atmospheric track of the
solar rays, since they were not affected in intensity
by the altitude of the Sun, and it was therefore
assumed that they denoted colours absent in sun-
light before it entered the Earth's atmosphere.
They were, in consequence, regarded as owing
their origin to a similar absorption of definite
colours in an atmosphere that was supposed to
envelop the incandescent surface of the Sun.
In the discovery of the origin of the telluric lines,
the first glimpse was obtained of the remarkable
power possessed by many gases of absorbing
colours so definitely as to more or less completely
extinguish them without affecting those immediately
on either side of them in the spectrum. In the
following year Brewster showed that, instead of
invariably necessitating an extensive atmosphere to
produce the effect, with some gases a few inches
were sufficient; for, on causing the light from a
candle-flame to traverse such small lengths of
certain gases before entering the slit of the spectro-
scope, the spectrum became ruled throughout by
dark lines and shaded bands. Upon introducing a
glass tube filled with the ruddy vapour of nitric
peroxide between a lamp-flame and a spectroscope,
the spectrum instantly became crossed by an
The Analysis of Sunlight. 167
enormous number of dark lines, some broad and
massive, and others most delicately fine. Each
dark line denoted the absence of a definite colour
that had been absorbed by the vapour. Some of
the lines appeared to coincide in their positions in
the spectrum with certain of the Fraunhofer lines
in the spectrum of the Sun, from which Brewster
was led to conclude that nitric peroxide was a con-
stituent of the Sun's atmosphere. Although this
conclusion has been disproved, in suggesting it
Brewster obtained the first glimpse of one of the
mo^t powerful and remarkable of the methods ot
modern scientific analysis.
The years that immediately followed Brewster's
observations marked the birth of the science of
photography. In 1838 Daguerre had discovered
the process, with which his name has been since
associated, for causing objects, by means of their
light radiations, to impress their pictures upon
specially prepared silver surfaces; in 1840 Dr.
Draper had effected the first application of the dis-
covery to astronomy in photographing the Moon ;
and two years later Becquerel succeeded in obtain-
ing a photograph of the solar spectrum by project-
ing it upon a sensitive plate. In this first photo-
graph of the spectrum the dark lines appeared as
surely as in eye observation, while the remarkable
fact became apparent, that the spectrum did not
terminate with the violet, but extended beyond it
to a distance far exceeding its visible limits, con-
tinuing, in its invisible extension, to be crossed by
lines of absent radiation. It appeared, therefore,
168 Recent Advances in Astronomy.
that the total radiation of the Sun contains rays
more refrangible than violet light, and which do
not possess the power of exciting the sense of
vision. A year later, Draper, also by the aid of
photography, similarly traced the solar spectrum
beyond its visible limit in the red, and there also
found Fraunhofer lines of absent radiation.
The time was now approaching when a successful
attack was to be made upon the great mystery of
the Fraunhofer lines. In 1849 M. Leon Foucault
devised an experiment with the view of determining
whether or not the coincidence as regards position
in the spectrum between the two components of
the Fraunhofer D line and the remarkable close
pair of yellow lines that appeared in the spectrum
of almost every flame, was exact. In this experi-
ment, which has become classical, the yellow pair
were obtained from the light of the electric arc.
The electric arc is formed by passing a current of
electricity across the space separating the ends of
two carbon rods that are almost in contact. In
passing through the rods the current experiences
but little resistance, and therefore develops but
little heat; but in its passage from one rod to the
other across the air-gap separating them enormous
heat is developed owing to the greatly increased
resistance, and the air is raised to a very high
temperature. The intensely hot bridge of air be-
tween the ends of the rods is technically known as
the " arc ". The temperature of the arc is so high
that impurities present in the carbon rods, and
indeed the carbon itself, volatilize and mix, in the
The Analysis of Sunlight. 169
state of gas, with the air in the gap. In spite of its
high temperature, the arc itself gives but little
light, owing to the poor radiating power of the
gases forming it; but the ends of the rods, bathed
in these highly-heated gases, are raised to the
vivid state of incandescence that is the source
of light in the arc lamp. The electric arc had
been discovered by Sir Humphry Davy in the year
1800.
On directing the spectroscope toward either of
the incandescent carbon ends, Foucault observed
that the light agreed with that radiated by all in-
candescent solid bodies in yielding a continuous
spectrum, but on deflecting the spectroscope towards
the gap, so that the light from the glowing gases
should be subjected to analysis, a number of separ-
ated bright lines were seen, indicating the existence,
in the radiations of the glowing gases of the arc, of
a corresponding number of isolated colours. Among
the lines so seen the familiar yellow pair shone out
conspicuously. To test whether these coincided
exactly in their spectral position with the components
of the D line in the solar spectrum, Foucault con-
densed the rays from the Sun upon the arc by means
of an ordinary condensing lens. The solar rays,
after traversing the arc, streamed onward, and
entered the slit of the spectroscope along with the
rays of the arc itself, and Foucault, anticipating
that the coincidence between the positions of the
yellow lines and the D lines would prove to be
exact, confidently expected that, in the light of the
Sun, poor in definite yellow rays, supplemented by
170 Recent Advances in Astronomy.
that of the arc, abnormally rich in them, the D lines
would altogether disappear.
On observing the spectrum, however, Foucault
witnessed a most remarkable and unexpected appear-
ance. Not only were the dark D lines not filled in
by the yellow lines of arc spectrum, but they appeared
to be both darker and wider than when the arc was
absent. Not only did the radiations of the arc fail
to supplement the deficiency of the similar radiations
in sunlight, but the deficiency at once became more
pronounced than before. Only one explanation
appeared possible. Not only were the gases of the
arc capable of radiating two definite shades of yel-
low, but they also possessed the power of absorbing
them. The gases of the arc had absorbed a greater
quantity of the yellow rays from the solar radiation
than they had added to it, and increased darkness
had been the result. Further, excluding the Sun
altogether, Foucault, by the aid of a mirror, re-
flected the light from one of the incandescent carbon
points through the gases occupying the gap be-
tween the pair; and on submitting the light thus
transmitted to analysis, observed a continuous spec-
trum crossed by a pair of fine dark lines in the
yellow. The arc had again absorbed the two
shades of yellow more abundantly than it had
radiated them, and the D lines had been produced
for the first time in a laboratory experiment.
There can be little doubt that if the origin of the
yellow pair had been known, the problem of the
Fraunhofer lines would have found its solution in
Foucault's experiments. The yellow lines had,
The Analysis of Sunlight. 171
however, proved a veritable stumbling-block to the
advance of spectrum analysis. In the greater num-
ber of cases it seemed probable that the appearance
of definite bright lines in spectra depended upon
the presence of definite glowing vapours in the
source of light, but the yellow pair seemed to defy
any such limitation. They flashed out in the
spectra of all flames, they seemed to be associated
with the burning of all substances; and it was
indeed suggested that they were developed in, and
inseparably connected with, the process of com-
bustion. For a few years after Foucault's obser-
vations they succeeded in evading the most refined
methods of scientific inquiry. By the year 1852,
however, Sir Gabriel Stokes had shown that they
were absent from the spectrum of a candle-flame
when the wick had been carefully snuffed clean and
so as not to project into the luminous envelope, as
well as from the spectrum of the flame of pure
alcohol when burned in a carefully-cleaned watch-
glass. On the other hand, they were most intensely
developed when common salt the chloride of so-
dium and other compounds of sodium were intro-
duced into flames. Gradually it became more and
more probable that they were due to the glowing
vapour of sodium, and that their almost universal
appearance in spectra arose from the extreme diffi-
culty of excluding a last trace of salt, and from their
very powerful development upon the presence of
the smallest possible quantity of it. Assuming
this to be the explanation of their appearance, Sir
Gabriel Stokes, in 1852, gave the correct explan-
172 Recent Advances in Astronomy.
ation of the appearance of the D lines in the spectrum
of the Sun.
Sir Gabriel Stokes's explanation was based upon
theoretical grounds the wave theory of light, and
the view of the structure of matter involved in its
acceptance. Since, in its later history, the most
important applications of the analysis of light to
astronomy have been directly due to the view of
the nature of light indicated in the wave theory, it
may be well to make a slight digression in a short
sketch of its general features.
According to the wave theory of light originally
enunciated by Christian Huygens in the latter part
of the seventeenth century, suppressed for a time
by the overpowering authority of Sir Isaac Newton,
but placed upon a sound scientific foundation early
in the present century by the labours of Dr. Thomas
Young light is due to the transmission of waves,
or undulations, from a luminous body to the eye.
For there to be undulations there must be some-
thing to undulate, and to this something the name
has been given of the "ether". To account for
the phenomena of light, it is necessary to regard
the ether, as not only existing throughout space, at
any rate to the farthest of the visible stars, but as
permeating all matter.
The display of iridescent colour frequently so
exquisitely developed in light reflected from thin
films, such as the envelope of a soap-bubble; the
coloured fringes visible upon either side of an
illuminated slit when viewed through a second and
similar slit at a moderate distance from it; and the
The Analysis of Sunlight. 173
spectrum formed by a diffraction grating, enable
us, if we interpret them according to the wave
theory by which alone they have so far received
a satisfactory explanation to measure the lengths
of the ether waves. Estimates deduced from the
different phenomena are in perfect accord, though
there is no doubt that the highest degree of accuracy
is obtainable in observations of the diffraction spec-
trum. From them it appears that colour, and there-
fore refrangibility, is determined by the length of
waves in free ether; the sensation of red being
excited by the longest and that of violet by the
shortest waves that affect the eye, while the pass-
age up the spectrum from red to violet is accom-
panied by a continual decrease of wave-length.
The actual wave-lengths are about a sixty-thou-
sandth of an inch for violet, and a thirty-thousandth
of an inch for red light, but they are more accu-
rately given in the table on p. 175.
It will scarcely be necessary to remind the reader
that the appearance of the transmission of matter by
wave motion is illusory. On the surface of water
disturbed by wave motion floating objects merely
rise and fall as the waves pass, which shows that
the movement of the wave-conveying medium con-
sists of a succession of oscillations up and down,
while the waves themselves continually pass them
horizontally. The illusion of water moving witn
waves results from each portion of the surface trans-
mitting its disturbance to the portion immediately
in front of it, but occupying a definite interval of
time in so doing. After a short interval, therefore,
174 Recent Advances in Astronomy.
the form of the water surface has been moved for-
ward, but so continuously that the appearance is
produced of the surface itself having been displaced
in the direction of the wave motion.
The double appearance of objects when viewed
through Iceland spar and other crystals, as well as
the chromatic effects and general properties of
polarized light, indicate that the motion of the ulti-
mate parts of the wave-conveying ether is transverse,
or across the direction of wave motion. In this
respect ether waves resemble waves upon the sur-
face of water, as well as those upon stretched
strings. They differ in character from waves of
sound, in that in these the motion of the air the
undulating medium in their case consists of oscil-
lations to and fro in the direction of wave motion.
During the passage of a single wave past a point
in the ether, the ether at the point executes a single
vibration or oscillation about its normal position,
this vibration being, according to the inference of
the preceding paragraph, across the direction of
wave motion. During the passage of a train, or
series, of similar waves, the ether, therefore, con-
tinues to oscillate, and the number of oscillations
executed every second a quantity known as the
"frequency of oscillation" is determined by, and
is equal to, the number of waves passing in a
second. Since all waves travel with the same speed,
the longer will pass in less rapid succession than
the shorter, and will therefore produce a less rapid
oscillation. The violet waves, for instance, being
only about half as long as the red, are associated
The Analysis of Sunlight.
175
with double the frequency of oscillation. From the
geometry of wave motion thus sketched it follows
from simple reasoning that in every case the velocity
of the waves is equal to the product of their length
into the frequency of oscillation. From this rela-
tion it is possible to determine the frequency of
oscillation of different kinds of light waves, since
their velocity the speed of light is known, and
their length may be determined in every case by a
diffraction grating. This has been carried out in
the following table, the speed of light being taken
as 187,000 miles per second. The way in which
the frequency of oscillation increases as the wave-
length decreases should be carefully noticed.
WAVE-LENGTHS AND FREQUENCIES OF OSCILLATION OF
ETHER WAVES.
Colour.
Fraunhofer
Line.
Wave-length
in free ether, 1
in millionth*
of an inch.
Vibrations per
second in
millions of
millions.
