by Draper, September 30, 1880.[1542] The marked approach towards a still
more perfectly satisfactory result shown by his plates of March, 1881
and 1882, was unhappily cut short by his death. Meanwhile, M. Janssen
was at work in the same field from 1881, with his accustomed
success.[1543] But Dr. A. Ainslie Common left all competitors far behind
with a splendid picture, taken January 30, 1883, by means of an exposure
of thirty-seven minutes in the focus of his 3-foot silver-on-glass
mirror.[1544] Photography may thereby be said to have definitely assumed
the office of historiographer to the nebulæ, since this one impression
embodies a mass of facts hardly to be compassed by months of labour with
the pencil, and affords a record of shape and relative brightness in the
various parts of the stupendous object it delineates which must prove
invaluable to the students of its future condition. Its beauty and merit
were officially recognised by the award of the Astronomical Society's
Gold Medal in 1884.
A second picture of equal merit, obtained by the same means, February
28, 1883, with an exposure of one hour, is reproduced in the
frontispiece. The vignette includes two specimens of planetary
photography. The Jupiter, with the great red spot conspicuous in the
southern hemisphere, is by Dr. Common. It dates from September 3, 1879,
and was accordingly one of the earliest results with his 36-inch, the
direct image in which imprinted itself in a fraction of a second, and
was subsequently enlarged on paper about twelve times. The exquisite
little picture of Saturn was taken at Paris by MM. Paul and Prosper
Henry, December 21, 1885, with their 13-inch photographic refractor. The
telescopic image was in this case magnified eleven times previous to
being photographed, an exposure of about five seconds being allowed; and
the total enlargement, as it now appears, is nineteen times. A trace of
the dusky ring perceptible on the original negative is lost in the
print.
A photograph of the Orion nebula taken by Dr. Roberts in 67 minutes,
November 30, 1886, made a striking disclosure of the extent of that
prodigious object. More than six times the nebulous area depicted on Dr.
Common's plates is covered by it, and it plainly shows an adjacent
nebula, separately catalogued by Messier, to belong to the same vast
formation.
This disposition to annex and appropriate has come out more strongly
with every increase of photographic power. Plates exposed at Harvard
College in March, 1888, with an 8-inch portrait-lens (the same used in
the preparation of the Draper Catalogue) showed the old-established
"Fish-mouth" nebula not only to involve the stars of the sword-handle,
but to be in tolerably evident connection with the most easterly of the
three belt-stars, from which a remarkable nebulous appendage was found
to proceed.[1545] A still more curious discovery was made by W. H.
Pickering in 1889.[1546] Photographs taken in three hours from the
summit of Wilson's Peak in California revealed the existence of an
enormous, though faint spiral structure, enclosing in its span of nearly
seventeen degrees the entire stellar and nebulous group of the Belt and
Sword, from which it most likely, although not quite traceably, issues
as if from a nucleus. A startling glimpse is thus afforded of the
cosmical importance of that strange "hiatus" in the heavens which
excited the wonder of Huygens in 1656. The inconceivable attenuation of
the gaseous stuff composing it was virtually demonstrated by Mr.
Ranyard.[1547]
In March, 1885, Sir Howard Grubb mounted for Dr. Isaac Roberts, at
Maghull, near Liverpool (his observatory has since been transferred to
Crowborough in Sussex), a silver-on-glass reflector of twenty inches
aperture, constructed expressly for use in celestial photography. A
series of nebula-pictures, obtained with this fine instrument, have
proved highly instructive both as to the structure and extent of these
wonderful objects; above all, one of the great Andromeda nebula, to
which an exposure of three hours was given on October 1, 1888.[1548] In
it a convoluted structure replaced and rendered intelligible the
anomalously rifted mass seen by Bond in 1847.[1549] The effects of
annular condensation appeared to have stamped themselves upon the plate,
and two attendant nebulæ presented the aspect of satellites already
separated from the parent body, and presumably revolving round it. The
ring-nebula in Lyra was photographed at Paris in 1886, and shortly
afterwards by Von Gothard with a 10-inch reflector,[1550] and he
similarly depicted in 1888 the two chief spiral and other nebulæ.[1551]
Photographs of the Lyra nebula taken at Algiers in 1890,[1552] and at
the Vatican observatory in 1892,[1553] were remarkable for the strong
development of a central star, difficult of telescopic discernment, but
evidently of primary importance to the annular structure around.
The uses of photography in celestial investigations become every year
more manifold and more apparent. The earliest chemical star-pictures
were those of Castor and Vega, obtained with the Cambridge refractor in
1850 by Whipple of Boston under the direction of W. C. Bond. Double-star
photography was inaugurated under the auspices of G. P. Bond, April 27,
1857, with an impression, obtained in eight seconds, of Mizar, the
middle star in the handle of the Plough. A series of measures from
sixty-two similar images gave the distance and position-angle of its
companion with about the same accuracy attainable by ordinary
micrometrical operations; and the method and upshot of these novel
experiments were described in three papers remarkably forecasting the
purposes to be served by stellar photography.[1554] The matter next fell
into the able hands of Rutherfurd, who completed in 1864 a fine object
glass (of 11-1/2 inches) corrected for the ultra-violet rays,
consequently useless for visual purposes. The sacrifice was recompensed
by conspicuous success. A set of measurements from his photographs of
nearly fifty stars in the Pleiades, and their comparison with Bessel's
places, enabled Dr. Gould to announce, in 1866, that during the
intervening third of a century no changes of importance had occurred in
their relative positions.[1555] And Mr. Harold Jacoby[1556] similarly
ascertained the fixity of seventy-five of Rutherfurd's Atlantids,
between the epoch 1873 and that of Dr. Elkin's heliometric triangulation
of the cluster in 1886,[1557] extending the interval to twenty-seven
years by subsequent comparisons with plates taken at Lick, September 27,
1900.[1558] Positive, however, as well as negative results have ensued
from the application of modern methods to that antique group.
On October 19, 1859, Wilhelm Tempel, a Saxon peasant by origin, later a
skilled engraver, discovered with a small telescope, bought out of his
scanty savings, an elliptical nebulosity, stretching far to the
southward from the star Merope. It attracted the attention of many
observers, but was so often missed, owing to its extreme susceptibility
to adverse atmospheric influences, as to rouse unfounded suspicions of
its variability. The detection of this evasive object gave a hint,
barely intelligible at the time, of further revelations of the same kind
by more cogent means.
A splendid photograph of 1,421 stars in the Pleiades, taken by the MM.
Henry with three hours' exposure, November 16, 1885, showed one of the
brightest of them to have a small spiral nebula, somewhat resembling a
strongly-curved comet's tail, attached to it. The reappearance of this
strange appurtenance on three subsequent plates left no doubt of its
real existence, visually attested at Pulkowa, February 5, 1886, by one
of the first observations made with the 30-inch equatoreal.[1559] Much
smaller apertures, however, sufficed to disclose the "Maia nebula,"
_once it was known to be there_. Not only did it appear greatly extended
in the Vienna 27-inch,[1560] but MM. Perrotin and Thollon saw it with
the Nice 15-inch, and M. Kammermann of Geneva, employing special
precautions, with a refractor of only ten inches aperture.[1561] The
advantage derived by him for bringing it into view, from the insertion
into the eye-piece of a uranium film, gives, with its photographic
intensity, valid proof that a large proportion of the light of this
remarkable object is of the ultra-violet kind.
The beginning thus made was quickly followed up. A picture of the
Pleiades procured at Maghull in eighty-nine minutes, October 23, 1886,
revealed nebulous surroundings to no less than four leading stars of the
group, namely, Alcyone, Electra, Merope, and Maia; and a second
impression, taken in three hours on the following night, showed further
"that the nebulosity extends in streamers and fleecy masses till it
seems almost to fill the spaces between the stars, and to extend far
beyond them."[1562] The coherence of the entire mixed structure was,
moreover, placed beyond doubt by the visibly close relationship of the
stars to the nebulous formations surrounding them in Dr. Roberts's
striking pictures. Thus Goldschmidt's notion that all the clustered
Pleiades constitute, as it were, a second Orion trapezium in the midst
of a huge formation of which Tempel's nebula is but a fragment,[1563]
has been to some extent verified. Yet it seemed fantastic enough in
1863.
Then in 1888 the MM. Henry gave exposures of four hours each to several
plates, which exhibited on development some new features of the
entangled nebulæ. The most curious of these was the linking together of
stars by nebulous chains. In one case seven aligned stars appeared
strung on a silvery filament, "like beads on a rosary."[1564] The "rows
of stars," so often noticed in the sky, may, then, be concluded to have
more than an imaginary existence. Of the 2,326 stars recorded in these
pictures, a couple of hundred among the brightest can, at the outside,
be reckoned as genuine Pleiades. The great majority were relegated, by
Pickering's[1565] and Stratonoff's[1566] counts of the stellar populace
_in_ and _near_ the cluster, to the position of outsiders from it. They
are undistinguished denizens of the abysmal background upon which it is
projected.
