CHAPTER XIII

_METHODS OF RESEARCH_

Comparing the methods now available for astronomical inquiries with

those in use forty years ago, we are at once struck with the fact that

they have multiplied. The telescope has been supplemented by the

spectroscope and the photographic camera. Now, this really involves a

whole world of change. It means that astronomy has left the place where

she dwelt apart in rapt union with mathematics, indifferent to all

things on earth save only to those mechanical improvements which should

aid her to penetrate further into the heavens, and has descended into

the forum of human knowledge, at once a suppliant and a patron,

alternately invoking help from and promising it to each of the sciences,

and patiently waiting upon the advances of all. The science of the

heavenly bodies has, in a word, become a branch of terrestrial physics,

or rather a higher kind of integration of all their results. It has,

however, this leading peculiarity, that the materials for the whole of

its inquiries are telescopically furnished. They are such as come very

imperfectly, or not at all, within the cognisance of the unarmed eye.

Spectroscopic and photographic apparatus are simply additions to the

telescope. They do not supersede or render it of less importance. On the

contrary, the efficacy of their action depends primarily upon the

optical qualities of the instruments they are attached to. Hence the

development, to their fullest extent, of the powers of the telescope is

of vital moment to the progress of modern physical astronomy, while the

older mathematical astronomy could afford to remain comparatively

indifferent to it.

The colossal Rosse reflector still marks, as to size, the _ne plus

ultra_ of performance in that line. A mirror four feet in diameter was,

however, sent out to Melbourne by the late Thomas Grubb of Dublin in

1870. This is mounted in the Cassegrainian manner, so that the observer

looks straight through it towards the object viewed, of which he really

sees a twice-reflected image. The dust-laden atmosphere of Melbourne is

said to impede very seriously the usefulness of this originally fine

instrument.

It may be doubted whether so large a spectrum will ever again be

constructed. A new material for the mirrors of reflecting telescopes,

proposed by Steinheil in 1856, and independently by Foucault in

1857,[1630] has in a great measure superseded the use of a metallic

alloy. This is glass upon which a thin film of silver has been deposited

by a chemical process originally invented by Liebig. It gives a

peculiarly brilliant reflective surface, throwing back more light than a

metallic mirror of the same area, in the proportion of about sixteen to

nine. Resilvering, too, involves much less risk and trouble than

repolishing a speculum. The first use of this plan on a large scale was

in an instrument of thirty-six inches aperture, finished by Calver for

Dr. Common in 1879. To its excellent qualities turned to account with

rare skill, his triumphs in celestial photography are mainly due. A more

daring experiment was the construction and mounting, by Dr. Common

himself, of a 5-foot reflector. But the first glass disc ordered from

France for the purpose proved radically defective. When figured,

polished, and silvered, towards the close of 1888, it gave elliptical

instead of circular star-images.[1631] A new one had to be procured, and

was ready for astronomical use in 1891. The satisfactory nature of its

performance is vouched for by the observations made with it upon

Jupiter's new satellite in December, 1892. This instrument, to which a

Newtonian form has been given, had no rival in respect of

light-concentration at the time when it was built. It has now two--the

Paris 50-inch refractor and the Yerkes 5-foot reflector.

It is, however, in the construction of refracting telescopes that the

most conspicuous advances have recently been made. The Harvard College

15-inch achromatic was mounted and ready for work in June, 1847. A

similar instrument had already for some years been in its place at

Pulkowa. It was long before the possibility of surpassing these

masterpieces of German skill presented itself to any optician. For

fifteen years it seemed as if a line had been drawn just there. It was

first transgressed in America. A portrait-painter of Cambridgeport,

Massachusetts, named Alvan Clark, had for some time amused his leisure

with grinding lenses, the singular excellence of which was discovered in

England by Mr. Dawes in 1853.[1632] Seven years passed, and then an

order came from the University of Mississippi for an object-glass of the

unexampled size of eighteen inches. An experimental glance through it to

test its definition resulted, as we have seen, in the detection of the

companion of Sirius, January 31, 1862. It never reached its destination

in the South. War troubles supervened, and it was eventually sent to

Chicago, where it served Professor Hough in his investigations of

Jupiter, and Mr. Burnham in his scrutiny of double stars.

The next step was an even longer one, and it was again taken by a

self-taught optician, Thomas Cooke, the son of a shoemaker at

Allerthorpe, in the East Riding of Yorkshire. Mr. Newall of Gateshead

ordered from him in 1863 a 25-inch object-glass. It was finished early

in 1868, but at the cost of shortening the life of its maker, who died

October 19, 1868, before the giant refractor he had toiled at for five

years was completely mounted. This instrument, the fine qualities of

which had long been neutralized by an unfavourable situation, was

presented by Mr. Newall to the University of Cambridge, a few weeks

before his death, April 21, 1889. Under the care of his son, Mr. Frank

Newall, it has proved highly efficient in the delicate work of measuring

stellar radial motions.