Red
A
29-9
395'8
Orange
Yellow
Green
6
C
D
E
27-0
25 '9
23-2
207
437
458
5io
57
Blue
F
19-1
618
Violet
G
H
I73
15-6
683
757
1 The necessity for the addition of the words "in free ether" is due to the
fact that, while traversing transparent substances, the speed of light is
reduced, doubtless as the result of the close association of the ether with
ordinary matter. The frequency of oscillation must, however, remain the
same, being always that of the source of the waves, so that the relation
speed of waves = wave-length x frequency indicates that the wave-length
must be reduced in proportion to the speed. It is customary to define a
particular kind of light by its wave-length. Since, however, this varies with
the medium through which the light travels, it would be far better to define
176 Recent Advances in Astronomy.
The most simple view to take with reference to
the generation of waves in the ether, is that the
luminous or wave-generating body contains os-
cillating portions of matter that possess a grip
upon the ether. As the oscillations of a hand or
tuning-fork may develop waves upon a stretched
cord, so these vibrating parts, gripping the ether,
and being thus able to transmit their movement to
those portions of it immediately round them, set up
waves, the frequency of oscillation of which is the
same as their own. What these oscillating parts
are is not of fundamental importance to the present
study, but they are generally regarded either as the
atoms of matter, or as portions of, or structures in,
the atoms, elastically attached and capable of oscil-
lation within them. The atom of sodium, capable
of emitting two yellow rays of nearly the same tint,
that is, of developing two series of waves of nearly
the same frequency, may be regarded as analogous
to a musical instrument with only two strings tuned
nearly to unison. Since the waves generated by
the incandescent vapour of sodium cause the ether
to oscillate about 510 millions of millions of times
in a second, this rate of vibration must also be that
of the structures themselves. Below the tempera-
ture at which the yellow light is emitted we must
suppose that the structures are not oscillating with
it by its frequency of oscillation. When light is denned by its wave-length,
that in free ether should always be understood. In glass the speed of light
is about % of its speed in free ether. In air it is scarcely affected. Re-
fraction is the direct result of alteration of speed in passing from one
medium into another, and from the extent of refraction the alteration in
speed, and therefore in wave-length, can be determined.
The Analysis of Sunlight. 177
this frequency, but that the oscillations may be
developed by sufficient addition of heat. The
higher the temperature, the more intense the radia-
tions, and therefore the more intense the oscilla-
tions of the structures. The reader is probably
aware that there is good reason for regarding such
atomic and molecular vibration as constituting the
sensible heat of matter.
Adopting this view as representing the mechan-
ism of radiation, that of absorption follows naturally.
Upon a series of ether waves traversing a space
throughout which atoms of matter are distributed,
the atomic structures gripping, and therefore
gripped by, the ether, will tend to be thrown to
and fro in harmony with its movement. Heat will
be represented by the motion generated in the
atoms, while the energy of the waves themselves
will be correspondingly decreased by the loss of the
motion transferred from them to the matter.
There is, however, one case in which the trans-
ference of the energy of motion from the ether
waves to the atoms will be especially pronounced.
It is a familiar fact, deducible from the first prin-
ciples of mechanics, that, if a body capable of
independent vibration is acted upon by a succes-
sion of impulses acting in unison with its own
oscillations, a far more extensive oscillation will
result than if such a coincidence did not exist. The
principle, generally known as that of sympathetic
vibration or resonance, is abundantly illustrated
throughout mechanics and physics. If a weight
be suspended by a cord, the upper end of which is
(M620) M
178 Recent Advances in Astronomy.
held in the hand, a succession of properly timed,
but scarcely appreciable, movements of the hand to
and fro may cause a very extensive oscillation of
the weight. If a large man be seated in a garden
swing, a little man, by properly timing his thrusts
into unison with the oscillations of the swing, may
develop in the large man an oscillation out of all
proportion to that which would otherwise result
from his most violent efforts. A musical note
sounded in the neighbourhood of a jar of such
dimensions that the natural period of oscillation of
the contained air coincides with that of the note,
will cause the air of the jar to sound loudly in
response. The recently discovered possibility of
electric signalling over considerable distances with-
out connecting wires depends upon the coincidence
between a succession of feeble electric impulses
applied to a distant conductor, and the normal
oscillations of electricity in the conductor.
If, now, white light that is, a number of wave-
trains of all possible frequencies between the limits
of the spectrum should traverse the vapour of
sodium, it should not be difficult to predict what
would occur. Those waves, the frequencies of
which did not agree with the natural vibrations of
the sodium atoms, would scarcely affect them,
and therefore they themselves would be scarcely
affected. Those waves, however, that possessed
vibration frequencies in unison with the normal
oscillations of the atoms, would apply impulses to
the atoms or atomic structures accurately timed to
their own oscillations; resonance would follow, and
The Analysis of Sunlight. 179
extensive motion would be developed in the atoms.
During the development of this motion, equal
energy of motion would be absorbed from the
waves. The waves, from which this energy would
be absorbed, would be damped, and the light, after
having traversed the vapour, would be found defi-
cient in precisely those waves that the vapour itself
could originate. Generally any vapour possessing
the power of emitting definite radiations must also
possess a special capacity for absorbing them.
The deficiency in sunlight of the two shades of
yellow emitted by the glowing vapour of sodium,
indicates, therefore, that the white light of the Sun
has traversed the vapour of sodium somewhere in
its passage to the surface of the Earth. Since the
vapour of sodium is not found in the atmosphere of
the Earth, and is assuredly not distributed in inter-
planetary space, it must be looked for in the atmos-
phere of the Sun.
Such was Sir Gabriel Stokes's explanation of the
double line D, and of other Fraunhofer lines, though
up to that time no other coincidence between Fraun-
hofer lines and bright lines in the spectra of glowing
terrestrial vapours has been established. With the
singular modesty and reticence that has character-
ized him through life, Stokes, while offering one of
the most remarkable of scientific theories as a sug-
gestion in private conversation with a friend, re-
frained from making it public. In the following
year (1853), however, a similar explanation was
given by Angstrom of Upsala. Angstrom, more-
over, established the coincidence between other of the
i8o Recent Advances in Astronomy.
Fraunhofer lines and certain bright lines, though of
unknown origin, in the spectrum of the electric arc.
In 1859 the publication of the classical researches
of Bunsen and Kirchhoff placed spectrum analysis
upon a sound foundation as a branch of science.
For the first time, bright lines in the spectra of
flames were definitely proved to arise from the
presence of glowing vapours in the flames. The
flame generally employed was that of a spirit-lamp,
or of gas which had been deprived of its luminosity
in the now familiar form of Bunsen burner. A
great number of different substances were made to
pass into the flame in the state of vapour, by intro-
ducing them in the solid or liquid state upon a
piece of platinum wire into the lower part of the
flame. It was found that every system of bright
lines was associated with the presence of a definite
vapour in the flame, and with such consistency
that the presence of the vapour could be inferred
with certainty from the appearance of its character-
istic lines in the spectrum. This fact, of course,
constitutes the very foundation of spectrum analysis.
The famous close yellow pair were traced to sodium;
and it was shown that their continual appearance
in all sorts and conditions of flames was due to the
universal distribution of common salt in the atmos-
phere, carried into it in all probability in the first
instance by sea spray, and to the marvellous de-
licacy of the spectral test, a delicacy so extreme
that the yellow lines appeared in the spectrum on
the introduction of a two-hundred-millionth part of
a grain of salt into the flame.
The Analysis of Sunlight. 181
Not only was the complete set of bright lines
yielded by one glowing vapour different from that
given by any other, but only rarely did any one of
the lines of one element appear to occupy in the
spectrum the position of a line of another. Whether
the few coincidences that have to the present time
been observed are in any case more than approxi-
mate, the result of insufficient spectroscopic power;
whether they are exact; and, if so, whether they are
more than accidental, has been very keenly dis-
cussed in recent years in connection with certain
modern speculations. In a great many instances
apparent coincidences have been shown, by the
application of more refined instrumental means, to
be only approximate, and the view is now generally
held that the few as yet unresolved will either yield
to higher dispersion or are merely accidental.
In 1859 the splendid results obtained by Bunsen
and Kirchhoff were brought to bear upon the
problem of the Fraunhofer lines by Kirchhoff.
Kirchhoff found no difficulty in obtaining the
sodium lines dark upon the background of a con-
tinuous spectrum, by interposing the flame of a
spirit-lamp, upon the wick of which a few grains
of salt had been sprinkled, in the path of rays pro-
ceeding from the incandescent lime of the lime-
light to the slit of a spectroscope. Further, he
failed to obtain the effect when the flame of a
bunsen burner similarly charged with salt was used
instead of the spirit-lamp, but perceived instead the
bright-yellow pair radiated by the flame superposed
upon the less brilliant continuous spectrum of the
182 Recent Advances in Astronomy.
lime-light. From this he suspected that to effect
"reversal" the temperature of the vapour must be
less than that of the radiating source.
The statement of the exact conditions under which
a vapour will effect the reversal of its spectral lines
was first given by Balfour Stewart in 1861. In 1791
Prevost of Geneva had published a suggestive paper
entitled " On the Equilibrium of Heat", in which,
according to a law then enunciated for the first time,
and since known as "the law of exchanges ", it was
shown that every body when at the same tempera-
ture as those surrounding it, must possess a power
of absorbing heat radiations in direct proportion to
its power of emitting them. At the time of Kirch-
hoffs researches, it was thought to be probable,
from the similarity in the laws by which they were
governed, that radiant heat and light were different
forms of one kind of radiation, and Balfour Stewart
perceived that the extension of Prevost's reasoning
to light radiation would account, not only for the
fact of reversal, but also for the condition, already
suggested by experiment, that to effect it, the ab-
sorbing vapour must be cooler than the source.
Limits of space, unfortunately, make it impossible
to introduce Balfour Stewart's reasoning here, but
an excellent outline is given in Balfour Stewart's
Heat, and will well repay the most careful study.
By a very simple process of reasoning, based upon
the obviously sound assumption that a body within
an opaque enclosure, all portions of the inner sur-
face of which are at the same constant temperature,
will ultimately acquire the temperature of the en-
The Analysis of Sunlight. 183
closure, Balfour Stewart showed that such a body,
when at the temperature of the enclosure, must ab-
sorb and emit any particular kind of radiation in
exactly equal amount. Since the rate of radiation
from a surface is directly dependent upon tempera-
ture, while the rate of absorption depends upon the
nature of the surface and is not directly affected by
its temperature, it follows that, under the conditions
imagined, any fall in the temperature of the body
will cause its radiation to fall short of its absorption,
while a rise in temperature will cause its radiation
to exceed the absorption.
From this conclusion it is possible to state defi-
nitely an essential condition for the appearance of
dark lines in the spectrum of the Sun. It is, that
the gases present in the solar atmosphere must be
at a lower temperature than the incandescent sur-
face or photosphere behind. If the temperature
of the gases were equal to that of the photosphere,
they would be in such a condition as to absorb from
its light precisely as much of any kind of radiation
as they would add to it, and, in consequence, the
light from the photosphere would, after traversing
them, be unchanged in composition. If the tem-
perature of the atmosphere were to fall below that
of the photosphere, the radiation of its gases would,
for each particular kind of ray, fall short of the
absorption, and dark lines would result in the
spectrum; while, if the temperature of the atmo-
sphere were to rise above that of the photosphere,
the radiation of its gases would exceed the absorp-
tion exercised by them, and bright lines would
184 Recent Advances in Astronomy.
appear upon the continuous spectrum of the photo-
sphere. From the general appearance of dark lines,
the Sun's atmosphere may be assumed to be cooler
than the photosphere, though, as will be seen later,
bright lines do occasionally appear.
It is clear from these principles that the Fraun-
hofer lines are not absolutely dark, but only appear
so by contrast with the more brilliant spectrum of
the photosphere upon which they are projected.
Even if a gaseous constituent of the solar atmo-
sphere were a perfect absorber of particular kinds of
light, its own radiation, which would be of the same
nature as the absorbed light, would travel on with
the photospheric rays that had escaped, and would
in part supply the place of those that had been ab-
sorbed. There is no doubt, that if the incandescent
surface of the Sun were for a moment to be extin-
guished, while its atmosphere remained unaffected,
the solar spectrum would appear as a crowd of
bright lines corresponding to actually existing dark
ones. Tb-e- 'existence of the photosphere behind
doesjaot detract from the light that we receive from
atmosphere, but, by filling the spaces between
its bright lines with more intense light, causes them
to appear dark by contrast. That the apparently
dark Fraunhofer lines are in reality brilliant can be
shown by carefully-arranged experiments.
The presence of sodium in the atmosphere of the
Sun having been established, Kirchhoff next en-
deavoured to discover other of its constituents, by
searching in the spectra of terrestrial elements for
bright lines that should coincide in their positions
doesnot
^^jffatmo
The Analysis of Sunlight. 185
with Fraunhofer lines. His efforts were entirely
successful. On passing the intense electric dis-
charge of an induction coil from one metal wire to
another across a short air gap, a brilliant spark was
obtained, which, when analysed, gave a spectrum
of bright lines clearly due to the glowing gases
filling the gap. Different sets of bright lines ap-
peared as different metals were employed to form
the spark, and it was clear that the spectrum con-
sisted of the bright lines given by incandescent air,
together with those due to the glowing vapours of
the metal wires, these being partially volatilized by
the intense heat of the discharge. Passing the dis-
charge between the ends of iron wires, the spectrum
given was that of a mixture of the vapour of iron
and air, and Kirchhoff succeeded in establishing
coincidences between no fewer than sixty of the
lines that were due to the iron and dark lines in the
spectrum of the Sun. It was, therefore, proved that
the vapour of iron was a constituent of the atmos-
phere of the Sun.