Investigations of its condition were carried a stage further by Barnard.
On November 14, 1890,[1567] he discovered visually with the Lick
refractor a close nebulous satellite to Merope, photographs of which
were obtained by Keeler in 1898.[1568] It appears in them of a rudely
pentagonal shape, a prominent angle being directed towards the adjacent
star. Finally, an exposure of ten hours made by Barnard with the Willard
lens indicated the singular fact that the entire group is embedded in a
nebulous matrix, streaky outliers of which blur a wide surface of the
celestial vault.[1569] The artist's conviction of the reality of what
his picture showed was confirmed by negatives obtained by Bailey at
Arequipa in 1897, and by H. C. Wilson at Northfield (Minnesota) in
1898.[1570]
With the Ealing 3-foot reflector, sold by Dr. Common to Mr. Crossley,
and by him presented to the Lick Observatory, Professor Keeler took in
1899 a series of beautiful and instructive nebula[1571] photographs; One
of the Trifid may be singled out as of particular excellence. An
astonishing multitude of new nebulæ were revealed by trial-exposures
with this instrument. A "conservative estimate" gave 120,000 as the
number coming within its scope. Moreover, the majority of those actually
recorded were of an unmistakable spiral character, and they included
most of Sir John Herschel's "double nebulæ," previously supposed to
exemplify the primitive history of binary stellar systems.[1572] Dr. Max
Wolf's explorations with a 6-inch Voigtländer lens in 1901 emphatically
reaffirmed the inexhaustible wealth of the nebular heavens. In one
restricted region, midway between Præsepe and the Milky Way, he located
135 nebulæ, where only three had until then been catalogued; and he
counted 108 such objects clustering round the star 31 Comæ
Berenices,[1573] and so closely that all might be occulted together by
the moon. The general photographic Catalogue of Nebulæ which Dr. Wolf
has begun to prepare[1574] will thus be a most voluminous work.
The history of celestial photography at the Cape of Good Hope began with
the appearance of the great comet of 1882. No special apparatus was at
hand; so Sir David Gill called in the services of a local artist, Mr.
Allis of Mowbray, with whose camera, strapped to the Observatory
equatoreal, pictures of conspicuous merit were obtained. But their
particular distinction lay in the multitude of stars begemming the
background. (See Plate III.) The sight of them at once opened to the
Royal Astronomer a new prospect. He had already formed the project of
extending Argelander's "Durchmusterung" from the point where it was left
by Schönfeld to the southern pole; and his ideas regarding the means of
carrying it into execution crystallised at the needle touch of the
cometary experiments. He resolved to employ photography for the purpose.
The exposure of plates was accordingly begun, under the care of Mr. Ray
Woods, in 1885; and in less than six years, the sky, from 19° of south
latitude to the pole, had been covered in duplicate. Their measurement,
and the preparation of a catalogue of the stars imprinted upon them,
were generously undertaken by Professor Kapteyn, and his laborious task
has at length been successfully completed. The publication, in 1900, of
the third and concluding volume of the "Cape Photographic
Durchmusterung"[1575] placed at the disposal of astronomers a
photographic census of the heavens fuller and surer than the
corresponding visual enumeration executed at Bonn. It includes 454,875
stars, nearly to the tenth magnitude, and their positions are reliable
to about one second of arc.
The production of this important work was thus a result of the Cape
comet-pictures; yet not the most momentous one. They turned the scale in
favour of recourse to the camera when the MM. Henry encountered, in
their continuation of Chacornac's half-finished enterprise of ecliptical
charting, sections of the Milky Way defying the enumerating efforts of
eye and hand. The perfect success of some preliminary experiments made
with an instrument constructed by them expressly for the purpose was
announced to the Academy of Sciences at Paris, May 2, 1885. By its means
stars estimated as of the sixteenth magnitude clearly recorded their
presence and their places; and the enormous increase of knowledge
involved may be judged of from the fact that, in a space of the Milky
Way in Cygnus 2° 15' by 3°, where 170 stars had been mapped by the old
laborious method, about five thousand stamped their images on a single
Henry plate.
These results suggested the grand undertaking of a general photographic
survey of the heavens, and Gill's proposal, June 4, 1886, of an
International Congress for the purpose of setting it on foot was
received with acclamation, and promptly acted upon. Fifty-six delegates
of seventeen different nationalities met in Paris, April 16, 1887, under
the presidentship of Admiral Mouchez, to discuss measures and organise
action. They resolved upon the construction of a Photographic Chart of
the whole heavens, comprising stars of a fourteenth magnitude, to the
surmised number of twenty millions; to be supplemented by a Catalogue,
framed from plates of comparatively short exposure, giving start to the
eleventh magnitude. These will probably amount to about one million and
a quarter. For procuring both sets of plates, instruments were
constructed precisely similar to that of the MM. Henry, which is a
photographic refractor, thirteen inches in aperture, and eleven feet
focus, attached to a guiding telescope of eleven inches aperture,
corrected, of course, for the visual rays. Each place covers an area of
four square degrees, and since the series must be duplicated to prevent
mistakes, about 22,000 plates will be needed for the Chart alone. The
task of preparing them was apportioned among eighteen observatories
scattered over the globe, from Mexico to Melbourne; but three in South
America having become disabled or inert, were replaced in 1900 by those
at Cordoba, Montevideo, and Perth, Western Australia. Meanwhile, the
publication of results has begun, and is likely to continue for at least
a quarter of a century. The first volume of measures from the Potsdam
Catalogue-plates was issued in 1899, and its successors, if on the same
scale, must number nearly 400. Moreover, ninety-six heliogravure
enlargements from the Paris Chart-plates, distributed in the same year,
supplied a basis for the calculation that the entire Atlas of the sky,
composed of similar sheets, will form a pile thirty feet high and two
tons in weight![1576] It will, however, possess an incalculable
scientific value. For millions of stars can be determined by its means,
from their imprinted images, with an accuracy comparable to that
attainable by direct meridian observations.
One of the most ardent promoters of the scheme it may be expected to
realise was Admiral Mouchez, the successor of Leverrier in the direction
of the Paris Observatory. But it was not granted to him to see the
fruition of his efforts. He died suddenly June 25, 1892.[1577] Although
not an astronomer by profession, he had been singularly successful in
pushing forward the cause of the science he loved, while his genial and
open nature won for him wide personal regard. He was replaced by M.
Tisserand, whose mathematical eminence fitted him to continue the
traditions of Delaunay and Leverrier. But his career, too, was unhappily
cut short by an unforeseen death on October 20, 1896; and the more
eminent among the many qualifications of his successor, M. Maurice
Loewy, are of the practical kind.
The sublime problem of the construction of the heavens has not been
neglected amid the multiplicity of tasks imposed upon the cultivators of
astronomy by its rapid development. But data of a far higher order of
precision, and indefinitely greater in amount, than those at the
disposal of Herschel or Struve must be accumulated before any definite
conclusions on the subject are possible. The first organised effort
towards realising this desideratum was made by the German Astronomical
Society in 1865, two years after its foundation at Heidelberg. The
original programme consisted in the _exact_ determination of the places
of all Argelander's stars to the ninth magnitude (exclusive of the polar
zone), from the reobservation of which, say, in the year 1950,
astronomers of two generations hence may gather a vast store of
knowledge--directly of the apparent motions, indirectly of the mutual
relations binding together the suns and systems of space. Thirteen
observatories in Europe and America joined in the work, now virtually
terminated. Its scope was, after its inception, widened to include
southern zones as far as the Tropic of Capricorn; this having been
rendered feasible by Schönfeld's extension (1875-1885) of Argelander's
survey. Thirty thousand additional stars thus taken in were allotted in
zones to five observatories. Another important undertaking of the same
class is the reobservation of the 47,300 stars in Lalande's _Histoire
Céleste_. Begun under Arago in 1855, its upshot has been the publication
of the great Paris Catalogue, issued in eight volumes, between 1887 and
1902. From a careful study of their secular changes in position, M.
Bossert has already derived the proper motions of a couple of thousand
out of nearly fifty thousand stars enumerated in it.
Through Dr. Gould's unceasing labours during his fifteen years'
residence at Cordoba, a detailed acquaintance with southern stars was
brought about. His _Uranometria Argentina_ (1879) enumerates the
magnitudes of 8,198 out of 10,649 stars visible to the naked eye under
those transparent skies; 33,160 down to 9-1/2 magnitude are embraced in
his "zones"; and the Argentine General Catalogue of 32,468 southern
stars was published in 1886. Valuable work of the same kind has been
done at the Leander McCormick Observatory, Virginia, by Professor O.