Close upon its construction followed that of the Washington 26-inch, for

which twenty thousand dollars were paid to Alvan Clark. The most

illustrious point in its career, entered upon in 1873, has been the

discovery of the satellites of Mars. Once known to be there, these were,

indeed, found to be perceptible with very moderate optical means (Mr.

Wentworth Erck saw Deimos with a 7-inch Clark); but the first detection

of such minute objects is a feat of a very different order from their

subsequent observation. Dr. See's perception with this instrument, in

1899, of Neptune's cloud-belts, and his refined series of micrometrical

measures of the various planets, attest the unimpaired excellence of its

optical qualities.

It held the primacy for more than eight years. Then, in December, 1880,

the place of honour had to be yielded to a 27-inch achromatic, built by

Howard Grubb (son and successor of Thomas Grubb) for the Vienna

Observatory. This, in its turn, was surpassed by two of respectively

29-1/2 and 30 inches, sent by Gautier of Paris to Nice, and by Alvan

Clark to Pulkowa; and an object-glass, three feet in diameter, was in

1886 successfully turned out by the latter firm for the Lick Observatory

in California. The difficulties, however, encountered in procuring discs

of glass of the size and purity required for this last venture seemed to

indicate that a term to progress in this direction was not far off. The

flint was, indeed, cast with comparative ease in the workshops of M.

Feil at Paris. The flawless mass weighed 170 kilogrammes, was over 38

inches across, and cost 10,000 dollars. But with the crown part of the

designed achromatic combination things went less smoothly. The

production of a perfect disc was only achieved after _nineteen_

failures, involving a delay of more than two years; and the glass for a

third lens, designed to render the telescope available at pleasure for

photographic purposes, proved to be strained, and consequently went to

pieces in the process of grinding. It has been replaced by one of 33

inches, with which a series of admirable lunar and other photographs

have been taken.

Nor is the difficulty in obtaining suitable material the only obstacle

to increasing the size of refractors. The "secondary spectrum," as it is

called, also interposes a barrier troublesome to surmount. True

achromatism cannot be obtained with ordinary flint and crown-glass; and

although in lenses of "Jena glass," outstanding colour is reduced to

about one-sixth its usual amount, their term of service is fatally

abridged by rapid deterioration. Nevertheless, a 13-inch objective of

the new variety was mounted at Königsberg in 1898; and discs of Jena

crown and flint, 23 inches across, were purchased by Brashear at the

Chicago Exhibition of 1893. An achromatic combination of three kinds of

glass, devised by Mr. A. Taylor[1633] for Messrs. Cooke of York, has

less serious drawbacks, but has not yet come into extensive use.

Meanwhile, in giant telescopes affected to the full extent by chromatic

aberration, such as the Lick and Yerkes refractors, the differences of

focal length for the various colours are counted by inches,[1634] and

this not through any lack of skill in the makers, but by the necessity

of the case. Embarrassing consequences follow. Only a small part of the

spectrum of a heavenly body, for instance, can be distinctly seen at one

time; and a focal adjustment of half an inch is required in passing from

the observation of a planetary nebula to that of its stellar nucleus. A

refracting telescope loses, besides, one of its chief advantages over a

reflector when its size is increased beyond a certain limit. That

advantage is the greater luminosity of the images given by it.

Considerably more light is transmitted through a glass lens than is

reflected from an equal metallic surface. But only so long as both are

of moderate dimensions. For the glass necessarily grows in thickness as

its area augments, and consequently stops a larger percentage of the

rays it refracts. So that a point at length arrives--fixed by the late

Dr. Robinson at a diameter a little short of 3 feet[1635]--where the

glass and the metal are, in this respect, on an equality; while above it

the metal has the advantage. And since silvered glass gives back

considerably more light than speculum metal, the stage of equalisation

with lenses is reached proportionately sooner where this material is

employed.[1636]