Continuing his researches by this method, Kirch-
hoff considered that he had demonstrated the exist-
ence in the atmosphere of the Sun of nine metals
known to terrestrial chemistry. They were sodium,
iron, calcium, magnesium, nickel, barium, copper,
zinc, and chromium. Further, it was regarded as
demonstrated, from the absence of their charac-
teristic lines, that twelve other metals, including
gold, silver, and mercury, were absent.
In 1862 Angstrom published the results of an
extensive series of observations, similar in principle
186 Recent Advances in Astronomy.
to those of Kirchhoff, on the chemistry of the Sun's
atmosphere. Experimental details differed from
those adopted by Kirchhoff in that the analysis of
the light was effected by the diffraction grating,
and in the substitution of the electric arc for the
discharge of an induction-coil as the source of heat.
The results were in general agreement with those
of Kirchhoff, but a few additional elements were
detected, among which the most interesting was
hydrogen. The spectrum of hydrogen is of the
highest importance in the astronomical applications
of light analysis. When enclosed within a glass
tube, under a pressure considerably less than that
of the atmosphere, and subjected to a discharge ot
electricity generally led into and from the gas by
platinum wires penetrating the glass hydrogen
gas becomes luminous, emitting a soft peach-like
glow. In 1859, Plucker, subjecting this glow to
analysis in the spectroscope, had found that it con-
sisted in the main of three bright colours, repre-
sented in the spectrum by three bright lines the
first of a magnificent crimson, the second of a
bluish-green, and the third of a deep-blue colour.
Angstrom found that each one of these had its
counterpart among the Fraunhofer lines, and in
1866 he detected a fourth line of a violet colour
in the hydrogen spectrum, and found that it also
was represented by a dark line in that of the Sun.
The important lines c and F in Fraunhofer's nomen-
clature are the reversals of the crimson and bluish-
green lines of hydrogen.
It is unnecessary to give more than the briefest
The Analysis of Sunlight. 187
outline of the later history of these methods. In
1872 Sir Norman Lockyer commenced a laborious
series of comparisons of photographs of the solar
spectrum with those of spectra of metals volatilized
and rendered incandescent by the electric discharge
of an induction-coil, and, as the result, succeeded
during the following four years in adding about
twenty new elements to the fourteen that had been
previously recognized in the atmosphere of the Sun.
In 1887 Messrs. Trowbridge and Hutchins demon-
strated the existence in the Sun of the vapour of
carbon, the first element of a non-metallic nature
that had been found in its atmosphere; while in
1891, also from photographic comparisons, silicon
was detected by Professor Rowland. Some idea
of the fulness of detail shown in Rowland's photo-
graphs may be gained from the fact that coinci-
dences were established in them of upwards of two
thousand of the Fraunhofer lines and bright lines
in the spectrum of iron.
By the series of researches that have been traced,
culminating in the work of Kirchhoff, spectrum
analysis was raised into the position of an exact
science. It appeared to be all-powerful in problems
to which the application of its methods was possible.
Every glowing gas was regarded as emitting and
absorbing definite radiations. By the appearance
of their radiations in the spectroscope gases could
be detected with certainty; and it was at first
not unnaturally concluded that by the absence of
their radiations from glowing matter their own
absence could be asserted with equal confidence.
188 Recent Advances in Astronomy.
Had this anticipation been realized, the determina-
tion of the presence or absence of any terrestrial
element or compound in the atmospheres of the
Sun and stars would have been only a matter of
careful and sufficiently prolonged observations, and
the later course of physical astronomy would have
been strangely different from its actual history.
From about ten years after the date of Kirchhoff s
work, it has become increasingly apparent, that,
although in every case the presence of a glowing
gas is directly demonstrated by the appearance of its
characteristic radiations, some caution is necessary
before the absence of the gas can be inferred with
equal certainty from the absence of those of its radia-
tions with which we are familiar. It has been found
that change of physical condition, change that may
result from alteration of temperature or pressure, or
even from the admixture of other substances, may
cause the familiar radiations of a gas to disappear
and to be replaced by others, frequently to such an
extent that its spectrum may assume an entirely
new and unfamiliar character, in which no relation
to its former self is apparent. Of the many instances
of such modifications of spectra that have been
studied, attention may be specially directed to two,
both of great importance in astronomical physics.
We have seen that the visible spectrum of hydro-
gen consists in the main of three bright lines a
crimson, a green, and a blue while there is a deeper
violet fourth that appears under strong electrical
excitement. Of these, the crimson is the one that
appeals most strongly to the eye. In 1869 Sir
The Analysis of Sunlight. 189
Edward Frankland and Sir Norman Lockyer made
some remarkable observations upon these lines and
those of nitrogen. Hydrogen gas was first enclosed
in a glass tube through which an electric current
was transmitted, and was then rarefied by the action
of an air-pump connected with the tube. The gas
within the tube became incandescent under the in-
fluence of the current, and as rarefaction proceeded,
the lines of its spectrum became finer and more
brilliant, and after a time a limit was reached at
which the first three were most conspicuous. The
gas was then subjected to a further rarefaction while
the electric discharge was maintained at a moderate
intensity. During the progress of the rarefaction
the spectrum was carefully observed, and it was
seen to undergo a striking alteration. The crimson
and blue lines became fainter, although the green
line was scarcely affected, while, ultimately, the
crimson and blue entirely vanished, leaving the
still strong green line as the sole representative of
the radiations of hydrogen. The more complicated
spectrum of nitrogen was, under similar conditions,
also reduced to a single green line ; and, as might
perhaps have been expected, a mixture of hydrogen
and nitrogen under these conditions yielded, when
traversed by an electric discharge, a spectrum con-
sisting of the two green lines already observed.
But, by now moderating the discharge so that the
temperature of the gases should be reduced, the
green nitrogen line in its turn disappeared, while
that due to hydrogen still shone out conspicuously;
so that, in a mixture known to contain nitrogen,
190 Recent Advances in Astronomy.
and traversed by a current of electricity sufficient in
intensity to cause a gas mixed with the nitrogen
to glow, no trace of nitrogen was recorded in the
spectrum.
Changes in the spectrum of calcium are no less
remarkable or important. Compounds of the metal
calcium the metallic base of lime when introduced
into the flame of a bunsen burner by the means
already described, cause the flame to acquire a brick-
red tinge. Observation of its spectrum shows this
light to be composed of rays of different colours,
conspicuous among which is red represented in the
spectrum by a broad red band. When introduced
into the electric arc, the temperature of which is
considerably higher than that of the bunsen flame,
the vapour of calcium gives a spectrum in which
the red band has become much reduced, while a
strong blue line, invisible in the flame spectrum, has
appeared, as well as a pair of brilliant violet lines.
In the spark from the induction-coil, which is pro-
bably at a still higher temperature, the spectrum
entirely loses its red ray, the blue becomes fainter,
while the violet pair are far more strongly developed
than before. The last spectrum of calcium is, there-
fore, as different from the first as if two different
metals had been subjected to examination. Passing
now to the Sun, we find among the Fraunhofer lines
the reversed images of the lines of the last the
spark spectrum : H and K, the great dusky pair lying
almost at the limit of the violet, corresponding in
their positions with the two broadened violet lines,
and a fine dark line to which no special name has
The Analysis of Sunlight. 191
been given with the blue line. It is further interest-
ing to notice that in light condensed from solar pro-
minences which are generally regarded as hotter
than the general atmosphere upon the slit of the
spectroscope (the method by which this is effected
will be described in a later chapter), the blue line
has in its turn disappeared, and the strong pair,
H and K, alone remain as the representatives of
calcium.
Sir Norman Lockyer has interpreted the change
in the spectrum of calcium, as well as similar in-
stances presented by spectra of other metals, as the
direct effect of increase in temperature, and main-
tains that they lend strong support to the view, of
which he has made himself the champion, that by
increase of temperature terrestrial elements become
"dissociated " or resolved into still more elementary
forms of matter. According to this view the pair of
violet lines are not radiated by the vapour of calcium,
but by the vapour of some element contained in cal-
cium and dissociated from it by the temperature of
the electric arc and that of the atmosphere of the
Sun, while the substance emitting the red rays
displayed in the bunsen burner has been entirely
decomposed at these temperatures. Similarly, the
substance, the radiation of which contains the blue
line, is first dissociated from calcium at the tempera-
ture of the arc, is partially dissipated in the hotter
spark, and is entirely destroyed in the still more
intense heat of the prominences. In 1897, however,
Sir William Huggins succeeded in effecting the
same changes in the spectrum of calcium by reduc-
192 Recent Advances in Astronomy.
ing the density of the vapour, without, as he con-
fidently believed, the change being accompanied by
any appreciable increase in temperature, so that it
appears probable that in the experiments first de-
scribed the changes appearing on increase of tem-
perature may have been only indirectly due to that
cause, the direct influence having been the reduction
in density consequent upon the expansion of the
heated vapours.
Although all of the more conspicuous of the
Fraunhofer lines have now been connected with
bright lines in the spectra of terrestrial elements,
the great majority of the whole are still unidentified.
It is, of course, possible that many or even all of
these may yet be found to correspond with the bright
lines of terrestrial elements if it should become
possible to produce in the laboratory conditions
more closely approximating to those that exist in
the atmosphere of the Sun. The failure to detect
the faintest trace of absorption by oxygen in the
solar radiations is very remarkable, in connection
with the extensive distribution and supreme impor-
tance of oxygen in the atmosphere and in the crust
of the Earth. It is true that oxygen is magnificently
represented among the Fraunhofer lines, the great
groups A and B being due to it, but there is no doubt
that these are entirely caused by absorption in the
atmosphere of the Earth. They become decreasingly
conspicuous in the solar spectrum as observations
are made from higher and higher altitudes, and at
such a rate as to indicate that beyond the farthest
limits of the atmosphere all trace of them would
The Analysis of Starlight. 193
disappear from the radiations of the Sun. It is of
course still possible that oxygen may exist in the
Sun's atmosphere under physical conditions so
differing from those with which we are familiar
that its spectrum is unrecognizable, or it is conceiv-
able that it exists dissociated into other and more
elementary forms of matter, the uninterpreted record
of which is before us among the many thousands of
the Fraunhofer lines whose language has still to be
read.
Chapter V.
The Analysis of Starlight.
In the previous chapter we have traced the suc-
cession of sure though laborious steps, by which,
from the decomposition of a beam of sunlight in
Newton's study, a new science has been constructed,
that has given us a revelation of the chemistry of a
body nearly a hundred million miles away across
apparently empty space, the very suggestion of
which would have seemed utterly preposterous a
hundred years ago. That a beam of sunlight, less
than a thousandth of a square inch in section, should
contain latent within it the record of the constitution
of the Sun's atmosphere may well induce hesitation
in imagining any limit to the powers of scientific
methods. While, moreover, steadily extending
astronomical discovery in this direction, the new
method has been developed with no less astounding
success along other lines. It was considered advis-
( M 520 ) Jf
194 Recent Advances in Astronomy.
able to ignore these for the time, and they will
form the subject of the present and the following
chapter.
We have seen that Fraunhofer in 1814 directed to
the light of the stars the method that he had already
applied with such remarkable results to that of the
Sun ; and that he found their spectra to be crossed,
like the spectrum of the Sun, by a number of dark
lines. He found, moreover, that while in some
cases the dark lines of stellar spectra agreed closely
with those seen in the spectrum of the Sun, they
were more often different from them both in their
positions and their relative intensity. The method
adopted by Fraunhofer in these investigations was
a modification of that applied to sunlight, differing
from it chiefly in that it did not involve the use of
a slit, and has since been generally followed in all
cases in which it has not been essential to determine
with a very high degree of accuracy the absolute
as contrasted with the relative positions of the dark
lines. A telescope is directed to the star, and the
practically parallel rays falling upon the object-glass
are by it condensed into a point-like image at its
principal focus. In the ordinary use of the telescope
these rays, continuing their courses, diverge again
after meeting at the focal image, and, after travers-
ing the eye-piece a system of lenses equivalent to
a magnifying-glass, enter the eye. It is convenient
to regard the eye as directly observing the image at
the focus of the object-glass by the aid of the mag-
nifying eye-piece. If now a prism be placed in the
path of the rays, either before or after they enter the
The Analysis of Starlight. 195
telescope, since different colours are deflected dif
ferently, the point-like focal image becomes expandeo
into a line of coloured light. Such a line is, how-
ever, obviously inconvenient for examination ; but
by further interposing a " cylindrical lens" a lens
having for its faces portions of cylindrical instead of
spherical surfaces anywhere in the path of the rays,
the line becomes broadened out into a band of de-
finite width, in which dark lines are clearly visible.