Stone; while the late Redcliffe observer's "Cape Catalogue for 1880"
affords inestimable aid to the practical astronomer south of the line,
which has been reinforced with several publications issued by the
present Astronomer Royal at the Cape. Moreover, the gigantic task
entered upon in 1860 by Dr. C. H. F. Peters, director of the Litchfield
Observatory, Clinton (N.Y.), and of which a large instalment was
finished in 1882, deserves honourable mention. It was nothing less than
to map all stars down to, and even below, the fourteenth magnitude,
situated within 30° on either side of the ecliptic, and so to afford "a
sure basis for drawing conclusions with respect to the changes going on
in the starry heavens."[1578]
It is tolerably safe to predict that no work of its kind and for its
purpose will ever again be undertaken. In a small part of one night
stars can now be got to register themselves more numerously and more
accurately than by the eye and hand of the most skilled observer in the
course of a year. Fundamental catalogues, constructed by the old,
time-honoured method, will continue to furnish indispensable
starting-points for measurement; and one of especial excellence was
published by Professor Newcomb in 1899;[1579] but the relative places of
the small crowded stars--the sidereal [Greek: hoi pholloi]--will
henceforth be derived from their autographic statements on the sensitive
plate. Even the secondary purpose--that of asteroidal discovery--served
by detailed stellar enumeration, is more surely attained by photography
than by laborious visual comparison. For planetary movement betrays
itself in a comparatively short time by turning the imprinted image of
the object affected by it from a dot into a trail.
In the arduous matter of determining star distances progress has been
steady, and bids fair to become rapidly accelerated. Together, yet
independently, Gill and Elkin carried out, at the Cape Observatory in
1882-83, an investigation of remarkable accuracy into the parallaxes of
nine southern stars. One of these was the famous Alpha Centauri,
the distance of which from the earth was ascertained to be just
one-third greater than Henderson had made it. The parallax of Sirius, on
the other hand, was doubled, or its distance halved; while Canopus
proved to be quite immeasurably remote--a circumstance which,
considering that, among all the stellar multitude, it is outshone only
by the radiant Dog-star, gives a stupendous idea of its real splendour
and dimensions.
Inquiries of this kind were, for some years, successfully pursued at the
observatory of Dunsink, near Dublin. Annual perspective displacements
were by Dr. Brünnow detected in several stars, and in others remeasured
with a care which inspired just confidence. His parallax for Alpha
Lyræ (0·13") was authentic, though slightly too large (Elkin's final
results gave Pi = 0·082"); and the received value for the
parallax of the swiftly travelling star "Groombridge 1,830" scarcely
differs from that arrived at by him in 1871 (Pi = 0·09"). His
successor as Astronomer-Royal for Ireland, Sir Robert Stawell Ball (now
Lowndean Professor of Astronomy in the University of Cambridge), has
done good service in the same department. For besides verifying
approximately Struve's parallax of half a second of arc for 61 Cygni, he
refuted, in 1811, by a sweeping search for (so-called) "large"
parallaxes, certain baseless conjectures of comparative nearness to the
earth, in the case of red and temporary stars.[1580] Of 450 objects thus
cursorily examined, only one star of the seventh magnitude, numbered
1,618 in Groombridge's Circumpolar Catalogue, gave signs of measurable
vicinity. Similarly, a reconnaissance among rapidly moving stars lately
made by Dr. Chase with the Yale heliometer[1581] yielded no really
large, and only eight appreciable parallaxes among the 92 subjects of
his experiments.
A second campaign in stellar parallax was undertaken by Gill and Elkin
in 1887. But this time the two observers were in opposite hemispheres.
Both used heliometers. Dr. Elkin had charge of the fine instrument then
recently erected in Yale College Observatory; Sir David Gill employed
one of seven inches, just constructed under his directions, in
first-rate style, by the Repsolds of Hamburg. Dr. Elkin completed in
1888 his share of the more immediate joint programme, which consisted in
the determination, by direct measurement, of the average parallax of
stars of the first magnitude. It came out, for the ten northern
luminaries, after several revisions, 0·098", equivalent to a
light-journey of thirty-three years. The deviations from this average
were, indeed, exceedingly wide. Two of the stars, Betelgeux and
Alpha Cygni, gave no certain sign of any perspective shifting; of the rest,
Procyon, with a parallax of 0·334", proved the nearest to our system. At
the mean distance concluded for these ten brilliant stars, the sun would
show as of only fifth magnitude; hence it claims a very subordinate rank
among the suns of space. Sir David Gill's definitive results were
published in 1900.[1582] As the average parallax of the eleven brightest
stars in the southern hemisphere, they gave 0·13", a value enhanced by
the exceptional proximity of Alpha Centauri. Yet four of these
conspicuous objects--Canopus, Rigel, Spica, and Beta Crucis--gave
no sign of perspective response to the annual change in our point of
view. The list included eleven fainter stars with notable proper
motions, and most of these proved to have fairly large parallaxes. Among
other valuable contributions to this difficult branch may be instanced
Bruno Peter's measurements of eleven stars with the Leipzig heliometer,
1887-92;[1583] Kapteyn's application of the method by differences in
right ascension to fifteen stars observed on the meridian 1885-89;[1584]
and Flint's more recent similar determinations at Madison,
Wisconsin.[1585]
The great merit of having rendered photography available for the
sounding of the celestial depths belongs to Professor Pritchard. The
subject of his initial experiment was 61 Cygni. From measurements of 200
negatives taken in 1886, he derived for that classic star a parallax of
0·438", in satisfactory agreement with Ball's of 0·468". A detailed
examination convinced the Astronomer-Royal of its superior accuracy to
Bessel's result with the heliometer. The Savilian Professor carried out
his project of determining all second magnitude stars to the number of
about thirty,[1586] conveniently observable at Oxford, obtaining as the
general outcome of the research an average parallax of 0·056", for
objects of that rank. But this value, though in itself probable, cannot
be accepted as authoritative, in view of certain inaccuracies in the
work adverted to by Jacoby,[1587] Hermann Davis, and Gill. The method
has, nevertheless, very large capabilities. Professor Kapteyn showed, in
1889,[1588] the practicability of deriving parallaxes wholesale from
plates exposed at due intervals, and applied his system, in 1900, with
encouraging success, to a group of 248 stars.[1589] The apparent absence
of spurious shiftings justified the proposal to follow up the completion
of the Astrographic Chart with the initiation of a photographic
"Parallax Durchmusterung."
Observers of double stars are among the most meritorious, and need to be
among the most patient and painstaking workers in sidereal astronomy.
They are scarcely as numerous as could be wished. Dr. Doberck,
distinguished as a computer of stellar orbits, complained in 1882[1590]
that data sufficient for the purpose had not been collected for above 30
or 40 binaries out of between five and six hundred certainly or probably
within reach. The progress since made is illustrated by Mr. Gore's
useful Catalogue of Computed Binaries, including fifty-nine entries,
presented to the Royal Irish Academy, June 9, 1890.[1591] Few have done
more towards supplying the deficiency of materials than the late Baron
Ercole Dembowski of Milan. He devoted the last thirty years of his life,
which came to an end January 19, 1881, to the revision of the Dorpat
Catalogue, and left behind him a store of micrometrical measures as
numerous as they are precise.
Of living observers in this branch, Mr. S. W. Burnham is beyond question
the foremost. While pursuing legal avocations at Chicago, he diverted
his scanty leisure by exploring the skies with a 6-inch telescope
mounted in his back-yard; and had discovered, in May, 1882, one thousand
close and mostly very difficult double stars.[1592] Summoned as chief
assistant to the new Lick Observatory in 1888, he resumed the work of
his predilection with the 36-inch and 12-inch refractors of that
establishment. But although devoting most of his attention to
much-needed remeasurements of known pairs, he incidentally divided no
less than 274 stars, the majority of which lay beyond the resolving
power of less keen and effectually aided eyesight. One of his many
interesting discoveries was that of a minute companion to Alpha
Ursæ Majoris (the first Pointer), which already gives unmistakable signs
of orbital movement round the shining orb it is attached to. Another
pair, Kappa Pegasi, detected in 1880, was found in 1892 to have
more than completed a circuit in the interim.[1593] Its period of a
little over eleven years is the shortest attributable to a _visible_
binary system, except that of Delta Equulei, provisionally
determined by Professor Hussey in 1900 at 5·7 years,[1594] and indicated
by spectroscopic evidence to be of uncommon brevity.[1595] Burnham's
Catalogue of 1,290 Double Stars, discovered by him from 1871 to
1899,[1596] is a record of unprecedented interest. Nearly all the 690
pairs included in it, 2" or less than 2" apart, must be physically
connected; and they offer a practically unlimited field for
investigation; while the notes, diagrams, and orbits appended profusely
to the various entries, are eminently helpful to students and computers.
The author is continuing his researches at the Yerkes Observatory,
having quitted the Lick establishment in 1892. The first complete
enrolment of southern double stars was made by Mr. R. T. A. Innes in
1899.[1597] The couples enumerated, twenty-one per cent. of which are
separated by less than one second of arc, are 2,140 in number. They
include 305 discovered by himself. Dr. See gathered a rich harvest of
nearly 500 new southern pairs with the Lowell 24-inch refractor in
1897.[1598] Professor Hough's discoveries in more northerly zones amount
to 623;[1599] Hussey's at Lick to 350; and Aitken's already to over 300.