The most distinctive faculty of reflectors, however, is that of bringing

rays of all refrangibilities to a focus together. They are naturally

achromatic. None of the beams they collect are thrown away in

colour-fringes, obnoxious both in themselves and as a waste of the chief

object of astrophysicists' greed--light. Reflectors, then, are in this

respect specially adapted to photographic and spectrographic use. But

they have a countervailing drawback. The penalties imposed by bigness

are for them peculiarly heavy. Perfect definition becomes with

increasing size, more and more difficult to attain; once attained, it

becomes more and more difficult to keep. For the huge masses of material

employed to form great object-glasses or mirrors tend with every

movement to become deformed by their own weight. Now, the slightest

bending of a mirror is fatal to its performance, the effect being

doubled by reflection; while in a lens alteration of figure is

compensated by the equal and contrary flexures of the opposing surfaces,

so that the emergent beams pursue much the same paths as if the curves

of the refracting medium had remained theoretically perfect. For this

reason work of precision must remain the province of refracting

telescopes, although great reflectors retain the primacy in the

portraiture of the heavenly bodies, as well as in certain branches of

spectroscopy. Professor Hale, accordingly, summarised a valuable

discussion on the subject by asserting[1637] "that the astrophysicist

may properly consider the reflector to be an even more important part of

his instrumental equipment than the refractor." A new era in its

employment west of the Atlantic opened with the transfer from Halifax to

Mount Hamilton of the Crossley reflector. Its prerogatives in nebular

photography were splendidly indicated in 1899 by Professor Keeler's

exquisite and searching portrayals of the cloud-worlds of space, and

those obtained two years later, with a similar, though smaller,

instrument, by Professor Ritchey of the Yerkes Observatory, were fully

comparable with them. The performances of the Yerkes 5-foot reflector

still belong to the future.

Ambition as regards telescopic power is by no means yet satisfied. Nor

ought it to be. The advance of astrophysical researches of all kinds

depends largely upon light-grasp. For the spectroscopic examination of

stars, for the measurement of their motions in the line of sight, for

the discovery and study of nebulæ, for stellar and nebular photography,

the cry continually is "more light." There is no enterprising head of an

observatory but must feel cramped in his designs if he can command no

more than 14 or 15 inches of aperture, and he aspires to greater

instrumental capacity, not merely with a view to the chances of

discovery, but for the steady prosecution of some legitimate line of

inquiry. Thus projects of telescope-building on a large scale are rife,

and some obtain realisation year by year. Sir Howard Grubb finished in

1893 a 28-inch achromatic for the Royal Observatory, Greenwich; the

Thompson equatoreal, mounted at the same establishment in 1897, carries

on a single axis a 26-inch photographic refractor and a 30-inch

silvered-glass reflector; the Victoria telescope, inaugurated at the

Cape in 1901, comprises a powerful spectrographic apparatus, together

with a chemically corrected 24-inch refractor, the whole being the

munificent gift of Mr. Frank McClean; at Potsdam, at Meudon, at Paris,

at Alleghany, engines for light-concentration have been, or shortly will

be, erected of dimensions which, two generations back, would have seemed

extravagant and impossible.

Perhaps the finest, though not absolutely the greatest, among them,

marked the summit and end of the performances of Alvan G. Clark, the

last survivor of the Cambridgeport firm.

In October, 1892, Mr. Yerkes of Chicago offered an unlimited sum for the

provision of the University of that city with a "superlative" telescope.

And it happened, fortunately, that a pair of glass discs, nearly 42

inches in diameter, and of perfect quality, were ready at hand. They had

been cast by Mantois for the University of Southern California, when the

erection of a great observatory on Wilson's Peak was under

consideration. In the Clark workshop they were combined into a superb

objective, brought to perfection by trials and delicate touches

extending over nearly five years. Then the maker accompanied it to its

destination, by the shore of a far Western Lake Geneva, and died

immediately after his return, June 9, 1897. Nor has the implement of

celestial research he just lived to complete been allowed to "rust

unburnished." Manipulated by Hale, Burnham, and Barnard, it has done

work that would have been impracticable with less efficient optical aid.

Its construction thus marks a noticeable enlargement of astronomical

possibilities, exemplified--to cite one among many performances--by

Barnard's success in keeping track of cluster-variables when below the

common limit of visual perception.

With the Lick telescope results have also been achieved testifying to

its unsurpassed excellence. Holden's and Schaeberle's views of planetary

nebulæ, Burnham's and Hussey's hair's-breadth star-splitting operations,

Keeler's measurements of nebular radial motion, Barnard's detections and

prolonged pursuit of faint comets, his discovery of Jupiter's tiny moon,

Campbell's spectroscopic determinations--all this could only have been

accomplished, even by an exceptionally able and energetic staff, with

the aid of an instrument of high power and quality. But there was

another condition which should not be overlooked.

The best telescope may be crippled by a bad situation. The larger it is,

indeed, the more helpless is it to cope with atmospheric troubles. These

are the worst plagues of all those that afflict the astronomer. No

mechanical skill avails to neutralise or alleviate them. They augment

with each increase of aperture; they grow with the magnifying powers

applied. The rays from the heavenly bodies, when they can penetrate the

cloud-veils that too often bar their path, reach us in an enfeebled,

scattered, and disturbed condition. Hence the twinkling of stars, the

"boiling" effects at the edges of sun, moon, and planets; hence

distortions of bright, effacements of feeble telescopic images; hence,

too, the paucity of the results obtained with many powerful

light-gathering machines.