The line of light necessary for the purpose of an-
alysis, instead of being obtained by a slit, is formed
by the extension of the point-like image of the star
into a line by the cylindrical lens. In Fraunhofer's
arrangement of apparatus, the rays passed through
the prism immediately before entering the telescope,
and the cylindrical lens was so adjusted as to be
just beyond the special line of light formed at the
principal focus.
No observations sufficiently delicate to add any-
thing material to Fraunhofer's discoveries in rela-
tion to stellar spectra were made before the
publication of Kirchhoff's researches into the origin
of the Fraunhofer lines in the solar spectrum. By
these researches, the dark lines in the spectrum of
the Sun were traced beyond doubt to the absorption
of the colours corresponding to them in a solar
atmosphere, and there could be little hesitation in
extending the same principle to the explanation of
the similar lines in the spectra of stars. The sun-
like character of the stars, already apparent with
respect to their total luminosity from the establish-
ment of the Copernican system, became more
196 Recent Advances in Astronomy.
intimately confirmed by the evidence revealed in
the analysis of their light that, like the Sun, their
glowing photospheres were enveloped in absorbent,
and therefore cooler, atmospheres.
Although observations of the spectra of stars had
been made for a few years previously by Father
Secchi at Rome, their examination was first attacked
systematically by Sir William Huggins and Dr.
Miller about the year 1863. At that time, and even
at the present, the visual observation of all but the
most brilliant stellar spectra was a most trying and
delicate task. The faint light of a star, even when
collected over the extended area of a large object-
glass and condensed to its focal point, is immeasur-
ably feeble when compared with sunlight. It is
necessary to further enfeeble it; first, by extending
it into a spectral line, and, secondly, by expanding
this line into a band; while, from the atmospheric
unsteadiness, of which the familiar appearance of
twinkling is a result, the excessively faint and, in
most cases, barely visible spectrum is, together with
its delicate system of lines, thrown into a continual
state of tremor. In spite of such difficulties, how-
ever, Huggins and Miller succeeded in detecting in
the spectra of several stars lines corresponding with
those in the spectrum of the Sun as well as with
many bright lines in the spectra of glowing vapours
of terrestrial elements. They also thoroughly con-
firmed Fraunhofer's observations that in many cases
the spectra of stars were strikingly different in the
arrangement and intensity of their dark lines from
that of the Sun.
The Analysis of Starlight. 197
While engaged in these observations, Sir William
Huggins applied the spectroscope to the investiga-
tion of the physical condition of the nebulae. We
have traced in an earlier chapter the development
of astronomical discovery and thought with refer-
ence to these cosmic clouds. We have seen that, at
the time of Sir William Huggins's observation, the
view was very generally entertained that they were
stellar systems, the constituent stars being, in the
great majority of cases, too faint to be individually
distinguishable, though the great telescope of the
Earl of Rosse appeared to have recently effected
the resolution of some of the nobler examples. We
have also followed the main features of Sir William
Huggins's discovery. The telescope was directed
to a small but rather bright nebula in the constella-
tion of the Dragon. The image of the nebula,
formed by the condensation of its rays by the object-
glass, was no longer a point, as with a star, but an
assemblage of points, one corresponding to each of
the luminous points of the nebula ; in fact, an image
of the nebula, such that, if a screen or photographic
plate had been placed behind the object-glass, at a
distance from it equal to its focal length, a perfect,
though excessively faint, picture of the nebula
would have been formed upon it. As a cylindrical
lens would not have expanded such an image into
a line, it was considered expedient to make use of
the more usual slit. The eye-piece of the telescope
was removed, and a spectroscope was fitted in its
place, the length of the collimator lying along the
main axis of the telescope, while the slit was ad-
198 Recent Advances in Astronomy.
justed to the position of the principal focus of the
object-glass. In this position the image of the
nebula was formed upon the "slit-plate" the pair
of metal plates, by the separation of which the slit
was produced. A thin slice of the nebula light thus
entered the slit, and was subjected to analysis and
subsequent examination in the ordinary way.
At a first glance the spectrum of the nebula
appeared to be entirely monochromatic, its light
being all condensed in a single green line. Closer
examination, however, revealed the presence of a far
fainter line rather higher up the spectrum that is,
toward the blue as well as a third, exceedingly
faint, and still higher in the spectrum. The failure
up to that time to observe a spectrum of isolated
bright lines otherwise than from the radiations of a
glowing gas, had caused such a spectrum to be re-
garded as a crucial test of gaseous constitution a
conclusion thoroughly supported by all later spectro-
scopic work and the observation was therefore
universally accepted as demonstrating the gaseous
constitution of the nebula. Of the three lines ob-
served in the spectrum, the third coincided in posi-
tion with the green line of hydrogen, the persistent
character of which was so strikingly illustrated by
Sir Edward Frankland and Sir Norman Lockyer
five years later ; neither of the other two probably
correspond with any lines that have been obtained
in terrestrial experiments, though, at the time, the
first was thought to occupy the position of a line of
nitrogen, to which, however, later measurements
have shown it to be only exceedingly close.
The Analysis of Starlight. 199
Of the many nebulae that have been subjected
to spectroscopic examination since the date of Sir
William Huggins's first observation, about one-
half have been found to yield a spectrum of bright
lines. It may be confidently asserted that the
incandescent matter of all these is gaseous. With
a few exceptions, the remainder yield faint continu-
ous spectra unmarked by any evidence of special
radiation or absorption, but it is not possible to
infer from this alone that they are not either partially
or entirely gaseous. Although a gas alone appears
to possess the power of giving rise to a spectrum
of bright lines, yet, under not abnormal conditions,
its light may yield a continuous spectrum indis-
tinguishable from that of an incandescent solid or
liquid body. Excessive pressure and great depth
of the radiating gas tend to bring about such a
result, but a continuous spectrum has been obtained
from a small quantity of oxygen contained in a
glass tube under considerably less than the at-
mospheric pressure. It is not at present possible
to interpret a continuous nebular spectrum.
The first and brightest of the nebular lines
detected by Sir William Huggins appears to be
specially characteristic of bright-line nebulas. In
every one of their spectra it appears as the brightest
line of the series, while in many it occurs as the
sole representative, other lines, though probably
present, being too faint for detection. In a few
instances other lines than the three originally seen
have been observed, nearly thirty having been
detected by visual and photographic observations
200 Recent Advances in Astronomy.
in that of the Great Nebula of Orion. Of these,
hydrogen lines, including the crimson c, and a
yellow line due to the element helium, the signi-
ficance of which will be seen later, are the only
ones that have been reproduced in laboratory
experiments, so that the chemistry of the nebulas
has only so far been connected with that of the
earth through the elements hydrogen and helium.
In its first application the spectroscope had been
essentially an instrument of chemical research. In
its demonstration of the physical condition of
nebulas it had been brought to bear upon problems
possessing a physical, as well as a purely chemical,
interest; but it was now to invade the domain of
physical science, pure and simple, and with the
most remarkable and far-reaching results.
In 1848 Christian Doppler of Prague had directed
attention to the fact that the apparent pitch of a
musical note became affected during any variation
in the distance separating the instrument emitting
it from the ear. A note appears to rise in pitch
as the source of sound approaches, and to fall in
pitch as it recedes from the ear. The effect was
recognized as a natural consequence of the wave
transmission of sound, and Doppler showed, that,
if light were also transmitted by wave motion, it
should follow from analogous reasoning that the
colour of an object should be affected by the motion
of the source, becoming more violet as the object
approached, and inclining toward red as it receded
from, the observer.
It is a well-known fact that the sensation of sound
The Analysis of Starlight. 201
is due to the transmission of vibrations from a
sounding body to the ear through the agency of
wave motion in the air; and that the pitch of a
note is the result of the frequency of the vibrations
the number executed in a second of time a
doubling of frequency causing a rise in pitch
recognized by the ear as an octave. At each
vibration of the sounding body a single wave is
generated in the immediately surrounding air; the
wave expands outward in an ever -increasing
spherical surface as a ripple on the surface of a
pool unruffled by wind extends in a continually
expanding circle and, upon arriving at the ear,
imparts to the auditory apparatus a vibration
similar to that by which it originated. The vibra-
tion of the sounding body continuing, waves are
continually generated, and follow in regular succes-
sion, so that, under normal conditions, as many
enter the ear every second as are generated by the
sounding body. The frequency of the note heard
is therefore that of the source of sound. If, how-
ever, the source is approaching the ear, this corre-
spondence is no longer maintained. The source
generates waves with the same rapidity as before,
a single wave being produced by each vibration ;
but it is important to notice that the speed with
which the waves travel through the air that is,
the velocity of sound is the same as when the
source was at rest, since it may be shown from
mechanical principles that the velocity of wave
motion is determined solely by the physical pro-
perties of the medium in which they exist, and is
202 Recent Advances in Astronomy.
entirely independent of any motion of the source.
Since, therefore, the waves are travelling through
the air with the same speed as when the source
was at rest, and the source is now following them,
they will be crowded together, and the length of
each will be decreased. The waves, being shorter
than before, and still forming a continuous series
travelling with the original speed, will enter the
ear in more rapid succession, the frequency of
vibration of the auditory apparatus will increase,
and the note will rise in pitch. From analogous
reasoning it will be seen without difficulty that a
recession of the source will result in a diminution
of frequency of vibration, and consequently in a
fall of pitch.
It may be well to give a further and more detailed
illustration of this very important principle. Let
us imagine a tuning-fork at rest, and radiating
waves in the air, each i foot in length waves
that would correspond to a note about two octaves
above the middle C of the piano. By the time ten
vibrations had occurred, and ten waves had conse-
quently been generated, the first wave would have
travelled 10 feet from the fork, since the whole
ten, each a foot in length, would form a continuous
series. Now imagine the fork to move forward,
and assume its velocity to be one-fifth that of the
waves that is, one-fifth the speed of sound and
under these new conditions imagine the original
ten vibrations to be repeated. As before, ten
waves will be generated, each following the last
in regular succession. At the instant of generation
The Analysis of Starlight. 203
of the tenth, the first wave will have reached the
same point as before, its speed being unaffected
by the motion of the fork; but, during its progress,
the fork has been following it with a speed one-
fifth of its own, so that the distance separating it
from the fork is four-fifths of its former value. As
there are the same number of waves lying between
it and the fork, each must therefore be four-fifths
as long as originally. The shorter waves, travel-
ling with the same speed as before, enter the ear
in more rapid succession, and the frequency of
vibration is increased to five-fourths of its former
value. In every case the change in frequency can
be calculated in this simple manner from the relation
between the speed of the moving source and that
of the waves.
It is quite easy to notice the fall in pitch in the
note of the whistle of an engine when passing the
observer at express speed. For a speed of 60 miles
an hour (88 feet per second), the speed of sound
being noo feet per second, the reader should find
little difficulty in showing that the frequency of the
whistle is raised to i -087 of its normal value while
approaching, and decreased to -926 of it while
receding. The relative frequencies of 1*087 an< ^
926 correspond to an interval of nearly three semi-
tones, that from do to la^ in music, and such a change,
which occurs at the instant that the engine passes
the observer, can scarcely escape the notice of the
least musical ear. The change is, of course, still
more strongly marked when the observer, instead of
being at rest, is travelling at express speed in the
204 Recent Advances in Astronomy.
direction opposite to that of the whistling engine.
The fall in pitch of the bell of a passing bicycle is
quite appreciable to anyone with a fairly sensitive
ear, even when at rest by the side of the road.
It is clear that if light depends upon the trans-
mission of waves in the ether, similar changes
must be produced in those waves by the motion
of the source of light towards or from the observer.
By a motion of approach, the waves entering the
eye must be reduced in length; they must arrive
in more rapid succession, and a colour more inclin-
ing to violet must result ; while, from a motion of
recession, the waves must be drawn out; they must
enter the eye in less rapid succession, and the
colour must appear lower in the spectral series.
Such was the prediction of Doppler, and he sug-
gested that the strongly-marked colours of certain
stars might originate in their rapid motions.
At the time of the enunciation of this principle,
the most serious objection to its suggested applica-
tion appeared to lie in the excessive speeds with
which it was necessary to suppose the stars to be
endued. By the rush of a star towards the Earth,
the whole series of spectral colours present in its
radiations were supposed to move up the spectrum,
so that the light received appeared to be abundantly
rich in violet and poor in red rays, the intermediate
colours appearing the same as before, since, as
each became displaced towards the violet, its place
would be supplied by the similarly affected rays
immediately below it. Owing to the high velocity
of light, it was clear, that, to effect any appreciable
The Analysis of Starlight. 205
change of colour by such a process, velocities of
many thousands of miles per second must be
imagined among the stars, and there appeared to
be no warrant for so extravagant an assumption.