There is as yet no certainty that the stars of 61 Cygni form a true
binary combination. Mr. Burnham, indeed, holds them to be in course of
definitive separation; and Professor Hall's observations at Washington,
1879 to 1891, although favouring their physical connection, are far from
decisive on the point.[1600] Dr. Wilsing, from certain anomalous
displacements of their photographed images, concluded in 1893[1601] the
presence of an invisible third member of the system, revolving in a
period of twenty-two months; but the effects noticed by him were
probably illusory.
Important series of double-star observations were made by Perrotin at
Nice in 1883-4;[1602] by Hall, with the 26-inch Washington equatoreal,
1874 to 1891;[1603] by Schiaparelli from 1875 onward; by Glasenapp, O.
Stone, Leavenworth, Seabroke, and many besides. Finally, Professor
Hussey's revision of the Pulkowa Catalogue[1604] is a work of the _teres
atque rotundus_ kind, which leaves little or nothing to be desired. The
methods employed in double-star determinations remain, at the beginning
of the twentieth century, essentially unchanged. The camera has scarcely
encroached upon this part of the micrometer's domain.[1605]
A research of striking merit into the origin of binary stars was
published in 1892 by Dr. T. J. J. See, in the form of an Inaugural
Dissertation for his doctor's degree in the University of Berlin. The
main result was to show the powerful effects of tidal friction in
prescribing the course of their development from double nebulæ,
revolving almost in contact, to double suns, far apart, yet inseparable.
The high eccentricities of their eventual orbits were shown to result
necessarily from this mode of action, which must operate with enormous
strength on closely conjoined, nearly equal masses, such as the rapidly
revolving pairs disclosed by the spectroscope. That these are still in
an early stage of their life-history is probable in itself, and is
re-affirmed by the exceedingly small density indicated for eclipsing
stars by the ratio of phase-duration to period.
Stellar photometry, initiated by the elder Herschel, and provided with
exact methods by his son at the Cape, by Steinheil and Seidel at Munich,
has of late years assumed the importance of a separate department of
astronomical research. Two monumental works on the subject, compiled on
opposite sides of the Atlantic, were thus appropriately coupled in the
bestowal of the Royal Astronomical Society's Gold Medal in 1886. Harvard
College Observatory led the way under the able direction of Professor E.
C. Pickering. His photometric catalogue of 4,260 stars,[1606]
constructed from nearly 95,000 observations of light-intensity during
the years 1879-82, constitutes a record of incalculable value for the
detection and estimation of stellar variability. It was succeeded in
1885 by Professor Pritchard's "Uranometria Nova Oxoniensis," including
photometric determinations of the magnitude of all naked-eye stars, from
the pole to ten degrees south of the equator to the number of 2,784. The
instrument employed was the "wedge photometer," which measures
brightness by resistance to extinction. A wedge of neutral-tint glass,
accurately divided to scale, is placed in the path of the stellar rays,
when the thickness of it they have power to traverse furnishes a
criterion of their intensity. Professor Pickering's "meridian
photometer," on the other hand, is based upon Zöllner's principle of
equalization effected by a polarising apparatus. After all, however, as
Professor Pritchard observed, "the eye is the real photometer," and its
judgment can only be valid over a limited range.[1607] Absolute
uniformity, then, in estimates made by various means, under varying
conditions, and by different observers, is not to be looked for; and it
is satisfactory to find substantial agreement attainable and attained.
Only in an insignificant fraction of the stars common to the Harvard and
Oxford catalogues discordances are found exceeding one-third of a
magnitude; a large proportion (71 per cent.) agree within one-fourth, a
considerable minority (31 per cent.) within one-tenth of a
magnitude.[1608] The Harvard photometry was extended, on the same scale,
to the opposite pole in a catalogue of the magnitudes of 7,922 southern
stars,[1609] founded on Professor Bailey's observations in Peru,
1889-91. Measurements still more comprehensive were subsequently
executed at the primary establishment. With a meridian photometer of
augmented power, the surprising number of 473,216 settings were made
during the years 1891-98, nearly all by the indefatigable director
himself, and they afforded materials for a "Photometric Durchmusterung,"
published in 1901, including all stars to 7·5 magnitude north of
declination -40°.[1610] A photometric zone, 20° wide, has for some time
been in course of observation at Potsdam by MM. Müller and Kempf. The
instrument employed by them is constructed on the polarising principle
as adapted by Zöllner.
Photographic photometry has meanwhile risen to an importance if anything
exceeding that of visual photometry. For the usefulness of the great
international star-chart now being prepared would be gravely compromised
by systematic mistakes regarding the magnitudes of the stars registered
upon it. No entirely trustworthy means of determining them have,
however, yet been found. There is no certainty as to the relative times
of exposure needed to get images of stars representative of successive
photometric ranks. All that can be done is to measure the proportionate
diameters of such images, and to infer, by the application of a law
learned from experience, the varied intensities of light to which they
correspond. The law is, indeed, neither simple nor constant. Different
investigators have arrived at different formulæ, which, being purely
empirical, vary their nature with the conditions of experiment. Probably
the best expedient for overcoming the difficulty is that devised by
Pickering, of simultaneously photographing a star and its secondary
image, reduced in brightness by a known amount.[1611] The results of its
use will be exhibited in a catalogue of 40,000 stars to the tenth
magnitude, one for each square degree of the heavens. A photographic
photometry of all the lucid stars, modelled on the visual photometry of
1884, is promised from the same copious source of novelties. The
magnitudes of the stars in the Draper Catalogue were determined, so to
speak, spectrographically. The quantity measured in all cases was the
intensity of the hydrogen line near G. By the employment of this
definite and uniform test, results were obtained, of special value
indeed, but in strong disaccord with those given by less exclusive
determinations.
Thought, meantime, cannot be held aloof from the great subject upon the
future illustration of which so much patient industry is being expended.
Nor are partial glimpses denied to us of relations fully discoverable,
perhaps, only through centuries of toil. Some important points in
cosmical economy have, indeed, become quite clear within the last fifty
years, and scarcely any longer admit of a difference of opinion. One of
these is that of the true status of nebulæ.
This was virtually settled by Sir J. Herschel's description in 1847 of
the structure of the Magellanic clouds; but it was not until Whewell, in
1853, and Herbert Spencer, in 1858,[1612] enforced the conclusions
necessarily to be derived therefrom that the conception of the nebulæ as
remote galaxies, which Lord Rosse's resolution of many into stellar
points had appeared to support, began to withdraw into the region of
discarded and half-forgotten speculations. In the Nubeculæ, as Whewell
insisted,[1613] "there coexist, in a limited compass and in
indiscriminate position, stars, clusters of stars, nebulæ, regular and
irregular, and nebulous streaks and patches. These, then, are different
kinds of things in themselves, not merely different to us. There are
such things as nebulæ side by side with stars and with clusters of
stars. Nebulous matter resolvable occurs close to nebulous matter
irresolvable."
This argument from coexistence in nearly the same region of space,
reiterated and reinforced with others by Mr. Spencer, was urged with his
accustomed force and freshness by Mr. Proctor. It is unanswerable. There
is no maintaining nebulæ to be simply remote worlds of stars in the face
of an agglomeration like the Nubecula Major, containing in its
(certainly capacious) bosom _both_ stars and nebulæ. Add the facts that
a considerable proportion of these perplexing objects are gaseous, and
that an intimate relation obviously subsists between the mode of their
scattering and the lie of the Milky Way, and it becomes impossible to
resist the conclusion that both nebular and stellar systems are parts of
a single scheme.[1614]
As to the stars themselves, the presumption of their approximate
uniformity in size and brightness has been effectually dissipated.
Differences of distance can no longer be invoked to account for
dissimilarity in lustre. Minute orbs, altogether invisible without
optical aid, are found to be indefinitely nearer to us than such radiant
objects as Canopus, Betelgeux, or Rigel. Moreover, intensity of light is
perceived to be a very imperfect index to real magnitude. Brilliant suns
are swayed from their course by the attractive power of massive yet
faintly luminous companions, and suffer eclipse from obscure
interpositions. Besides, effective lustre is now known to depend no less
upon the qualities of the investing atmosphere than upon the extent and
radiative power of the stellar surface. Red stars must be far larger in
proportion to the light diffused by them than white or yellow
stars.[1615] There can be no doubt that our sun would at least double
its brightness were the absorption suffered by its rays to be reduced to
the Sirian standard; and, on the other hand, that it would lose half its
present efficiency as a light-source if the atmosphere partially veiling
its splendours were rendered as dense as that of Aldebaran.