No sooner had the Parsonstown telescope been built than it became

obvious that the limit of profitable augmentation of size had, under

climatic conditions at all nearly resembling those prevailing there,

been reached, if not overpassed; and Lord Rosse himself was foremost to

discern the need of pausing to look round the world for a clearer and

stiller air than was to be found within the bounds of the United

Kingdom. With this express object Mr. Lassell transported his 2-foot

Newtonian to Malta in 1852, and mounted there, in 1860, a similar

instrument of fourfold capacity, with which, in the course of about two

years, 600 new nebulæ were discovered. Professor Piazzi Smyth's

experiences during a trip to the Peak of Teneriffe in 1856 in search of

astronomical opportunities[1638] gave countenance to the most sanguine

hopes of deliverance, at suitable elevated stations, from some of the

oppressive conditions of low-level star-gazing; yet for a number of

years nothing effectual was done for their realisation. Now, at last,

however, mountain observatories are not only an admitted necessity but

an accomplished fact; and Newton's long forecast of a time when

astronomers would be compelled, by the developed powers of their

telescopes, to mount high above the "grosser clouds" in order to use

them,[1639] had been justified by the event.

James Lick, the millionaire of San Francisco, had already chosen, when

he died, October 1, 1876, a site for the new observatory, to the

building and endowment of which he had devoted a part of his large

fortune. The situation of the establishment is exceptional and splendid.

Planted on one of the three peaks of Mount Hamilton, a crowning summit

of the Californian Coast Range, at an elevation of 4,200 feet above the

sea, in a climate scarce rivalled throughout the world, it commands

views both celestial and terrestrial which the lover of nature and

astronomy may alike rejoice in. Impediments to observation are there

found to be most materially reduced. Professor Holden, who was

appointed, in 1885, president of the University of California and

director of the new observatory affiliated to it, stated that during six

or seven months of the year an unbroken serenity prevails, and that half

the remaining nights are clear.[1640] The power of continuous work thus

afforded is of itself an inestimable advantage; and the high visual

excellences testified to by Mr. Burnham's discovery, during a two

months' trip to Mount Hamilton in the autumn of 1879, of forty-two new

double stars with a 6-inch achromatic, gave hopes since fully realised

of a brilliant future for the Lick establishment. Its advantages are

shared, as Professor Holden desired them to be, by the whole

astronomical world.[1641] A sort of appellate jurisdiction was at once

accorded to the great equatoreal, and more than one disputed point has

been satisfactorily settled by recourse to it.

Its performances, considered both as to quality and kind, are unlikely

to be improved upon by merely outbidding it in size, unless the care

expended upon the selection of its site be imitated. Professor Pickering

thus showed his customary prudence in reserving his efforts to procure a

great telescope until Harvard College owned a dependent observatory

where it could be employed to advantage. This was found by Mr. W. H.

Pickering, after many experiments in Colorado, California, and Peru, at

Arequipa, on a slope of the Andes, 8,000 feet above the sea-level. Here

the post provided for by the "Boyden Fund" was established in 1891,

under ideal meteorological conditions. Temperature preserves a "golden

mean"; the barometer is almost absolutely steady; the yearly rainfall

amounts to no more than three or four inches. No wonder, then, that the

"seeing" there is of the extraordinary excellence attested by Mr.

Pickering's observations. In the absence of bright moonlight, he tells

us,[1642] eleven Pleiades can always be counted; the Andromeda nebula

appears to the naked eye conspicuously bright, and larger than the full

moon; third magnitude stars have been followed to their disappearance at

the true horizon; the zodiacal light spans the heavens as a complete

arch, the "Gegenschein" forming a regular part of the scenery of the

heavens. Corresponding telescopic facilities are enjoyed. The chief

instrument at the station, a 13-inch equatoreal by Clark, shows the

fainter parts of the Orion nebula, photographed at Harvard College in

1887, by which the dimensions given to it in Bond's drawing are doubled;

stars are at times seen encircled by half a dozen immovable diffraction

rings, up to twelve of which have been counted round Alpha Centauri;

while on many occasions no available increase of magnifying power

availed to bring out any wavering in the limbs of the planets. Moreover,

the series of fine nights is nearly unbroken from March to November.

The facilities thus offered for continuous photographic research

rendered feasible Professor Bailey's amazing discovery of variable

star-clusters. They belong exclusively to the "globular" class, and the

peculiarity is most strikingly apparent in the groups known as

Omega Centauri, and Messier 3, 5, and 15. A large number of their minute

components run through perfectly definite cycles of change in periods

usually of a few hours.[1643] Altogether, about 500 "cluster-variables"

have been recorded since 1895. It should be mentioned that Mr. David

Packer and Dr. Common discerned, about 1890, some premonitory symptoms

of light-fluctuation among the crowded stars of Messier 5.[1644] With

the Bruce telescope, a photographic doublet 24 inches in diameter, a

store of 5,686 negatives was collected at Arequipa between 1896 and

1901. Some were exposed directly, others with the intervention of a

prism; and all are available for important purposes of detection or

investigation.