A still more fatal objection to Doppler's theory
became apparent, when, in the years immediately
following, the extension of the spectrum into its
invisible ultra-violet and infra-red regions was
discovered. From the existence of these invisible
radiations, it would follow that the colours of the
visible spectrum of a star should be unchanged
by its approach or recession. A motion of ap-
proach would cause the frequency of each radiation
to increase, and, in consequence, each colour would
become more violet in hue, and would experience
greater deviation by the prism. Each colour would,
therefore, be displaced towards the violet end of
the spectrum, assuming the previous tint, while
occupying the former position, of the colour im-
mediately before it. The lowest red rays would
move farther into the spectrum, their colour at the
same time becoming brighter; but the frequency
of the rays immediately below them, previously
just too low to excite the sensation of vision, would
be so increased that they would appear in the
spectrum just within its lowest limits, and would
take the place of those deep rays that had been
displaced upwards. Similarly, the frequency of
the extreme violet waves would be so increased
that they would enter the invisible region beyond.
The colours of the visible spectrum would there-
fore be the same as when the star was at rest.
206 Recent Advances in Astronomy.
There is no doubt that the colour of a star is a
physical fact, initially impressed upon its radiations,
and that it cannot be explained by any theory of
optical illusion.
So far the attempt to apply Doppler's principle
to physical astronomy had been attended with
failure; but in 1848 Fizeau indicated a method by
which it might still be possible to detect by its aid
evidence of the approach or recession of stars in
their analysed light. It was suggested that, instead
of attempting to detect the evidence of motion in
the entire light of stars, close attention should be
directed to the exact positions of the dark lines by
which their spectra were crossed. We have seen
that a dark line in a spectrum indicates the position
in it of colour absent from the radiations; and it
follows that, by a motion of approach of a star,
since the colours that are just more and just less
refrangible than the missing one will be equally
displaced up the spectrum, the gap separating
them will be similarly displaced ; in other words,
every dark line in the spectrum of an approaching
star should be displaced toward the violet of the
spectrum, while, from analogous reasoning, every
dark line in the spectrum of a receding star should
be displaced toward the red.
After several unsuccessful experiments, Sir
William Huggins felt justified in announcing in
1868 that he had succeeded in detecting in the
spectrum of Sirius such a displacement of one of
its spectral lines as would result from a motion of
recession of the star. The selection of Sirius for
The Analysis of Starlight. 207
the purpose of the research was due to the great
intensity of its light, as well as from the strongly-
marked character and undoubted origin of its
spectral lines. It has been found, however, more
recently, that these advantages are seriously dis-
counted by the grave disadvantage arising from
the ill-defined character of the lines. The visual
spectrum of Sirius differs from that of the Sun in
the extraordinary emphasis of the dark lines of
hydrogen absorption indeed, under ordinary con-
ditions these are all that are visible, though, when
the atmosphere is steady and an instrument of high
optical quality is employed, a great number of fine
dark lines, many of them corresponding with
bright lines in the spectrum of iron, may also be
distinguished. The dark hydrogen lines in the
spectrum of Sirius are about six times as broad as
those in the solar spectrum, and, unlike the latter,
which are clearly marked and sharply defined,
those of the star are hazy and pass by insensible
degrees into the bordering light of the spectrum.
There is good reason to regard this difference as
indicating a greater density of the hydrogen in the
atmosphere of the star; since, while at a low
density, as in the ordinary vacuum-tube, glowing
hydrogen gives a spectrum consisting of bright
lines that resemble the dark lines in the spectrum
of the Sun in being fine and sharp, with increased
density of the gas the lines become broad and
badly defined at their edges; and when the gas is
under a pressure approaching, though still distinctly
below, that of the atmosphere, its bright lines very
208 Recent Advances in Astronomy.
closely resemble in their definition the dark lines of
the Sirian spectrum.
To determine whether a dark line in the spectrum
of a star coincides exactly with a corresponding
bright line in the spectrum of an incandescent
terrestrial vapour, it is absolutely necessary that
both should appear in the field of view at the same
time, and this condition necessitates the rejection of
the more convenient cylindrical lens in favour of
the slit. In Huggins's experiment the spectro-
scope was so adjusted that the slit was very near
but not coincident with the principal focus of the
object-glass of a telescope of 8 inches of aperture,
so that a small length of it became illuminated by
the nearly condensed rays of the star, and a spec-
trum of a corresponding width was produced. At
the same time, rarefied hydrogen, contained in a
glass tube placed just beyond the object-glass, was
made to glow by the electric discharge from an in-
duction-coil. Under these conditions the narrow
spectrum of the star became visible, while extending
right across it were the bright lines of the glowing
hydrogen. Attention was specially directed to the
most conspicuous of the dark lines that in the
green part of the spectrum, the far more delicate
bright-green line of the glowing gas appearing to
traverse it in the direction of its length. The most
careful observations of the lines were made many
times, and Huggins felt confident that the bright
line, though it appeared projected upon the dark
one, did not lie along the middle of it, but was
placed rather lower down toward the red of the
The Analysis of Starlight. 209
spectrum. Since, however, it was quite conceivable
that expansion of the dark line in the spectrum of
the star, presumably due to increased density of the
absorbing gas, might not have taken place equally
upon both sides, the observation was not quite
conclusive, but Huggins carefully examined the
spectrum of glowing hydrogen at varying densities,
and found that the broadening accompanying
increase of density was in every case entirely
symmetrical upon either side. The want of co-
incidence between the centres of the lines was
therefore confidently attributed to motion in the
line of sight, and, from its extent, after allowing for
the motion of the Earth in its orbit at the time,
Huggins estimated that the star was receding from
the Sun at the rate of 29^ miles per second.
In the following year Huggins extended his
observations to other stars, and was successful in
detecting displacements of spectral lines in thirty
instances. With some stars, such as Rigel and
Castor, the hydrogen lines were displaced towards
the red end of the spectrum, an indication of reces-
sion ; with others, including Arcturus and Vega,
they were raised towards the violet, and denoted
approach. Further, with the view of confirming
beyond doubt both the soundness of the principle
and the possibility of its practical application, the
spectrum of Venus was observed at times when,
from the position of the planet in its orbit, its
motion was known to be directed towards and away
from the Earth. Since the light of Venus is re-
flected sunlight, the ordinary Fraunhofer lines are
(M520) O
210 Recent Advances in Astronomy.
represented in its spectrum, but a careful examina-
tion of the selected hydrogen lines showed, as had
been confidently anticipated, displacements from
the positions of the lines of terrestrial hydrogen,
and precisely such displacements as were demanded
by theory from the speed of the planet relatively to
that of light.
The approach or recession of an object is known
technically as its motion in the line of sight. It is
clearly but one component of the whole movement,
the other being a drift at right angles to or athwart
the line of sight, and the determination of the com-
plete motion demands a knowledge of both of the
components. Motion of a star across the line of
sight is indicated in its so-called " proper motion ",
or apparent rapidity of drift across the face of the
sky, but, since the apparent drift represented by a
given velocity obviously depends inversely upon
the distance of the star, it is essential to know the
distance before translating proper motion into
definite velocity. Our present knowledge of the
distances of stars is so imperfect that only in a very
few instances is it possible to make the application
with any approach to accuracy, but in a few in-
stances some rough approximation has probably
been possible. The unique power of the spectro-
scopic method of determining motion lies in the fact
that the exact interpretation of its record is entirely
independent of the distance of a star, the same
displacement of spectral lines resulting from a
definite movement in the line of sight, whether the
luminous body is a member of the Solar System,
The Analysis of Starlight. 211
or whether it lies at the extreme limits of fathomable
space.
There can assuredly be but little hesitation in
placing the detection and measurement of the
motions of stars in the line of sight as among the
greatest achievements of physical science. Before
the statement of Doppler's principle, the mere detec-
tion of such movements must have appeared to be
beyond the very possibility of human endeavour, at
any rate without observations extending over many
thousands of years. From the motion of a star in
line of sight, its position upon the face of the
heavens is unaffected. It is true that by the con-
tinuance of such movement, in the course of time
a change in the apparent brightness of a star would
result, as also an alteration of its parallax; but so
many thousands of years must elapse before either
would become appreciable to the most refined
observation, that but little enthusiasm could be
aroused by the contemplation of the ultimate possi-
bility of the successful application of either method.
By the discovery of Huggins, however, an observa-
tion, demanding, it is true, the utmost delicacy, but
which need not extend over more than a few
minutes, has proved sufficient. Again, few dis-
coveries have furnished a finer illustration of the
debt that one science so frequently owes to its
sisters, and of the unexpected nature of the con-
junction towards which accumulation of knowledge
is tending. Upon one line we see the story of
sunlight first roughly sketched in the ray dispersed
by Newton's prism; told with more detail in the
212 Recent Advances in Astronomy.
improved conditions of experiment devised by
Wollaston and Fraunhofer; and again with a still
fuller meaning in the shrewd conjectures of Stokes
and in the experiments of Kirchhoff. Upon a
converging line we trace the first conception of the
wave theory of light in the genius of Huygens,
and, after a century of neglect, its restoration and
establishment upon a firm foundation by Young.
Anon comes Doppler still discovering, though
mistaken as to the exact course he was directing;
then the direction of the course towards its proper
goal by Fizeau ; and, later, its magnificent attain-
ment in the experimental skill of Sir William
Huggins.
In 1870, two years after Huggins's discovery, Dr.
Hermann Vogel, who then had charge of a private
observatory at Bothcamp, was attracted to the
measurement of stellar motions in the line of sight.
For four years at Bothcamp, and for a further
period of thirteen years at Potsdam, where he be-
came possessed of instrumental means of greater
power, Vogel carried out measurements upon
practically the same method as that originally
adopted by Huggins. In 1887, however, by which
time the photographic gelatine dry plate first in-
troduced by Mr. Kennet in 1876 had reached a
high degree of perfection, and had been introduced
with remarkable success in other branches of
astronomy, Vogel applied it to the purpose of his
work, and soon became convinced, that, by photo-
graphing the spectrum of a star together with the
bright lines of glowing hydrogen or some other
The Analysis of Starlight. 213
terrestrial vapour introduced for the purpose of
exact comparison, and by subjecting the com-
pound spectrum so photographed to microscopic
examination, a far higher degree of accuracy could
be attained than was possible in visual observation.
From that date to the year 1891 the photographic
method was consistently followed at Potsdam, and
from the consistency between independent observa-
tions of the same star at different times there can be
no hesitation in regarding the results as constituting
the most exact record that has so far been acquired
of the motions of stars in the line of sight. The
speeds of approach and recession of the following
eight more familiar stars are taken from Vogel's
results, the velocities being given in miles per
second. Due allowance has been made in every
instance for the direction of the speed of the Earth
in its orbit 18*7 miles per second at the time of
observation, so that the numbers are actually the
velocities of the stars relatively to the Sun. So far,
no star has been found to possess a higher velocity
in the line of sight than Aldebaran.
VELOCITIES OF APPROACH AND RECESSION OF STARS
(Miles per second).
Approaching Stars. j Receding Stars.
Sirius, 9-8 ! Aldebaran, 30-2
The Pole Star, ... i6'i , Rigel, 10-2
Arcturus, 4*8 | Capella, 15*2
Vega, 9'5 , Betelgeux, 107
It was during the course of these observations
that the last step was taken in the demonstration of
the existence of the dark companion of Algol. The
2i4 Recent Advances in Astronomy.
essential principles other than the spectroscopic
ones that underlie the investigation have already
been given in an earlier chapter, 1 and the reader
will now have no difficulty in completing the story
of the discovery. It had been shown to follow from
the laws of motion that, if the regular and con-
tinually repeated fading observed in the light of
Algol were due, as was strongly suspected, to its
periodic eclipse by a dark star revolving round it
in an orbit presented edgeways to the Earth, Algol
itself should be in revolution in a similar orbit, and
in the same plane, and should therefore during each
revolution alternately approach and recede from the
Earth. Such approach and recession should pro-
duce an oscillation of the dark lines in the spectrum
of the star, as they became by Doppler's principle
displaced higher and lower in the spectrum, and
precisely such an oscillation of the lines as was
demanded by theory was actually detected in twelve
photographs of the spectrum of Algol taken at
intervals between 1889 and 1891.
An oscillation of spectral lines precisely similar
to that presented by Algol was discovered during
the same period in Spica, the most brilliant star in
the constellation of the Virgin. In this case the
complete oscillation was effected in just over four
days, and the orbital speed of the star indicated by
the displacement of its spectral lines is 56-7 miles
per second. It can, therefore, scarcely be doubted
that, like Algol, Spica is accompanied by a dark
companion, but that the plane of its orbit is inclined
1 See pp. 22-31.
The Analysis of Starlight. 215
to the line of sight to such an extent that, at each
conjunction, the dark star passes either just over
or under it, thus avoiding an eclipse. Vogel is
indeed of opinion that faint traces of the spectrum
of the companion can be detected in the photo-
graphs.