Thus, variety of all kinds is seen to abound in the heavens; and it must
be admitted that the consequent insecurity of all hypotheses as to the
relative distances of individual stars singularly complicates the
question of their allocation in space. Nevertheless, something has been
learnt even on that point; and the tendency of modern research is, on
the whole, strongly confirmatory of the views expressed by Herschel in
1802. He then no longer regarded the Milky Way as the mere visual effect
of an enormously extended stratum of stars, but as an actual
aggregation, highly irregular in structure, made up of stellar clouds
and groups and nodosities. All the facts since ascertained fit in with
this conception, to which Proctor added arguments favouring the view,
since adopted by Barnard[1616] and Easton,[1617] that the stars forming
the galactic stream are not only situated more closely together, but are
also really, as well as apparently, of smaller dimensions than the lucid
orbs studding our skies. By the laborious process of isographically
charting the whole of Argelander's 324,000 stars, he brought out in
1871[1618] signs of relationship between the distribution of the
brighter stars and the complex branchings of the Milky Way, which has
been stamped as authentic by Newcomb's recent statistical
inquiries.[1619] There is, besides, a marked condensation of stars,
especially in the southern hemisphere, towards a great circle inclined
some twenty degrees to the galactic plane; and these were supposed by
Gould to form with the sun a subordinate cluster, of which the
components are seen projected upon the sky as a zone of stellar
brilliants.[1620] The zone has, however, galactic rather than solar
affinities, and represents, perhaps, not a group, but a stream.
The idea is gaining ground that the Milky Way is designed, in its main
outlines, on a spiral pattern, and that its various branches and
sections are consequently situated at very different distances from
ourselves. Proctor gave a preliminarily interpretation of their
complexities on this principle, and Easton of Rotterdam[1621] has
renewed the attempt with better success.
A most suggestive delineation of the Milky Way, completed in 1889, after
five years of labour, by Dr. Otto Boedicker, Lord Rosse's astronomer at
Parsonstown, was published by lithography in 1892. It showed a curiously
intricate structure, composed of dimly luminous streams, and shreds, and
patches, intermixed with dark gaps and channels. Ramifications from the
main trunk ran out towards the Andromeda nebula and the "Bee-hive"
cluster in Cancer, involved the Pleiades and Hyades, and, winding round
the constellation of Orion, just attained the Sword-handle nebula. The
last delicate touches had scarcely been put to the picture, when the
laborious eye-and-hand method was, in this quarter, as already in so
many others, superseded by a more expeditious process. Professor Barnard
took the first photographs ever secured of the true Milky Way, July 28,
August 1 and 2, 1889, at the Lick Observatory. Special conditions were
required for success; above all, a wide field and a strong light-grasp,
both complied with through the use of a 6-inch portrait-lens. Even thus,
the sensitive plate needed some hours to pick out the exceedingly faint
stars collected in the galactic clouds. These cannot be photographed
under the nebulous aspect they wear to the eye; the camera takes note of
their real nature, and registers their constituent stars rank by rank.
Hence the difficulty of disclosing them. "In the photographs made with
the 6-inch portrait-lens," Professor Barnard wrote, "besides myriads of
stars, there are shown, for the first time, the vast and wonderful
cloud-forms, with all their remarkable structure of lanes, holes, and
black gaps, and sprays of stars. They present to us these forms in all
their delicacy and beauty, as no eye or telescope can ever hope to see
them."[1622] In Plate VI. one of these strange galactic landscapes is
reproduced. It occurs in the Bow of Sagittarius, not far from the Trifid
nebula, where the aggregations of the Milky Way are more than usually
varied and characteristic. One of their distinctive features comes out
with particular prominence. It will be noticed that the bright mass near
the centre of the plate is tunnelled with dark holes and furrowed by
dusky lanes. Such interruptions recur perpetually in the Milky Way. They
are exemplified on the largest scale in the great rift dividing it into
two branches all the way from Cygnus to Crux; and they are reproduced in
miniature in many clusters.
PLATE VI.
[Illustration: Region of the Milky Way in Sagittarius--showing a double
black aperture.
Photographed by Professor E. E. Barnard.]
Mr. H. C. Russell, at Sydney in 1890, successfully imitated Professor
Barnard's example.[1623] His photographs of the southern Milky Way have
many points of interest. They show the great rift, black to the eye, yet
densely star-strewn to the perception of the chemical retina; while the
"Coal-sack" appears absolutely dark only in its northern portion. His
most remarkable discovery, however, was that of the spiral character of
the two Nubeculæ. With an effective exposure of four and a half hours,
the Greater Cloud came out as "a complex spiral, with two centres";
while the similar conformation of its minor companion developed only
after eight hours of persistent actinic action. The revelation is full
of significance.
Scarcely less so, although after a different fashion, is the disclosure
on plates exposed by Dr. Max Wolf, with a 5-inch lens, in June, 1891, of
a vastly extended nebula, bringing some of the leading stars in Cygnus
into apparently organic connection with the piles of galactic star-dust
likewise involved by it.[1624] Barnard has similarly found great tracts
of the Milky Way to be photographically nebulous, and the conclusion
seems inevitable that we see in it a prodigious mixed system, resembling
that of the Pleiades in point of composition, though differing widely
from it in plan of structure. Of corroborative testimony, moreover, is
the discovery independently resulting from Gill's and Pickering's
photographic reviews, that stars of the first type of spectrum largely
prevail in the galactic zone of the heavens.[1625] With approach to that
zone, Kapteyn noticed a steady growth of actinic intensity relative to
visual brightness in the stars depicted on the Cape Durchmusterung
plates.[1626] In other words, stellar light is, in the Milky Way,
_bluer_ than elsewhere. And the reality of the primitive character hence
to be inferred for the entire structure was, in a manner, certified by
Mr. McClean's observation that Helium stars--the supposed immediate
products of nebulous matter--crowd towards its medial plane.
The first step towards the unravelment of the tangled web of stellar
movements was taken when Herschel established the reality, and indicated
the direction of the sun's journey. But the gradual shifting backward of
the whole of the celestial scenery amid which we advance accounts for
only a part of the observed displacements. The stars have motions of
their own besides those reflected upon them from ours. All attempts,
however, to grasp the general scheme of these motions have hitherto
failed. Yet they have not remained wholly fruitless. The community of
slow movement in Taurus, upon which Mädler based his famous theory, has
proved to be a fact, and one of very extended significance.
In 1870 Mr. Proctor undertook to chart down the directions and
proportionate amounts of about 1,600 proper motions, as determined by
Messrs. Stone and Main, with the result of bringing to light the
remarkable phenomenon termed by him "star-drift."[1627] Quite
unmistakably, large groups of stars, otherwise apparently disconnected,
were seen to be in progress together, in the same direction and at the
same rate, across the sky. An example of this kind of unanimity was
alleged by him in the five intermediate stars of the Plough; and that
the agreement in thwartwise motion is no casual one is practically
demonstrated by the concordant radial velocities determined at Potsdam
for four out of the five objects in question. All of these approach the
earth at the rate of about eighteen miles a second; and the fifth and
faintest, Delta Ursæ, though not yet measured, may be held to share
their advance. One of them, moreover, Zeta Ursæ, alias Mizar,
carries with it three other stars--Alcor, the Arab "Rider" of the horse,
visible to the naked eye, besides a telescopic and a spectroscopic
attendant. So that the group may be regarded as octuple. It is of vast
compass. Dr. Höffler assigned to it in 1897[1628]--although on grounds
more or less hypothetical--a mean parallax corresponding to a
light-journey of 192 years, which would give to the marching squadron a
total extent of at least fourteen times the distance from the sun to
Alpha Centauri, while implying for its brightest member--Eta
Ursæ Majoris--the lustre of six hundred suns. The organising principle
of this grand scheme must long remain mysterious.
It is no solitary example. Particular association, indeed--as was
surmised by Michell far back in the eighteenth century--appears to be
the rule rather than an exception in the sidereal system. Stars are
bound together by twos, by threes, by dozens, by hundreds. Our own sun
is, perhaps, not exempt from this gregarious tendency. Yet the search
for its companions has, up to the present, been unavailing. Gould's
cluster[1629] seems remote and intangible; Kapteyn's collection of solar
stars proved to have been a creation of erroneous data, and was
abolished by his unrelenting industry. Rather, we appear to have secured
a compartment to ourselves for our long journey through space. A
practical certainty has, at any rate, been gained that whatever
aggregation holds the sun as a constituent is of a far looser build than
the Pleiades or Præsepe. Of all such majestic communities the laws and
revolutions remain, as yet, inaccessible to inquiry; centuries may
elapse before even a rudimentary acquaintance with them begins to
develop; while the economy of the higher order of association, which we
must reasonably believe that they unite to compose, will possibly
continue to stimulate and baffle human curiosity to the end of time.
FOOTNOTES:
[Footnote 1369: _Report Brit. Assoc._, 1868, p. 166. Rutherfurd gave a
rudimentary sketch of a classification of the kind in December, 1862,
but based on imperfect observation. See _Am. Jour. of Sc._, vol. xxxv.,
p. 77.]