Vapours and air-currents do not alone embarrass the use of giant

telescopes. Mechanical difficulties also oppose a formidable barrier to

much further growth in size. But what seems a barrier often proves to be

only a fresh starting-point; and signs are not wanting that it may be

found so in this case. It is possible that the monumental domes and huge

movable tubes of our present observatories will, in a few decades, be as

much things of the past as Huygens's "aerial" telescopes. It is certain

that the thin edge of the wedge of innovation has been driven into the

old plan of equatoreal mounting.

M. Loewy, the present director of the Paris Observatory, proposed to

Delaunay in 1871 the direction of a telescope on a novel system. The

design seemed feasible, and was adopted; but the death of Delaunay and

the other untoward circumstances of the time interrupted its execution.

Its resumption, after some years, was rendered possible by M.

Bischoffsheim's gift of 25,000 francs for expenses, and the _coudé_ or

"bent" equatoreal has been, since 1882, one of the leading instruments

at the Paris establishment.

Its principle is briefly this: The telescope is, as it were, its own

polar axis. The anterior part of the tube is supported at both ends, and

is thus fixed in a direction pointing towards the pole, with only the

power of twisting axially. The posterior section is joined on to it at

right angles, and presents the object-glass, accordingly, to the

celestial equator, in the plane of which it revolves. Stars in any other

part of the heavens have their beams reflected upon the object-glass by

means of a plane rotating mirror placed in front of it. The observer,

meanwhile, is looking steadfastly down the bent tube towards the

invisible _southern_ pole. He would naturally see nothing whatever were

it not that a second plane mirror is fixed at the "elbow" of the

instrument, so as to send the rays which have traversed the object-glass

to his eye. He never needs to move from his place. He watches the stars,

seated in an arm-chair in a warm room, with as perfect convenience as if

he were examining the seeds of a fungus with a microscope. Nor is this a

mere gain of personal ease. The abolition of hardship includes a vast

accession of power.[1645]

Among other advantages of this method of construction are, first, that

of added stability, the motion given to the ordinary equatoreal being

transferred, in part, to an auxiliary mirror. Next, that of increased

focal length. The fixed part of the tube can be made almost indefinitely

long without inconvenience, and with enormous advantage to the optical

qualities of a large instrument. Finally, the costly and unmanageable

cupola is got rid of, a mere shed serving all purposes of protection

required for the "coudé."

The desirability of some such change as that which M. Loewy has realised

has been felt by others. Professor Pickering sketched, in 1881, a plan

for fixing large refractors in a permanently horizontal position, and

reflecting into them, by means of a shifting mirror, the objects desired

to be observed.[1646] The observations for his photometric catalogues

are, in fact, made with a "broken transit," in which the line of sight

remains permanently horizontal, whatever the altitude of the star

examined. Sir Howard Grubb, moreover, set up, in 1882, a kind of

siderostat at the Crawford Observatory, Cork. In a paper read before the

Royal Society, January 21, 1884, he proposed to carry out the principle

on a more extended scale;[1647] and shortly afterwards undertook its

application to a telescope 18 inches in aperture for the Armagh

Observatory.[1648] The chief honours, however, remain to the Paris

inventor. None of the prognosticated causes of failure have proved

effective. The loss of light from the double reflection is

insignificant. The menaced deformation of images is, through the

exquisite skill of the MM. Henry in producing plane mirrors of all but

absolute perfection, quite imperceptible. The definition was admitted to

be singularly good. Sir David Gill stated in 1884 that he had never

measured a double star so easily as he did Gamma Leonis by its

means.[1649] Sir Norman Lockyer pronounced it to be "one of the

instruments of the future"; and the principle of its construction was

immediately adopted by the directors of the Besançon and Algiers

Observatories, as well as for a 17-inch telescope destined for a new

observatory at Buenos Ayres. At Paris, it has since been carried out on

a larger scale. A "coudé," of 23-1/2 inches aperture and 62 feet focal

length was in 1890 installed at the National Observatory, and has served

M. Loewy for his ingenious studies on refraction and aberration--above

all, for taking the magnificent plates of his lunar atlas. The "bent"

form is capable of being, but has not yet been, adapted to

reflectors.[1650]