While these refined observations were in pro-
gress at Potsdam, a very beautiful application of
Doppler's principle was effected at the observatory
of Harvard in the United States. From 1886 to
1890 the energy of Professor E. C. Pickering and
his assistants was mainly directed to effecting a
photographic record of the spectra of stars as far as
the eighth magnitude. The method adopted con-
sisted in accurately adjusting a photographic plate
at the principal focus of the object-glass of a tele-
scope, while immediately in front of the object-
glass, a large prism known as an objective prism
was fixed. In the absence of the prism the
sensibly parallel rays of a star would have been
condensed into a point-like image upon the plate,
but by the prism the image was elongated into a
spectral line. The method so far was, as we have
seen, that devised by Fraunhofer, who expanded
the spectral line into a band of sensible width by a
cylindrical lens. In the work at Harvard, however,
no cylindrical lens was employed. The line was
directly photographed, but, by causing the tele-
scope to slowly move relatively to the star so that
the spectral line drifted in a direction at right angles
to its length over the plate a band was produced in
which the dark lines were distinctly visible. All
216 Recent Advances in Astronomy.
stars included within a certain small area of the
heavens toward which the instrument was directed
formed images, and therefore spectra, upon the
plate, which therefore frequently contained a con-
siderable number of stellar spectra. The method
was inferior to that followed at Potsdam in that, in
the necessary absence of a comparison spectrum
from the plate, it was impossible to determine the
absolute positions of lines in their respective spectra
with great accuracy, such, for instance, as would
have been essential to the detection of motion in the
line of sight, but it enabled a far greater number of
stars to be examined in the time, the spectra of over
ten thousand being in fact photographed and ex-
amined in four years. Upon examining several
photographs of the spectrum of Mizar, the middle
star of the three forming the handle of the
" Plough" or the tail of the " Great Bear", the
singular fact appeared that while upon some plates
the dark lines presented a normal appearance, on
others they were doubled ; and upon a more critical
examination it appeared that they opened and
closed with perfect regularity in successive periods
of fifty-two days. A simple and complete explana-
tion of these appearances is found in the assumption
that we are here presented with a system of two
stars, similar to that of Algol and its companion,
except that in the case of Mizar both of the stars are
bright. The actual photographed spectrum would
therefore be a combination of the spectra of the two
stars. The pair being in continual revolution
round their common centre of mass, at the instant
The Analysis of Starlight. 217
of their conjunction with the direction of the Earth
they would be drifting across the line of sight
in opposite directions; neither would be ap-
proaching or receding; the spectral lines of both,
being in their normal positions, would coincide;
and the appearance of a single spectrum would
result. A quarter of the complete period of revolu-
tion later, however, one of the stars would be rush-
ing towards and the other from the Earth, their
lines would consequently experience displacement
in opposite directions in their spectra, and would
appear as separated. In another quarter period
half a revolution would have been accomplished
from the time of the first observation, conjunction
with the Earth would again occur, and the com-
bined spectrum would once more assume its normal
appearance. There can be little hesitation in
accepting this explanation.
From the impracticability of determining the
exact positions of the lines in the spectrum it was
not possible to form an estimate of the actual speeds
of the component stars in the line of sight, but, from
the widest distance to which the lines open out it is
a simple matter to determine for the instant of their
greatest separation the speed of one star relatively
to the other in the line of sight. It appeared to be
about 100 miles per second. If we assume that the
orbits are presented edgewise to the Earth, the
movement of the stars at the time of the widest
separation of their spectral lines would be directly
towards and away from the Earth, and this velocity
would be the actual speed of one star relatively to
2i8 Recent Advances in Astronomy.
the other. From the knowledge of this relative
velocity, and of the complete period of revolution
in this case 104 days it is possible from the laws of
mechanics and gravitation to calculate the combined
mass of the stars. In the result a mass is indicated
of forty times that of the Sun. If the orbits are
merely inclined to the direction of the Earth and are
not presented edgewise, the actual relative speeds,
being at the time of greatest separation of the lines
only in part directed to the Earth, must be greater
than those assumed, and the masses of the stars
must exceed the value deduced from the first
assumptions. Since there are no means of deter-
mining the inclination of the orbits to the line of
sight, it becomes therefore only possible to deter-
mine a limit above which the mass of the double
star must He.
From the spectroscopic examination of an object
so remote that its distance is incapable of determin-
ation, and that in the field of view of the most
powerful telescope appears but as an absolute point
of light, to see a pair of revolving suns; to measure
the period of their mutual revolution ; to trace over
their blazing surfaces cooler atmospheres, and in
these to recognize gases familiar upon the surface
of the Earth ; and to assign a minimum limit to the
mass of the entire system, is an achievement that
can scarcely fail to appeal even to those many and
most hardened of sinners against intellectual light
who would value scientific investigation only in
exact proportion to the monetary equivalent of its
technical application.
The Red Flames of the Sun. 219
Chapter VI.
The Red Flames of the Sun.
We have traced in a previous chapter the course
of discovery resulting from the spectroscopic exam-
ination of the general light of the Sun. From
the time of Fraunhofer's observations to those of
Angstrom, the light submitted to examination was
indeed that of the Sun, but it is important to notice
that no pains had been taken to differentiate the
radiations thrown off by different parts of the Sun's
surface. From the centre, as well as from the edge
of the disc ; from the dark spots and brilliant faculae,
as well as from the delicate extensions of its atmos-
pheric surroundings, only so far recognized during
the brief moments of totality of solar eclipse ; rays
entered the spectroscope and mingled their story in
the resulting spectrum. In 1866, however, Ang-
strom for the first time adopted a different method,
one by which it became possible to examine in
detail the radiations of different parts of the Sun's
surface, and which has during the years that have
followed yielded a veritable harvest of interesting
and valuable results. It will only be possible in
the present chapter to pass under review very briefly
a few of the more remarkable of these in their bear-
ing upon the Physics of the Sun.
Angstrom's device consisted in forming, by means
of a convex lens, a sharply-defined image or picture
of the Sun upon the slit-plate of a spectroscope.
220 Recent Advances in Astronomy.
The result is generally and most conveniently
obtained by adjusting a spectroscope so that its
collimator lies along the axis of an astronomical
telescope, and so that its slit-plate is removed from
the object-glass by a distance equal to its focal
length. Under these conditions, and when the
telescope is directed towards the Sun, the rays from
different portions of the Sun's surface are focussed
at corresponding points upon the plate, and there
is formed upon it a perfectly-defined picture of the
solar disc, in which the sun-spots are clearly visible.
The size of the image is, by elementary optical
laws, in direct proportion to the focal length of the
lens forming it, a picture of the Sun an inch in
diameter necessitating a focal length of about 9
feet. To observe the spectrum of any selected por-
tion of the Sun's surface, the slit is so adjusted that
the image of that particular portion falls upon it.
Under these conditions, the converged rays from the
selected region of the Sun, instead of illuminating
the plate, pass directly through the opening of the
slit, and travelling through the spectroscope, are
subjected to analysis. It will be noticed that the
method is essentially the same as that applied by
Huggins two years earlier to the examination of
the spectra of nebulas. It had also been employed
by Donati in 1864 for the purpose of examining the
spectrum of a comet.
It is important to observe that different portions
of the length of the slit being illuminated by differ-
ent parts of the Sun's surface, the light filling it
may, and indeed frequently does, differ in quality
The Red Flames of the Sun. 221
in different parts of its length. As in the resulting
spectrum the light from each small part of the slit
is spread out into a spectral band, the appearance
is produced of the several and frequently differing
spectra of those portions of the Sun's surface by
which the slit is illuminated arranged as a series of
parallel strips, each being in contact with those
immediately above and below it.
In 1868 the new method was applied to the study
of the " red flames " or " prominences " of the Sun.
At a total eclipse of the Sun, during the few minutes
at most in which the glowing photosphere is covered
by the Moon, there are seen, apparently projecting
from behind the dark disc of the Moon, the delicate
appendages of the Sun known as the "promin-
ences" and the "corona". The corona appears as
an exquisitely beautiful and generally irregular halo
of silvery light surrounding the black circle of the
Moon. It is full of the most delicate detail, and
appears to the eye to consist chiefly of streamers,
some of which frequently extend from the surface
of the Sun to a distance greater than its diameter.
The far smaller but more brilliant "prominences"
are rose-tinted projections that frequently assume
the most fantastic forms, and occasionally extend
to a height of a quarter of the Sun's diameter from
its surface. Though at one time generally regarded
as belonging to the Moon, the prominences had
for some years been recognized as of solar origin,
from the fact that during the progress of an eclipse,
the dark disc of the Moon had been seen to travel
over them, gradually covering those in front, and
222 Recent Advances in Astronomy.
at the same time unveiling those behind the direc-
tion of its motion. A sufficient explanation of their
invisibility upon the limb of the uneclipsed Sun,
even when viewed with the telescope, is that their
fainter light is entirely overwhelmed by the far
more brilliant illumination of the Earth's atmos-
phere produced by the direct solar rays.
In 1868 there occurred an eclipse of the Sun in
which the track of the Moon's shadow travelled
across India. It was observed from several stations
situated at different points of the shadow's path by
scientific men collected from all parts of the civilized
world, and among them the French astronomer
M. Janssen, who had made special arrangements
to examine the spectrum of the prominences. As
they flashed out at the instant of totality, Janssen
rapidly brought the slit of his spectroscope across
the telescopic image of one of the finest, and, on
applying his eye to the instrument, perceived at
once a number of bright separated lines, the certain
indication of gaseous constitution. But this was
not all. Janssen was so impressed with the extreme
brightness of the lines, that, as the prominences
themselves melted from view in the reappearing
sunlight, he perceived the possibility of recognizing
them on the edge of the uneclipsed Sun. Clouds
prevented him from attempting the observation
during the remainder of that day, but on the follow-
ing morning, not long after sunrise, Janssen care-
fully searched the immediate neighbourhood of
the Sun's limb with the spectroscope, and had no
difficulty in again recognizing the brilliant lines of
The Red Flames of the Sun.
223
the spectra of prominences entirely invisible in the
telescopic view of the Sun.
The principle underlying this discovery, a dis-
covery that initiated an entirely new application of
the spectroscope, is extremely beautiful, as well as
simple. The atmospheric glare in the direction of
the Sun, by which the prominences are usually con-
cealed, is scattered sunlight, and as such, yields the
ordinary solar spectrum. The amount of light dis-
tributed over the whole of this spectrum is derived
from that entering the slit, so that the intensity ot
illumination, or brightness, of the spectrum becomes
less and less in direct proportion to its extension in
length, and must continue to do so until the dis-
persion becomes so great that the spectrum, ceasing
to be continuous, breaks up into a number of separ-
ate images of the slit. We have seen, however,
that with sunlight this has never been effected.
Since, by the employment of a number of prisms,
through all of which the light is transmitted in suc-
cession, any required degree of spectral extension
may be obtained, the brightness of the spectrum
may by the same means be reduced to any required
extent. With the prominences, however, this is not
the case. From the fact of their being gaseous, their
light consists of a finite number of, and practically
of a very few, pure colours. The first separation of
these in the spectroscope is, of course, accompanied
by a decrease in brightness, but since the colours
are now pure, they are not further enfeebled to
whatever extent the spectrum is extended ; the only
effect of such extension being to place them farther
224 Recent Advances in Astronomy.
apart. We have then the general result, that the
brightness of the spectrum of the prominences is
only slightly, and to a limited degree, enfeebled by
spectral extension; whereas the spectrum of the
atmospheric glare is enfeebled without limit in
direct proportion to spectral extension or dispersion.
After a certain dispersion, therefore, the general
illumination of the greatly weakened spectrum of the
air glare is no longer sufficiently bright to conceal
the scarcely reduced light of the spectrum of the
prominences, and their bright lines become visible
upon the background of the enfeebled spectrum of
the sunlight scattered in the Earth's atmosphere.
Previously to Janssen's discovery, however, the
principle by which it was effected had been clearly
recognized by the English astronomers. It had
been plainly stated in 1866 by Sir Norman Lockyer,
who, at the time of the eclipse, had a powerful spec-
troscope in process of construction for the purpose
of attempting its application. The instrument was
completed shortly afterwards; and, by its means,
Lockyer detected the spectra of prominences before
the news of Janssen's success had reached Eng-
land. Sir William Huggins had, moreover, already
searched the limb of the Sun for spectra of promi-
nences with a spectroscope of moderate power,
though without success. Upon the announcement
of the discovery, however, he repeated the obser-
vations with the same instrument; and, now that
he was aware of the exact part of the spectrum
toward which to direct his attention, had no diffi-
culty in recognizing the bright lines.