[Footnote 1370: _Publicationem_, Potsdam, No. 14, 1884, p. 31.]
[Footnote 1371: Von Konkoly _once_ derived from a slow-moving meteor a
hydro-carbon spectrum. A. S. Herschel, _Nature_, vol. xxiv., p. 507.]
[Footnote 1372: _Phil. Trans._, vol. cliv., p. 429.]
[Footnote 1373: _Am. Jour. of Sc._, vol. xix., p. 467.]
[Footnote 1374: _Photom. Unters._, p. 243.]
[Footnote 1375: _Spectre Solaire_, p. 38.]
[Footnote 1376: Mr. J. Birmingham, in the Introduction to his Catalogue
of Red Stars, adduced sundry instances of colour-change in a direction
the opposite to that assumed by Zöllner to be the inevitable result of
time. _Trans. R. Irish Acad._, vol. xxvi., p. 251. A learned discussion
by Dr. T. J. J. See, moreover, enforces the belief that Sirius was
absolutely _red_ eighteen hundred years ago. _Astr. and Astroph._, vol.
xi., p. 269.]
[Footnote 1377: _Phil. Trans.,_ vol. clxiv., p. 492.]
[Footnote 1378: _Astr. Nach._, No. 2,000.]
[Footnote 1379: _Proc. Roy. Soc._, vols. xvi., p. 31; xvii., p. 48.]
[Footnote 1380: _Annalen der Physik_, Bd. xx., p. 155.]
[Footnote 1381: _Ibid._, p. 153.]
[Footnote 1382: _Knowledge_, vol. xiv., p. 101.]
[Footnote 1383: _Meteoritic Hypothesis_, p. 380.]
[Footnote 1384: _Phil. Trans._, vol. cxci. A., p. 128; _Spectra of
Southern Stars_, p. 3.]
[Footnote 1385: See the author's _System of the Stars_, p. 84.]
[Footnote 1386: A designation applied by Sir Norman Lockyer to
third-type stars.]
[Footnote 1387: See _ante_, p. 198.]
[Footnote 1388: _Bothkamp Beobachtungen_, Heft ii., p. 146.]
[Footnote 1389: _Astr. Nach._, No. 2,539.]
[Footnote 1390: _Ibid._, No. 2,548; _Observatory_, vol. vi., p. 332.]
[Footnote 1391: _Month. Not._, vol. xlvii., p. 92.]
[Footnote 1392: _Publ. Astr. Pac. Soc._, vol. i., p. 80; _Observatory_,
vol. xiii., p. 46.]
[Footnote 1393: _Lockyer, Proc. Roy. Soc._, vol. lvii., p. 173.]
[Footnote 1394: _Astr. Nach._, No. 3,129.]
[Footnote 1395: _Month. Not._, vol. lix., p. 505.]
[Footnote 1396: _Astr. Nach._, No. 2,581.]
[Footnote 1397: _Ibid._, Nos. 2,651-2.]
[Footnote 1398: _Ibid._, No. 3,051; _Astr. and Astrophysics_, vol. xi.,
p. 25; Bélopolsky, _Astr. Nach._, No. 3,129.]
[Footnote 1399: _Comptes Rendus_, t. lxv., p. 292.]
[Footnote 1400: _Copernicus_, vol. iii., p. 207.]
[Footnote 1401: _System of the Stars_, p. 70; _Harvard Annals_, vol.
xxviii., pt. ii., p. 243 (Miss Cannon).]
[Footnote 1402: _Potsdam Publ._, No. 14, p. 17.]
[Footnote 1403: _Proc. Roy. Soc._, vol. xlix., p. 33.]
[Footnote 1404: Miss A. J. Cannon, _Harvard Annals_, vol. xxviii., pt.
ii., p. 141.]
[Footnote 1405: _Astr. and Astroph._, vol. xiii., p. 448.]
[Footnote 1406: _Potsdam Publ._, No. 2.]
[Footnote 1407: The results of Von Konkoly's extension of Vogel's work
to 15° of south declination were published in _O Gyalla Beobachtungen_,
Bd. viii., Th. ii., 1887.]
[Footnote 1408: _Astroph. Jour._, vols. viii., p. 237; ix., p. 271.]
[Footnote 1409: _Ibid._, vol. ix., p. 119.]
[Footnote 1410: _Phil. Trans._, vol. cliv., p. 413. Some preliminary
results were embodied in a "note" communicated to the Royal Society,
February 19, 1863 (_Proc. Roy. Soc._, vol. xii., p. 444).]
[Footnote 1411: _Bothkamp Beob._, Heft i., p. 25.]
[Footnote 1412: _Astroph. Jour._, vol. vi., p. 423.]
[Footnote 1413: _Phil. Trans._, vol. cliv., p. 429, _note_.]
[Footnote 1414: _Month. Not._, vol. xxiii., p. 180.]
[Footnote 1415: _Proc. Roy. Soc._, vol. xxv., p. 446.]
[Footnote 1416: _Phil. Trans._, vol. clxxi., p. 669; _Atlas of Stellar
Spectra_, p. 22.]
[Footnote 1417: _Astr. Nach._, No. 2,301; _Monatsb._, Berlin, 1879, p.
119; 1880, p. 192.]
[Footnote 1418: _Jour. de Physique_, t. v., p. 98.]
[Footnote 1419: _System of the Stars_, p. 39.]
[Footnote 1420: See _ante_, p. 198.]
[Footnote 1421: _Proc. Roy. Soc._, vol. xlviii., p. 314.]
[Footnote 1422: _Harvard Circulars_, Nos. 12, 18; _Astroph. Jour._, vol.
v., p. 92.]
[Footnote 1423: _Astroph. Jour._, vol. vi., p. 233.]
[Footnote 1424: McClean, _Phil. Trans._, vol. cxci. A., p. 129.]
[Footnote 1425: _Proc. Roy. Soc._, vol. lxii., p. 417.]
[Footnote 1426: _Ibid._, April 27, 1899; _Astroph. Jour._, vol. x., p.
272.]
[Footnote 1427: _Astr. Nach._, No. 3,565.]
[Footnote 1428: _Ibid._, No. 3,583.]
[Footnote 1429: Lunt, _Astroph. Jour._, vol. xi., p. 262; _Proc. Roy.
Soc._, vol. lxvi., p. 44; Lockyer, _ibid._, November 23, 1899; _Nature_,
vol. lxi., p. 263.]
[Footnote 1430: _Die Spectralanalyse_, p. 314.]
[Footnote 1431: _Henry Draper Memorial, First Ann. Report_, 1887.]
[Footnote 1432: _Mem. Amer. Acad._, vol. xi., p. 215.]
[Footnote 1433: _Harvard Annals_, vol. xxvii.]
[Footnote 1434: _Harvard Annals_, vol. xxviii., parts i. and ii.]
[Footnote 1435: See _ante_, p. 201.]
[Footnote 1436: _Phil. Trans._, vol. clviii., p. 529.]
[Footnote 1437: Schellen, _Die Spectralanalyse_, Bd. ii., p. 326 (ed.
1883).]
[Footnote 1438: _Proc. Roy. Soc._, vol. xx., p. 386.]
[Footnote 1439: _System of the Stars_, p. 199.]
[Footnote 1440: Pickering, _Am. Jour. of Sc._, vol. xxxix., p. 46;
Vogel, _Astr. Nach._ No. 3,017.]
[Footnote 1441: _Sitzungsberichte_, Berlin, May 2, 1901; _Astroph.
Jour._, vol. xiii., p. 324.]
[Footnote 1442: The "relative orbit" of a double star is that described
by one round the other as a fixed point. Micrometrical measures are
always thus executed. But in reality both stars move in opposite
directions, and at rates inversely as their masses round their common
centre of gravity.]
[Footnote 1443: Vogel, _Astr. Nach._, Nos. 3,017, 3,039.]
[Footnote 1444: Huggins, _Pres. Address_, 1891; Cornu, _Sur la Méthode
Doppler-Fizeau_ p. D. 38.]
[Footnote 1445: _Sitzungsb._, Berlin, 1890, p. 401; _Astr. Nach._, No.
2,995.]
[Footnote 1446: _Ibid._]
[Footnote 1447: _Astroph. Jour._, vol. v., p. 1; Newall, _Month. Not._,
vol. lvii., p. 575.]
[Footnote 1448: _Bull. de l'Acad. de St. Pétersb._, tt. vi., viii.]
[Footnote 1449: _Astroph. Jour._, vol. x., p. 177; _Month. Not._, vol.
lx., p. 418; Vogel, _Sitzungsb._, Berlin, April 19, 1900.]
[Footnote 1450: _Month. Not._, vol. lx., p. 595.]
[Footnote 1451: Hussey, _Astr. Jour._, No. 484.]
[Footnote 1452: _Astroph. Jour._, vols. x, p. 180; xiv., p. 140; _Lick
Bulletin_, No. 4; Bélopolsky, _Astr. Nach._, No. 3,637.]