The "coelostat," in the form given to it by Professor Turner, has

proved an invaluable adjunct to eclipse-equipments. It consists

essentially of a mirror rotating in forty-eight hours on an axis in its

own plane, and parallel to the earth's axis. In the field of a telescope

kept rigidly pointed towards such a mirror, stars appear immovably

fixed. The employment of long-focus lenses for coronal photography is

thus facilitated, and the size of the image is proportional to the

length of the focus. Professor Barnard, accordingly, depicted the

totality of 1900 with a horizontal telescope 61-1/2 feet long, fed by a

mirror 18 inches across, the diameter of the moon on his plates being 7

inches. The largest siderostat in the world is the Paris 50-inch

refractor, which formed the chief attraction of the Palais d'Optique at

the Exhibition of 1900. It has a focal length of nearly 200 feet, and

can be used either for photographic or for visual purposes.

Celestial photography has not reached its grand climacteric; yet its

earliest beginnings already seem centuries behind its present

performances. The details of its gradual yet rapid improvement are of

too technical a nature to find a place in these pages. Suffice it to say

that the "dry-plate" process, with which such wonderful results have

been obtained, appears to have been first made available by Sir William

Huggins in photographing the spectrum of Vega in 1876, and was then

successively adopted by Common, Draper, and Janssen. Nor should Captain

Abney's remarkable extension of the powers of the camera be left

unnoticed. He began his experiments on the chemical action of red and

infra-red rays in 1874, and at length succeeded in obtaining a

substance--the "blue" bromide of silver--highly sensitive to these

slower vibrations of light. With its aid he explored a vast, unknown,

and for ever invisible region of the solar spectrum, presenting to the

Royal Society, December 5, 1879,[1651] a detailed map of its infra-red

portion (wave-lengths 7,600 to 10,750), from which valuable inferences

may yet be derived as to the condition of the various kinds of matter

ignited in the solar atmosphere. Upon plates rendered "orthochromatic"

by staining with alizarine, or other dye-stuffs, the whole visible

spectrum can now be photographed; but those with their maximum of

sensitiveness near G are found preferable, except where the results of

light-analysis are sought to be completely recorded. And since

photographic refractors are corrected for the blue rays, exposures with

them of orthochromatic surfaces would be entirely futile.

The chemical plate has two advantages over the human retina:[1652]

First, it is sensitive to rays which are utterly powerless to produce

any visual effect; next, it can accumulate impression almost

indefinitely, while from the retina they fade after one-tenth part of a

second, leaving it a continually renewed _tabula rasa_.

It is, accordingly, quite possible to photograph objects so faint as to

be altogether beyond the power of any telescope to reveal--witness the

chemical disclosure of the invisible nebula encircling Nova Persei--and

we may thus eventually learn whether a blank space in the sky truly

represents the end of the stellar universe in that direction, or whether

farther and farther worlds roll and shine beyond, veiled in the

obscurity of immeasurable distance.

Of many ingenious improvements in spectroscopic appliances the most

fundamentally important relate to what are known as "gratings." These

are very finely striated surfaces, by which light-waves are brought to

interfere, and are thus sifted out, strictly according to their

different lengths, into "normal" spectra. Since no universally valid

measures can be made in any others, their production is quite

indispensable to spectroscopic science. Fraunhofer, who initiated the

study of the diffraction spectrum, used a real grating of very fine

wires: but rulings on glass were adopted by his successors, and were by

Nobert executed with such consummate skill that a single square inch of

surface was made to contain 100,000 hand-drawn lines. Such rare and

costly triumphs of art, however, found their way into very few hands,

and practical availability was first given to this kind of instrument by

the inventiveness and mechanical dexterity of two American

investigators. Both Rutherfurd's and Rowland's gratings are

machine-ruled, and reflect instead of transmitting the rays they

analyse; but Rowland's present to them a very much larger diffractive

surface, and consequently possess a higher resolving power. The first

preliminary to his improvements was the production, in 1882, of a

faultless screw, those previously in use having been the inevitable

source of periodical errors in striation, giving, in their turn,

ghost-lines as subjects of spectroscopic study.[1653] Their abolition

was not one of Rowland's least achievements. With his perfected machine

a metallic area of 6-1/4 by 4-1/4 inches can be ruled with exquisite

accuracy to almost any degree of fineness; he considered, however,

43,000 lines to the inch to be the limit of usefulness.[1654] The ruled

surface is, moreover, concave, and hence brings the spectrum to a focus

without a telescope. A slit and an eye-piece are alone needed to view

it, and absorption of light by glass lenses is obviated--an advantage

especially sensible in dealing with the ultra- or infra-visible rays.