The Red Flames of the Sun. 225
The bright lines of the spectra of prominences
had been observed by several astronomers in India
during the progress of the eclipse. Although other
fainter lines had appeared, one observer having
indeed detected as many as nine, by far the most
conspicuous were three a crimson, a green, and a
yellow. The duration of the eclipse had been too
brief to allow of an accurate determination of the
positions of the lines, but it was believed from their
general appearance that the red and green would
be found to be due to the radiations of hydrogen,
and the yellow to those of sodium. On subse-
quently examining the spectra of prominences at
leisure in the uneclipsed Sun, the coincidences of
the red and green lines with those of glowing
hydrogen were established, but the yellow line was
found to be rather more refrangible than the yellow
of sodium, and not to correspond in position with
any line that had so far been recognized in the
spectrum of a terrestrial gas. It was consequently
assumed to arise from the radiations of a gaseous
constituent of the prominences unfamiliar to terres-
trial chemistry. Later, this hypothetical gas re-
ceived, at the suggestion of Frankland, the name
of helium. For a long time subsequently, helium
remained unrecognized, save as a constituent of the
Sun, stars, and nebulas, until, in 1895, it was dis-
covered by Professor William Ramsay among the
gases extracted from a terrestrial mineral, clevite.
For some days after the eclipse Janssen remained
at his station in India, fascinated with the applica-
tion of the new discovery. He soon found that the
(M520) P
226 Recent Advances in Astronomy.
bright lines originally seen in the spectra of pro-
minences could be traced, though to a far less
distance from it, round the entire limb of the Sun.
There could be no hesitation in ascribing their
continual appearance to the radiations of the in-
candescent atmosphere of the Sun, and it con-
sequently appeared that the prominences were
essentially enormous and local extensions of the
solar atmosphere. Janssen also found that the pro-
minences were essentially unstable, and that they
were subject to changes upon the most stupendous
scale. During the eclipse an enormous red flame,
estimated as being at least 89,000 miles in height,
had been directly seen upon the edge of the Sun :
but, upon the following day, scarcely a trace of
bright lines could be detected by the spectroscope
in the place that it had occupied. From day to
day Janssen traced, from the occurrence and the
varying distance to which they could be followed
from its limb, the mighty surging now recognized
for the first time in the atmosphere of the Sun.
In this, the earliest method of studying pro-
minences upon the limb of the uneclipsed Sun,
observation was confined to a narrow section of
the prominence the image of which was at that
instant formed upon the slit of the spectroscope.
It was possible, however, from the examination
of a number of such sections, to trace the complete
form of the prominence. For this purpose it
was most convenient to adjust the slit so that it
lay "radially", or at right angles to the edge
of the solar image formed upon the slit plate,
The Red Flames of the Sun. 227
and so that a small portion of it penetrated the
image itself. The appearance in the spectroscope
then consisted of the ordinary solar spectrum of
that portion of its surface that illuminated part of
the slit, while above it was extended the bright-
line spectrum of the solar atmosphere and pro-
minences, the length of the lines depending upon
the extension of the radiating gases above the solar
surface. By moving the spectroscope so that the
slit travelled round the solar image while always
maintaining its radial position with reference to it,
the bright lines were seen to contract or extend as
the level of the atmosphere and prominences rose
and fell. From measurements of the lengths of the
lines, it therefore became possible to trace the
varying height of the incandescent gases produc-
ing them, and thus to construct the complete out-
line of a prominence.
In the early part of the following year a further
simplicity was effected in the observation of pro-
minences. In 1869 Sir William Huggins, having
by its spectrum detected a prominence upon the
limb of the Sun, carefully adjusted the slit so as to
lie within the image of the prominence but just
outside of that of the Sun. He then boldly opened
the slit, and saw, not the line spectrum of the pro-
minence, but, in the position where its red line had
appeared, the prominence itself, splendidly dis-
played as a cloud of crimson fire.
The explanation of this very beautiful discovery
is again extremely simple. The opened slit may
be regarded as a narrow window directed towards
228 Recent Advances in Astronomy.
the prominence. The light of the sky entering the
window along with that of the prominence, is
spread out into a very impure spectrum, which,
retaining its continuous character, is greatly en-
feebled in brightness by its extension. The pro-
minence light, however, consisting almost entirely
of three pure colours, is at once resolved, but is
not further weakened. The crimson rays of the
prominence are deflected in block by the prism,
and, being scarcely reduced in intensity, convey to
the eye the appearance of a crimson prominence
superposed upon the enfeebled crimson of the spec-
trum of the atmospheric glare. In a similar man-
ner, a green picture of the prominence is produced
by its green rays, and appears upon the green
region of the air spectrum, while each pure colour
present in the radiations of the prominences must,
in like manner, develop a picture of the prominence
in its own colour. Until very recently this con-
stituted the most powerful method for studying the
varying forms of the prominences. From the
great visual intensity of the crimson light, the
picture formed by it has been the one generally
selected for observation.
The picture presented by solar prominences,
when viewed with fine instrumental means, is of
such extreme beauty as to lend a special charm to
their study. As was first indicated by Lockyer in
1870, they appear to be divisible into two distinct
classes. Those of the first, which are generally
known as quiescent, are commonly the larger. In
appearance they closely resemble terrestrial clouds
The Red Flames of the Sun. 229
bathed in the crimson glory of sunset. Their soft
outlines experiencing but gradual change, they
seem to float, often for days together, at a great
elevation above the glowing surface of the Sun,
being sometimes connected with it by delicate
filaments of light, but at other times entirely sepa-
rated from it. They are to be traced round the
entire limb of the Sun, and appear to have no
direct relation to sun-spots. The spectroscope
shows their light to consist almost entirely of the
radiations of hydrogen and helium.
The prominences of the second class, which are
known as eruptive, appear, as their name implies,
to be the results of veritable explosions from be-
neath the cloud-surface of the photosphere. They
are intensely brilliant. They are subject to the
most violent and rapid changes, and are short-
lived. Their spectra indicate, that, while their light
is, in the main, due to the glowing hydrogen and
helium, the vapours of other metals, and conspicu-
ously those of sodium, magnesium, and iron, are
generally present in them. They are clearly con-
nected with the unrecognized physical cause to
which sun-spots are due, for they are most densely
distributed over the zones on either side of the Sun's
equator to which spots are almost entirely limited;
they appear in greatest abundance at the approxi-
mately regular periods of eleven years that mark
the maximum richness of spot distribution ; and, in
several instances, they have been seen in direct con-
nection with large spots, that have, by the slow
rotation of the Sun, just reached the edge of the
230 Recent Advances in Astronomy.
disc. When it has been possible to trace the con-
nection most exactly, their glowing matter has
appeared to have been projected from the edges of
spots, and later, in its descent, to have fallen
towards their dark and probably depressed centres.
Violent commotions, evidenced by rapid changes
in appearance, are invariably associated with pro-
minences of the eruptive class, but they have
been, from the first days of prominence study, re-
cognized in another manner. The spectrum of a
prominence is usually observed with the slit, which
is now very narrow, lying radially across the limb
of the Sun's image. With such an arrangement
the bright lines of the prominence spectrum seem,
under normal conditions, to be the continuations
of the corresponding dark Fraunhofer lines in the
solar spectrum appearing immediately below and
in contact with them. Frequently, however, this
coincidence is not maintained, a prominence line
appearing to be displaced to one side or the
other of the position of the corresponding Fraun-
hofer line, while occasionally displacements in
different directions are observed as the slit is ad-
justed to different parts of the image of the same
prominence. The reader will have no hesitation in
assigning these displacements to their true cause
to a Doppler effect, due to the rush of glowing pro-
minence matter either directly from or towards the
observer. In this manner velocities in the line of
sight have been recognized of from 200 to 300
miles a second, a velocity upon much the same
scale as that traced across the direction of vision
The Red Flames of the Sun. 231
by the rapidity of changes in prominence forms.
From the detailed method of observation, however,
spectral changes, due to movement in the line of
sight, are now more complicated and fuller of
meaning. Since different parts of the slit are
illuminated by light from different regions of a
prominence, and as each element of length of the
slit gives rise to a correspondingly narrow strip of
the spectrum, it frequently happens that, in different
parts of its length, a bright line displays different
displacements, due to differing velocities in the line
of sight in the corresponding regions of the pro-
minence. Thus, one part of a line may be dis-
placed to the red, while another is displaced to the
violet, and the whole line not unfrequently assumes
a curiously curved and irregular form. It is clear
that, from the study of these contorted lines, the
motion of the glowing matter in the line of sight
may be determined at different levels of the pro-
minence.
For twenty-two years the method discovered by
Huggins for observing prominences remained
supreme. In 1891, however, Professor Hale of
Chicago devised an extremely beautiful instrument,
which he has termed the spectro-heliograph, by
which it has now become possible to record their
pictures by photography in a far more expeditious
and perfect manner. The principle of the spectro-
heliograph is as follows. Although, in the total
quantity of their light, the prominences are so
much less brilliant than the glare produced by
the Sun's rays in the atmosphere of the Earth,
232 Recent Advances in Astronomy.
that they are, excepting when the Sun is totally
eclipsed, permanently invisible ; yet, in their
special radiations, they are so much brighter,
that, when observed under such conditions that
attention is only directed to these, they become
clearly visible. Professor Hale's method consists
essentially in photographing the Sun and its
immediate surroundings by light of one colour
only, selecting as that colour one especially strongly
represented in the radiations of prominences. The
prominence ray that appeals most powerfully to the
eye is, as we have seen, the crimson light of hydro-
gen; but this is quite unsuitable for the purpose
of photography, since it produces no effect upon the
ordinary photographic plate, and those plates that
are specially prepared so as to be affected by it are
very slow in their action. The more refrangible
green light of hydrogen might conceivably be
employed ; but, still better than either of these, are
either of two deep violet rays present in all promin-
ence radiations, and corresponding in their positions
in the spectrum with the two broad dark Fraun-
hofer bands H and K that lie almost at the violet
extremity of the visible spectrum. Owing to the
extreme position of these rays in the spectrum they
scarcely affect the eye, and for this reason would be
entirely unsuitable for visual observation, but they
are extremely energetic in their action upon the
photographic plate. Their radiations are, as we
have already seen, due to the incandescent vapour
of calcium. Although always present in the light
of prominences, owing to the feebleness of their
The Red Flames of the Sun. 233
visual effect they remained undiscovered until
1882, when they were recognized in photographs of
the spectra of prominences seen upon the edge of
the dark moon during the famous total solar eclipse
of that year. Although the dark bands H and K are
very broad, the corresponding violet lines of the
prominence spectra are fine; hence there is this
further advantage in making use of them, that,
since the colours immediately next them in the
spectrum of sunlight are absent or weakly repre-
sented, they are more easily isolated for the pur-
poses of experiment. In practical work it has been
found most convenient to make use of the violet
line that corresponds with K.
In photographing prominences with the spec-
tro-heliograph, an image of the Sun is formed by
the object-glass of a telescope upon the slit plate
of a spectroscope in the usual manner, the image
being, in the apparatus actually employed, about
2 inches in diameter. The extremely narrow slit,
which is somewhat longer than this, lies right
across the picture, and is therefore illuminated by
light from a narrow strip of the Sun and its at-
mospheric surroundings on either side. The
spectrum is focussed upon a small screen which
forms a part of the instrument, and it of course
consists of a succession of images of the slit, one
formed by each colour present in the light that
penetrates it. In the screen there is a second slit,
and this is carefully adjusted so as to be in the
exact position of the bright K line. The violet K
light, therefore, and that only, penetrates the second
234 Recent Advances in Astronomy.
slit, and forms an image of the first upon a photo-
graphic plate that is placed immediately beyond.
There being formed upon the surface of the plate
an exact picture of the illuminated slit, so far as its
K radiation is concerned, the picture of the slice of
the Sun, the image of which is focussed upon the
slit, is therefore reproduced by these rays. By the
regular action of a water-clock, the slit of the
spectroscope is now made to travel across the image
of the Sun, thus successively including images of
all portions of its surface, while, by a proper
mechanism, at the same time, and with such a
perfectly sympathetic movement that the K line of
the moving spectrum continually coincides in posi-
tion with the second slit, the screen in front of the
photographic plate is carried over it. Adjacent
pictures of adjacent strips of the Sun are therefore
photographed by K light, and thus, in the result, a
complete picture of the Sun is produced, while the
prominences are so rich in K rays that they appear
beautifully defined upon it. The passage of the slit
over the whole image of the Sun occupies but a
fraction of a minute, in which time a survey is
effected that would occupy an observer several
hours to complete less perfectly by visual observa-
tion.
Professor Hale's method is invaluable in recording
not only prominences, but other features of the solar
surface to which we have not as yet referred. Near
the limb of the Sun, and most richly displayed in
the neighbourhood of sun-spots, there are always
visible in telescopic observation irregular bright
The Red Flames of the Sun. 235
masses, commonly forming a rough network over
the surface. The visibility of these masses, which
have received the name of " faculag ", near the limb,
combined with the fact of their disappearance as they
are carried by the rotation of the Sun farther on to
its disc, is satisfactorily explained by the assumption
that they are not appreciably brighter than the clouds
of the photosphere, but that they float at a higher
level. The brightness of the solar disc is readily
seen through a telescope to diminish towards the
limb, a consequence of absorption exercised upon its
light by its atmosphere, the absorption being spe-
cially pronounced in rays coming from the limb to
the eye, by reason of their oblique passage through
the atmosphere, and the consequent great length of
their path involved in it. The faculag being at a
higher level than the general surface, their light
does not experience the effects of absorption in so
marked a manner, and when near the limb they
therefore become visible upon the background of
the dimmed photosphere.