[Footnote 1453: The significance of the name "El Ghoul" leaves little
doubt that the Arab astronomers took note of this star's variability. E.
M. Clerke, _Observatory_, vol. xv., p. 271.]
[Footnote 1454: _Phil. Trans._, vol. lxxiii., p. 484.]
[Footnote 1455: _Proc. Amer. Acad._, vol. xvi., p. 17; _Observatory_,
vol. iv., p. 116. For a preliminary essay by T. S. Aldis, see _Phil.
Mag._, vol. xxxix., p. 363, 1870.]
[Footnote 1456: _Astr. Nach._, No. 2,947.]
[Footnote 1457: _Astr. Jour._, Nos. 165-6, 255-6, 509. See also
_Knowledge_, vol. xv., p. 186.]
[Footnote 1458: Bauschinger, _V. J. S. Astr. Ges._, Jahrg. xxix.; but
_cf._ Searle, _Harvard Annals_, vol. xxix., p. 223; Boss, _Astr. Jour._,
No. 343.]
[Footnote 1459: _Comptes Rendus_, t. cxx., p. 125.]
[Footnote 1460: Myers, _Astroph. Jour._, vol. vii., p. 1; A. W. Roberts,
_Ibid._, vol. xiii., p. 181.]
[Footnote 1461: _Proc. R. Irish Ac._, July, 1884.]
[Footnote 1462: _Ibid._, vol. i., p. 97.]
[Footnote 1463: _Astr. Jour._, Nos. 179, 180.]
[Footnote 1464: _Ibid._, Nos. 300, 379.]
[Footnote 1465: _Astr. Jour._, Nos. 491-2.]
[Footnote 1466: _System of the Stars_, p. 125.]
[Footnote 1467: _Proc. Roy. Soc._, vol. xv., p. 146.]
[Footnote 1468: Weiss, _Astr. Nach._, No. 1,590; Espin, _Ibid._, No.
3,200.]
[Footnote 1469: _Comptes Rendus_, t. lxxxiii., p. 1172.]
[Footnote 1470: _Monatsb._, Berlin, 1877, pp. 241, 826.]
[Footnote 1471: _Copernicus_, vol. ii., p. 101.]
[Footnote 1472: Burnham, _Month. Not._, vol. lii., p. 457.]
[Footnote 1473: _Astr. Nach._, No. 2,682.]
[Footnote 1474: A. Hall, _Am. Jour. of Sc._, vol. xxxi., p. 301.]
[Footnote 1475: Young, _Sid. Messenger_, vol. iv., p. 282; Hasselberg,
_Astr. Nach._, No. 2,690.]
[Footnote 1476: _Report Brit. Assoc._, 1885, p. 935.]
[Footnote 1477: _Month. Not._, vol. xlvii., p. 54.]
[Footnote 1478: _Nature_, vol. xxxii., p. 522.]
[Footnote 1479: _Astr. Nach._, Nos. 1,267, 2,715.]
[Footnote 1480: _Month. Not._, vol. xxi., p. 32.]
[Footnote 1481: _Observatory_, vol. viii., p. 335.]
[Footnote 1482: _Astr. Nach._, No. 3,118; _Astr. and Astroph._, vol.
xi., p. 907.]
[Footnote 1483: _Cape Results_, p. 137.]
[Footnote 1484: _Trans. R. Soc. of Edinburgh_, vol. xxvii., p. 51;
_Astr. and Astroph._, August, 1892, p. 593.]
[Footnote 1485: Vogel, _Astr. Nach._, No. 3,079.]
[Footnote 1486: _Observatory_, vol. xv., p. 287; Seeliger, _Astr.
Nach._, No. 3,118; _Astr. and Astroph._, vol. xi., p. 906.]
[Footnote 1487: Ranyard, _Knowledge_, vol. xv., p. 110.]
[Footnote 1488: _Proc. Roy. Soc._, vol. li., p. 492.]
[Footnote 1489: Burnham, _Month. Not._, vol. liii., p. 58.]
[Footnote 1490: _Astr. Nach._, Nos. 3,118, 3,143.]
[Footnote 1491: Renz, _Ibid._, Nos. 3,119, 3,238; Huggins, _Astr. and
Astroph._, vol. xiii., p. 314.]
[Footnote 1492: _Astr. Nach._, No. 3,111.]
[Footnote 1493: Bélopolsky, _Astr. Nach._, No. 3,120.]
[Footnote 1494: _Nature_, September 15, 1892.]
[Footnote 1495: _Astr. Nach._, Nos. 3,122, 3,129.]
[Footnote 1496: _Ibid._, No. 3,133; _Astr. and Astroph._, vol. xi., p.
715.]
[Footnote 1497: _Publ. Astr. Pac. Soc._, vol. iv., p. 244.]
[Footnote 1498: Barnard, _Astroph. Jour._, vol. xiv., p. 152; Campbell,
_Observatory_, vol. xxiv., p. 360.]
[Footnote 1499: _Pop. Astr._, March, 1895, p. 307.]
[Footnote 1500: _Harvard Circular_, No. 4, December 20, 1895. The first
Nova Persei was spectrographically recorded in 1887.]
[Footnote 1501: Vogel, _Sitzungsb._, Berlin, April 19, 1900, p. 389.]
[Footnote 1502: Sidgreaves, _Observatory_, vol. xxiv., p. 191.]
[Footnote 1503: _Ibid._, _Knowledge_, vol. xxv., p. 10.]
[Footnote 1504: _Lick Bulletin_, No. 8.]
[Footnote 1505: _Astr. Nach._, No. 3,736.]
[Footnote 1506: _Astroph. Jour._, vol. xiv., p. 167.]
[Footnote 1507: _Lick Bulletin_, No. 10.]
[Footnote 1508: _Astroph. Jour._, vols. xiv., p. 293; xv., p. 129.]
[Footnote 1509: _Cf._ the theories on the subject of M. Wolf, _Astr.
Nach._, Nos. 3,752, 3,753; Kapteyn, _Ibid._, No. 3,756; F. W. Very,
_Ibid._, No. 3,771; and W. E. Wilson, _Proc. Roy. Dublin Soc._, No. 45,
p. 556.]
[Footnote 1510: _Phil. Trans._, vol. cliv., p. 437.]
[Footnote 1511: _Phil. Trans._, vol. clviii., p. 540. The true
proportion seems to be about one-tenth (_Harvard Annals_, vol. xxvi.,
pt. ii., p. 205), the Tulse Hill working-list having been formed of
specially selected objects.]
[Footnote 1512: Scheiner, _Astr. Nach._, No. 3,476; _Astroph. Jour._,
vol. vii., p. 231; Campbell, _Ibid._, vols. ix., p. 312; x., p. 22.]
[Footnote 1513: _Proc. Roy. Soc._, vols. xlvi., p. 40; xlviii., p. 202.]
[Footnote 1514: _Publ. Astr. Pac. Soc._, vol. ii., p. 265; _Proc. Roy.
Soc._, vol. xlix., p. 399.]
[Footnote 1515: _Astr. Nach._, No 3,549.]
[Footnote 1516: _Atlas of Stellar Spectra_, p. 125.]
[Footnote 1517: _Knowledge_, vol. xix., p. 39.]
[Footnote 1518: _Astr. Nach._, Nos. 1,366, 1,391, 1,689; Chambers,
_Descriptive Astr._ (3rd ed.), p. 543; Flammarion, _L'Univers Sidéral_,
p. 818.]
[Footnote 1519: _Month. Not._, vol. li., p. 94.]
[Footnote 1520: _Ibid._, vol. lix., p. 372.]
[Footnote 1521: _Ibid._, vol. lx., p. 424.]
[Footnote 1522: Dreyer, _Ibid._, vol. lii., p. 100.]
[Footnote 1523: _Wash. Obs._, vol. xxv., App. 1.]
[Footnote 1524: _Am. Jour. of Sc._, vol. xiv., p. 433; C. Dreyer,
_Month. Not._, vol. xlvii., p. 419.]
[Footnote 1525: _Ibid._, vol. li., p. 496.]
[Footnote 1526: Reproduced in _Knowledge_, April, 1893.]
[Footnote 1527: Unless an exception be found in the Pleiades nebulæ,
which may be assumed to share the small apparent movement of the stars
they adhere to.]
[Footnote 1528: _Abhandl. Akad. der Wiss._, Leipzig, 1857, Bd. iii., p.
295.]
[Footnote 1529: _Month. Not._, vol. lii., p. 31.]
[Footnote 1530: _Proc. Roy. Soc._, 1874, p. 251.]
[Footnote 1531: _Publ. Astr. Pac. Soc._, vol. ii., p. 278.]
[Footnote 1532: _System of the Stars_, p. 257.]
[Footnote 1533: _Proc. Roy. Soc._, vol. xlix., p. 399.]
[Footnote 1534: _Potsdam Publ._, Bd. vii., Th. i.]
[Footnote 1535: _Astr. Nach._, No. 2,714; Schönfeld, _V. J. S. Astr.