The high qualities of Rowland's great photographic map of the solar

spectrum were thus based upon his previous improvement of the

instrumental means used in its execution. The amount of detail shown in

it is illustrated by the appearance on the negatives of 150 lines

between H and K; and many lines depict themselves as double which, until

examined with a concave grating, had passed for one and indivisible. A

corresponding hand-drawing, for which M. Thollon received in 1886 the

Lalande Prize, exhibits, not the diffractive, but the prismatic spectrum

as obtained with bisulphide of carbon prisms of large dispersive power.

About one-third of the visible gamut of the solar radiations (A to _b_)

is covered by it; it includes 3,200 lines, and is over ten metres

long.[1655] The grating is an expensive tool in the way of light. Where

there is none to spare, its advantages must be foregone. They could not,

accordingly, be turned to account in stellar spectroscopy until the Lick

telescope was at hand to supply more abundant material for research. By

the use thus made possible of Rowland's gratings, Professor Keeler was

able to apply enormous dispersion to the rays of stars and nebulæ, and

so to attain a previously unheard-of degree of accuracy in their

measurement. His memorable detection of nebular movement in line of

sight ensued as a consequence. Professor Campbell, his successor, has

since obtained, by the same means, the first satisfactory photographs of

stellar diffraction-spectra.

The means at the disposal of astronomers have not multiplied faster than

the tasks imposed upon them. Looking back to the year 1800, we cannot

fail to be astonished at the change. The comparatively simple and serene

science of the heavenly bodies known to our predecessors, almost perfect

so far as it went, incurious of what lay beyond its grasp, has developed

into a body of manifold powers and parts, each with its separate mode

and means of growth, full of strong vitality, but animated by a restless

and unsatisfied spirit, haunted by the sense of problems unsolved, and

tormented by conscious impotence to sound the immensities it perpetually

confronts.

Knowledge might be said, when the _Mécanique Céleste_ issued from the

press, to be bounded by the solar system; but even the solar system

presented itself under an aspect strangely different from what it now

wears. It consisted of the sun, seven planets, and twice as many

satellites, all circling harmoniously in obedience to a universal law,

by the compensating action of which the indefinite stability of their

mutual relations was secured. The occasional incursion of a comet, or

the periodical presence of a single such wanderer chained down from

escape to outer space by planetary attraction, availed nothing to impair

the symmetry of the majestic spectacle.

Now, not alone the ascertained limits of the system have been widened by

a thousand millions of miles, with the addition of one more giant planet

and seven satellites to the ancient classes of its members, but a

complexity has been given to its constitution baffling description or

thought. Five hundred circulating planetary bodies bridge the gap

between Jupiter and Mars, the complete investigation of the movements of

any one of which would overtask the energies of a lifetime. Meteorites,

strangers, apparently, to the fundamental ordering of the solar

household, swarm, nevertheless, by millions in every cranny of its

space, returning at regular intervals like the comets so singularly

associated with them, or sweeping across it with hyperbolic velocities,

brought, perhaps, from some distant star. And each of these cosmical

grains of dust has a theory far more complex than that of Jupiter; it

bears within it the secret of its origin, and fulfils a function in the

universe. The sun itself is no longer a semi-fabulous, fire-girt globe,

but the vast scene of the play of forces as yet imperfectly known to us,

offering a boundless field for the most arduous and inspiring

researches. Among the planets the widest variety in physical habitudes

is seen to prevail, and each is recognised as a world apart, inviting

inquiries which, to be effective, must necessarily be special and

detailed. Even our own moon threatens to break loose from the trammels

of calculation, and commits "errors" which sap the very foundations of

the lunar theory, and suggest the formidable necessity for its complete

revision. Nay, the steadfast earth has forfeited the implicit confidence

placed in it as a time-keeper, and questions relating to the stability

of the earth's axis and the constancy of the earth's rate of rotation

are among those which it behoves the future to answer. Everywhere there

is multiformity and change, stimulating a curiosity which the rapid

development of methods of research offers the possibility of at least

partially gratifying.

Outside the solar system, the problems which demand a practical solution

are virtually infinite in number and extent. And these have all arisen

and crowded upon our thoughts within less than a hundred years. For

sidereal science became a recognised branch of astronomy only through

Herschel's discovery of the revolutions of double stars in 1802. Yet

already it may be, and has been called, "the astronomy of the future,"

so rapidly has the development of a keen and universal interest attended

and stimulated the growth of power to investigate this sublime subject.

What has been done is little--is scarcely a beginning; yet it is much in

comparison with the total blank of a century past. And our knowledge

will, we are easily persuaded, appear in turn the merest ignorance to

those who come after us. Yet it is not to be despised, since by it we

reach up groping fingers to touch the hem of the garment of the Most

High.

FOOTNOTES:

[Footnote 1630: _Comptes Rendus_, t. xliv., p. 339.]

[Footnote 1631: A. A. Common, _Memoirs R. Astr. Soc._, vol. i., p. 118.]