In 1872 Professor Young of New Jersey observed,
that, in the spectra of faculas, fine bright lines always
appeared down the centres of the broad bands H
and K. The appearance probably indicates that the
faculae contain the incandescent vapour of calcium
at a lower density but at a higher temperature than
the same vapour that, in the atmosphere at a lower
level, produces by its absorption of the light of the
photosphere the bands H and K. According to this
view, the light of the photosphere is robbed of the
H and K radiations while traversing the cool and
236 Recent Advances in Astronomy.
dense mass of calcium vapour lying immediately
above it; at a still greater height more intensely
heated clouds of the same vapour in part restore the
rays, but the glowing matter being now at a lower
density, the light is more truly monochromatic, and
narrower spectral lines are the result. If, therefore,
it were possible to view the Sun by its K light, and
that alone, we should in all probability be able to
distinguish the faculas not only near the limb but
over the whole disc; and the method applied by
Janssen to the prominences would probably be suc-
cessful but for the fact that these extreme violet
radiations affect the eye to so slight an extent. The
photographs of the spectro-heliograph are, however,
entirely taken by K radiation, and it is not therefore
surprising to find in them representations of faculae
over the entire picture of the Sun. It should be
added, that when it is desired to photograph the
faculae the operation is carried out precisely in the
manner described, but that in photographing the
more delicate prominences upon the limb, it is better
to exclude the light of the photosphere by covering
its image by a circular disc. In the resulting pic-
ture the Sun consequently appears black, and the
whole strikingly resembles a photograph of the
eclipsed Sun.
In our brief study of the work of the spectroscope
we have but touched upon those of its applications
that have so far proved the most important, probably
because it has been found possible to interpret them.
It has been necessary to pass over a vast accumula-
tion of its records, in some of which a meaning is
The Red Flames of the Sun. 237
indicated with less certainty, but which, in greater
part, have utterly baffled rational conjecture. It
cannot be imagined, however, for a moment that the
story of the spectroscope is as a tale that is told.
Year by year its record is accumulating, while year
by year advance in other branches of physical science
aids in the task of dealing effectively with it. The
story of its work during the past fifty years is, how-
ever, alone a noble record of scientific achievement ;
and it is with feelings of highest interest and keenest
expectation that the astronomer is now watching its
continual development. But the path that we have
followed with some care is one only of a number
along which knowledge is advancing with no less
success and promise of future triumph. At the close
of the nineteenth century as never before does the
music of Nature resound with a soul-inspiring har-
mony, and never in the past have the paths of science
appeared so exquisitely attractive to her children.
Index.
Aberration of light, 50, 51.
Absorption, mechanism of, 177.
Algol, discovery of dark companion,
22, 213.
Alpha Centauri, distance of, 6.
Alpha Crucis, relation to Milky Way,
go.
Alpha Cygni, relation to Milky Way,
86.
Andrews, critical temperature of
gases, 37.
Andromeda, nebula in, 16.
Angstrom, explanation of dark lines
in solar spectrum, 179; method of
observing spectrum of Sun, 219;
researches on chemistry of the
Sun, 1 86.
Arcturus, motion of, 46, 70.
Barnard, observations of Mars, 120,
121, 129; photographs of the
Milky Way, 66, 67.
Becquerel, photographs ultra-violet
region of spectrum, 167.
Bessel, first detects parallax of a
star, 5, 50.
Bifurcation of Milky Way, 60.
Boeddicker, drawing of the Milky
Way as seen by the naked eye,
89, 131-
Break in the Milky Way, 62.
Brewster, discovers telluric lines in
solar spectrum, 166.
Bunsen and Kirchhoff, researches
in spectrum analysis, 180.
Calcium, incandescent vapour of, in
faculae, 235; variation of spectrum
under differentphysical conditions,
190.
Carbon, discovery of vapour in at-
mosphere of Sun, 187.
Carbonic acid gas, suggested as a
constituent of atmosphereof Mars,
142.
Clustering of stars in neighbourhood
of Sun, 97.
Coal Sack, in Milky Way, 61, 76,
81, 83, 89.
Collisions between stars, 45, 48.
Colour, effect of motion of source
upon, 29.
Colour, relation to length and fre-
quency of ether waves, 173, 175.
Copernicus, his view regarding the
nature of the stars, a.
Corona, 221 ; drawings of, 131.
Daguerre, his discoveries in photo-
graphy, 167.
Dark matter in space, possible exis-
tence of, 13, 32, 67.
Dark stars, 13, 32; possibility of
detection, 22, 35.
Diffraction grating, 160.
Doppler, enunciation of principle
regarding the effect of motion of
source on generated waves, 29,
200.
Doppler's principle, application to
the discovery of Algol's com-
panion, 29.
Douglass, his observations of Mars,
I2O, 126.
240
Recent Advances in Astronomy.
Draper, photographs infra-red region
of spectrum, 168; photographs the
Moon, 167.
Earth, appearance of, as seen from
a planet, 114.
Eclipse of 1868, 222.
Energy, conservation of, 45.
Ether of space, 172.
Faculae, solar, 234, 235.
Fizeau, indicates correct application
of Doppler's principle, 29, 206.
Foucault, observes absorption of D
light by gases of electric arc,
168, 170.
Fraunhofer, his improvements in the
spectroscope, 156; observes dark
lines in spectra of stars, 157;
observes dark lines in spectrum of
Sun, 156.
Fraunhofer lines, first observed by
Wollaston, 155; studied by Fraun-
hofer, 156.
Gamma Cygni, relation to Milky
Way, 87.
Goodricke, suggests eclipse theory
of Algol, 23.
Groombridge 1830, 46, 99.
Hale, photographs prominences and
faculae in uneclipsed Sun, 232.
Heliometer, 54.
Helium, discovered in clevite by
Ramsay, 225 ; a constituent of
some nebulae, 200; in solar atmo-
sphere and prominences, 225.
Henderson, detects parallax of alpha
Centauri, 6.
Herschel, Sir John, his drawing of
the Eta Argus nebula, 130; his
views regarding the structure of
the sidereal system, 80; observes
the relation between the nebulae
and Milky Way, 71.
Herschel, Sir William, observes
antipathy between nebulae and
stats, 71; observes relation of
stars to the Milky Way, 68 ; views
regarding the structure of the
system of the stars, 73, 77 ; views
regarding the Sun, 36.
Huggins, Sir William, demonstrates
gaseous nature of certain nebulae,
21 ; determines motion of stars in
the line of sight, 30, 206; ob-
serves variations in the spectrum
of calcium, 191 ; observes spectra
of stars, 196 ; observes promi-
nences in uneclipsed Sun, 227.
Huygens, enunciates wave theory of
light, 172.
Hydrogen, in atmosphere of Sun
and in solar prominences, 225;
spectrum of, 186.
Janssen, observes spectra of promi-
nences in uneclipsed Sun, 222.
Keeler, his observations of Mars,
129.
Kirchhoff, his researches on the
chemistry of the Sun, 184; his
researches in spectrum analysis,
180.
Lane's law regarding the variation
of temperature of a cooling gas,
38-40-
Light, possible absorption of, in
space, 79 ; refraction of, 147 ; wave
theory of, 172-175.
Lockyer, Sir Norman, his researches
in the chemistry of the Sun, 187 ;
his dissociation hypothesis, 191;
meteoritic hypothesis, 43 {note).
Lowell, his observations on Mars,
166.
Mars, 101; atmosphere of, no, 113,
114; canals of, 102, 124, 132;
climate of, 132, 142; clouds on,
122 ; dark belt surrounding polar
Index.
241
cap on, 121 ; distance of, 103;
gravitation on, 103; gray -green
"seas" on, 117, 120; limb-light
on, no, 112; " a miniature of the
Earth", 102; mass of, 103; op-
positions of, 108; orange con-
tinents on, 117; phases of, 104;
polar caps of, 1 16 ; rotation of,
108 ; seasons on, 109 ; size of, 103 ;
speculations on possible inhabi-
tants, 127; telescopic appearance
of, 108 ; vapour of water in atmo-
sphere of, 115.
Milky Way, Barnard's photographs
of, 66, 67; a collection of faint
stars, 60 ; dark rifts in, 67, 81 ; a
definite formation, 64, 82; dis-
tance of, 62, 92; early views re-
garding, 59; general appearance
of, 59 ; lines of stars in, 67 ; nebu-
lous matter in, 64, 68, 87 ; photo-
graphs of, 65 ; structure apparent
in, 63; telescopic appearance of,
60,64.
Moon, nature of motion round
Earth, 25.
Nebulae, constitution of, 197; de-
monstration of gaseous nature of,
19, 197; early conjectures as to
nature of, 16 ; Herschel's views
regarding, 16 ; regarded as exter-
nal galaxies, 18; relation of, to
Milky Way, 71; temperature of,
38.
Newton, Sir Isaac, his analysis of
sunlight, 144.
Oppositions of a planet, 104.
Orion, nebula in, 13 ; star streams
in, related to Milky Way, 88.
Oxygen, its apparent absence from
the atmosphere of the Sun, 192.
Parallax, method of relative, 3, 52,
56; of a star, 5, 51, 52.
(M620)
Photographs, of infra region of
spectrum, 168 ; method of taking,
of celestial objects, 66 ; of Milky
Way, 66, 67; of spectrum, 167,
168, 187; of ultra-violet region
of spectrum, 167.
Photography, discovery by Da-
guerre, 167; invention of gelatine
plate, 212.
Photosphere, solar, 36.
Pickering, E. C., his discovery of
spectroscopic double stars, 215.
Pickering, W. H., his observations
of Mars, 121, 126.
Prism, action of, upon light, 149.
Proctor, maintains relation of lucid
stars to Milky Way, 69, 86, 89 ;
suggests possible structure of
Milky Way, 83.
Prominences, solar, 228 ; connection
with sun-spots, 229.
Radiation, mechanism of, 176.
Ramsay, discovers helium, 225.
Ranyard, maintains intimate rela-
tion between stars and Milky
Way, 86.
Refraction, by atmosphere, 51 ; of
light, 147.
Resonance, 177.
Reversal of spectral lines, first ob-
served by Foucault, 168. 170 ;
theory of, enunciated by Stokes,
171-179.
Rowland, his photographs of solar
spectrum, 187.
Schiaparelli, discovers the canals of
Mars, 124.
Selective absorption, 135, 138.
Simms, applies collimator to spec-
troscope, 157.
Sirius, brightness of, 8 ; distance of,
7 ; motion of, in line of sight, 206,
209 ; spectrum of, 207.
Sky, cause of appearance of, no.
Q
242 Recent Advances in Astronomy.
Spectra, conditions of purity of, 152,
157, 158 ; of flames, 163, 165 ; of
solar prominences, 222 ; variation
of, with different physical condi-
tions, 188; of nebulae, 198; of stars,
194 ; of Sun, 144.
Spectro-heliograph, 236.
Spectroscope, principle of, 153;
prismatic, 158.
Stars, death stage of, 22 ; distances
of, 3-8, 50 ; hypothesis of uniform
distribution of, in space, 77, 78,
94, 96; magnitudes of, 92; motion
of, 45; motion of, in the line of
sight, 213; relation of, to Milky
Way, 68, 71, 82, 86, 91, 92, 98;
spectra of, method of obtaining,
194; spectroscopic double, 215;
sun-like nature of, 2, 8.
Stewart, Balfour, states condition
necessary for reversal of spectral
lines, 182.
Stokes, Sir Gabriel, explains reversal
of spectral lines, 171-179.
Struve, Wilhelm, his views on struc-
ture of sidereal system, 81.
Sun, death stage of, n ; life history
of, 42 ; motion of, in space, 47;
source of heat of, n ; telescopic
appearance of, 35.
Sympathetic vibration, 177.
Telluric lines in solar spectrum, 166.
Tyndall, his researches on selective
absorption, 137; illustrates the
theory of sky formation, no.
Universe, last catastrophe of, 49.
Vogel, demonstrates existence of
dark companion of Algol, 24, 31 ;
measures motions of stars in the
line of sight, 212.
Waters, Sydney, his map of nebulae
in their relation to Milky Way, 72.
Waves, formation of, 176 ; of light,
172-175 ; of sound, 174, 200.
Wollaston, first observes dark lines
in solar spectrum, 156; improves
conditions for viewing spectra,
I 55-
Wright, of Durham, his theory re-
garding the structure of the stellar
system, 73.
Young, C. A., observes reversal of
calcium lines in faculae, 235.
Young, Thomas, establishes the
wave theory of light, 172.
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