Ges._, Jahrg. xxi., p. 58.]
[Footnote 1536: _Astroph. Journ._, vol. xiii., p. 80.]
[Footnote 1537: _Proc. Roy. Soc._, vol. xxxiii., p. 425; _Report Brit.
Assoc._, 1882, p. 444. An impression of the four lower lines in the same
spectrum was almost simultaneously obtained by Dr. Draper. _Comptes
Rendus_, t. xciv., p. 1243.]
[Footnote 1538: _Proc. Roy. Soc._, vol. xlviii., p. 213.]
[Footnote 1539: _Month. Not._, vol. xlviii., p. 360.]
[Footnote 1540: _Proc. Roy. Soc._, vol. xlvi., p. 40; _System of the
Stars_, p. 79.]
[Footnote 1541: _Sitzungsb._, Berlin, February 13, 1890.]
[Footnote 1542: _Wash. Obs._, vol. xxv., App. i., p. 226.]
[Footnote 1543: _Comptes Rendus_, t. xcii., p. 261.]
[Footnote 1544: _Month. Not._, vol. xliii., p. 255.]
[Footnote 1545: _Harvard Annals_, vol. xviii., p. 116.]
[Footnote 1546: _Sid. Mess._, vol. ix., p. 1.]
[Footnote 1547: _Knowledge_, vol. xv., p. 191.]
[Footnote 1548: _Month. Not._, vol. xlix., p. 65.]
[Footnote 1549: _System of the Stars_, p. 269.]
[Footnote 1550: _Astr. Nach._, Nos. 2,749, 2,754.]
[Footnote 1551: Vogel, _Astr. Nach._, 2,854.]
[Footnote 1552: _Nature_, vol. xliii., p. 419.]
[Footnote 1553: _L'Astronomie_, t. xl., p. 171.]
[Footnote 1554: _Astr. Nach._, Bände xlvii., p. 1; xlviii., p. 1; xlix.,
p. 81. Pickering, _Mem. Am. Ac._, vol. xi., p. 180.]
[Footnote 1555: Gould on Celestial Photography, _Observatory_, vol. ii.,
p. 16.]
[Footnote 1556: _Annals N. Y. Acad. of Sciences_, vol. vi., p. 239,
1892; Elkin, _Publ. Astr. Pac. Soc._, vol. iv., p. 134.]
[Footnote 1557: _Trans. Yale Observatory_, vol. i., pt. i.]
[Footnote 1558: _Astroph. Jour._, vol. xiii., p. 56.]
[Footnote 1559: _Astr. Nach._, No. 2,719.]
[Footnote 1560: _Ibid._, No. 2,726.]
[Footnote 1561: _Ibid._, No. 2,730.]
[Footnote 1562: _Month. Not._, vol. xlvii., p. 24.]
[Footnote 1563: _Les Mondes_, t. iii., p. 529.]
[Footnote 1564: Mouchez, _Comptes Rendus_, t. cvi., p. 912.]
[Footnote 1565: _Astr. Nach._, No. 3,422.]
[Footnote 1566: _Ibid._, No. 3,441.]
[Footnote 1567: _Ibid._, Nos. 3,018, 3,032.]
[Footnote 1568: _Journ. Brit. Astr. Assoc._, vol. ix., p. 133.]
[Footnote 1569: _Astr. Nach._, No. 3,253.]
[Footnote 1570: _Observatory_, vol. xxi., pp. 351, 386.]
[Footnote 1571: Reproduced in _Astroph. Journ._, vol. xi., p. 324.]
[Footnote 1572: _Ibid._, p. 347.]
[Footnote 1573: _Astr. Nach._, No. 3,704.]
[Footnote 1574: _Sitzungsb. Bayer. Akad._, March 23, 1901.]
[Footnote 1575: _Annals of the Cape Observatory_, vols. iii., iv., v.]
[Footnote 1576: _Month. Not._, vol. lx., p. 381.]
[Footnote 1577: D. Klumpke, _Observatory_, vol. xv., p. 305.]
[Footnote 1578: Gilbert, _Sid. Mess._, vol. i., p. 288.]
[Footnote 1579: _Astr. Papers for the Amer. Ephemeris_, vol. viii., pt.
ii.]
[Footnote 1580: _Nature_, vol. xxiv., p. 91; _Dunsink Observations_, pt.
v., 1884.]
[Footnote 1581: Elkin, _Report for 1891-92_, p. 25; Newcomb, _The
Stars_, p. 151.]
[Footnote 1582: _Annals of the Cape Observatory_, vol. viii., pt. ii.
Some of the measures were made by Messrs. Finlay and de Sitter.]
[Footnote 1583: _Astr. Nach._, No. 3,483; _Observatory_, vol. xxi., p.
180.]
[Footnote 1584: _Annalen der Sternwarte in Leiden_, Bd. vii.]
[Footnote 1585: _Report of Harvard Conference in 1898_ (Snyder).]
[Footnote 1586: _Researches in Stellar Parallax_, pt. ii., 1892.]
[Footnote 1587: _V. J. S. Astr. Ges._, Jahrg., xxviii., p. 117.]
[Footnote 1588: _Bulletin de la Carte du Ciel_, No. 1, p. 262.]
[Footnote 1589: _Publ. of the Astr. Laboratory at Groningen_, No. 1.]
[Footnote 1590: _Nature_, vol. xxvi., p. 177.]
[Footnote 1591: _Proc. R. Irish Acad._, vol. i., p. 571, ser. iii.]
[Footnote 1592: _Mem. R. A. S._, vol. xlvii., p. 178.]
[Footnote 1593: _Astr. Nach._, No. 3,142.]
[Footnote 1594: _Publ. Astr. Pac. Soc._, No. 76.]
[Footnote 1595: Campbell, _Lick Bulletin_, No. 4.]
[Footnote 1596: _Publ. Yerkes Observatory_, vol. i., 1900.]
[Footnote 1597: _Annals Cape Observatory_, vol. ii., pt. ii.]
[Footnote 1598: _Astr. Jour._, Nos. 431-2.]
[Footnote 1599: W. J. Hussey, _Publ. Astr. Pac. Soc._, No. 74.]
[Footnote 1600: _Astr. Jour._, No. 258.]
[Footnote 1601: _Sitzungsberichte_, Berlin, October 26, 1893.]
[Footnote 1602: _Annales de l'Obs. de Nice_, t. ii.]
[Footnote 1603: _Washington Observations_, 1888, App. i.]
[Footnote 1604: _Publ. Lick Observatory_, vol. v., 1901.]
[Footnote 1605: T. Lewis, _Observatory_, vol. xvi., p. 312.]
[Footnote 1606: _Harvard Annals_, vol. xiv., pt. i., 1884.]
[Footnote 1607: _Observatory_, vol. viii., p. 309.]
[Footnote 1608: _Month. Not._, vol. xlvi., p. 277.]
[Footnote 1609: _Harvard Annals_, vol. xxxiv.]
[Footnote 1610: _Ibid._, vol. xlv.]
[Footnote 1611: _Carte Phot. du Ciel. Réunion du Comité Permanent_,
Paris, 1891, p. 100.]
[Footnote 1612: _Essays_ (2nd ser.), _The Nebular Hypothesis_.]
[Footnote 1613: _On the Plurality of Worlds_, p. 214 (2nd ed.).]
[Footnote 1614: Proctor, _Month. Not._, vol. xxix., p. 342.]
[Footnote 1615: This remark was first made by J. Michell, _Phil.
Trans._, vol. lvii., p. 25 (1767).]
[Footnote 1616: _Pop. Astr._, No. 45.]
[Footnote 1617: _Astroph. Jour._, vol. i., p. 220.]
[Footnote 1618: _Month. Not._, vols. xxxi., p. 175; xxxii., p. 1.]
[Footnote 1619: _The Stars_, p. 273.]
[Footnote 1620: _System of the Stars_, p. 384; _Old and New Astronomy_,
p. 749 (Ranyard).]
[Footnote 1621: _Astroph. Jour._, vol. xii., p. 156.]
[Footnote 1622: _Publ. Astr. Pac. Soc._, vol. ii., p. 242.]
[Footnote 1623: _Month. Not._, vol. li., pp. 40, 97. For reproductions
of some of the photographs in question, see _Knowledge_, vol. xiv., p.
50.]
[Footnote 1624: _Astr. Nach._, No. 3,048; _Observatory_, vol. xiv., p.
301.]
[Footnote 1625: _Proc. Roy. Inst._, May 29, 1891 (Gill).]
[Footnote 1626: _Annals Cape Obs._, iii., Introduction, p. 22.]
[Footnote 1627: _Proc. Roy. Soc._, vol. xviii., p. 169.]
[Footnote 1628: _Astr. Nach._, No. 3,456; _Observatory_, vol. xxi., p.
65; Newcomb, _The Stars_, p. 80.]
[Footnote 1629: _Month. Not._, vol. xl., p. 249.]