[Footnote 1632: Newcomb, _Pop. Astr._, p. 137.]

[Footnote 1633: _Month. Not._, vol. liv., p. 67.]

[Footnote 1634: Keeler, _Publ. Astr. Pac. Soc._, vol. ii., p. 160.]

[Footnote 1635: H. Grubb, _Trans. Roy. Dub. Soc._, vol. i. (new ser.),

p. 2.]

[Footnote 1636: Hale, nevertheless (_Astroph. Jour._, vol. v., p. 128),

considers that refractors preserve their superiority of visual

light-grasp over Newtonian reflectors up to an aperture of 52-1/2, while

equalisation is reached for the photographic rays at 34 inches.]

[Footnote 1637: _Astroph. Jour._, vol. v., p. 130.]

[Footnote 1638: _Phil. Trans._, vol. cxlviii., p. 465.]

[Footnote 1639: _Optics_, p. 107 (2nd ed., 1719).]

[Footnote 1640: _Observatory_, vol. viii., p. 85.]

[Footnote 1641: Holden on Celestial Photography, _Overland Monthly_,

Nov., 1886.]

[Footnote 1642: _Observatory_, vol. xv., p. 283.]

[Footnote 1643: Bailey, _Astroph. Jour._, vol. x., p. 255.]

[Footnote 1644: _Harvard Circulars_, Nos. 2, 18, 24, 33;]

[Footnote 1645: Loewy, _Bull. Astr._, t. i., p. 286; _Nature_, vol.

xxix., p. 36.]

[Footnote 1646: _Nature_, vol. xxiv., p. 389.]

[Footnote 1647: _Ibid._, vol. xxix., p. 470.]

[Footnote 1648: _Trans. R. Dublin Soc._, vol. iii., p. 61.]

[Footnote 1649: _Observatory_, vol. vii., p. 167.]

[Footnote 1650: Loewy, _Bull. Astr._, t. i., p. 265.]

[Footnote 1651: _Phil. Trans._, vol. clxxi., p. 653.]

[Footnote 1652: Janssen, _L'Astronomie_, t. ii., p. 121.]

[Footnote 1653: Rev. A. L. Cortie, _Astr. and Astrophysics_, vol. xi.,

p. 400.]

[Footnote 1654: _Phil. Mag._, vol. xiii., 1882, p. 469.]

[Footnote 1655: _Bull. Astr._, t. iii., p. 331.]

APPENDIX

TABLE I

CHRONOLOGY, 1774-1893

1774, March 4 Herschel's first observation. Subject, the Orion Nebula.

1774 Sun-spots geometrically proved to be depressions by

Wilson.

1774 First experimental determination of the earth's mean

density by Maskelyne.

1781, March 13 Discovery of Uranus.

1782 Herschel's first Catalogue of Double Stars.

1783 Herschel's first investigation of the sun's movement

in space.

1783 Goodricke's discovery of Algol's law of variation.

1784 Analogy between Mars and the Earth pointed out by

Herschel.

1784 Construction of the Heavens investigated by Herschel's

method of star-gauging. "Cloven-disc" plan of the

Milky Way.

1784 Discovery of binary stars anticipated by Michell.

1786 Herschel's first Catalogue of Nebulæ.

1787, Jan. 11 Discovery by Herschel of two Uranian moons

(Oberon and Titania).

1787, Nov. 19 Acceleration of the moon explained by Laplace.

1789 Herschel's second Catalogue of Nebulæ, and

classification by age of these objects.

1789 Completion of Herschel's forty-foot reflector.

1789, Aug. 28 His discovery with it of the two inner Saturnian

and Sept. 17 satellites.

1789 Repeating-circle invented by Borda.

1789 Five-foot circle constructed by Ramsden for Piazzi.

1790 Maskelyne's Catalogue of thirty-six fundamental stars.

1791 Herschel propounds the hypothesis of a fluid

constitution for nebulæ.

1792 Atmospheric refraction in Venus announced by Schröter.

1794 Rotation-period of Saturn fixed by Herschel at 10h. 16m.

1795 Herschel's theory of the solar constitution.

1796 Herschel's first measures of comparative stellar

brightness.

1796 Laplace's Nebular Hypothesis published in _Exposition

du Système du Monde_.

1797 Publication of Olbers's method of computing

cometary orbits.

1798 Retrograde motions of Uranian satellites detected

by Herschel.

1799 Publication of first two volumes of _Mécanique Céleste_.

1799, May 7 Transit of Mercury observed by Schröter.

1799, Nov. 12 Star-shower observed by Humboldt at Cumana.

1800 _Monatliche Correspondenz_ started by Von Zach.

1800 Invisible heat-rays detected in the solar spectrum

by Herschel.

1801, Jan. 1 Discovery of Ceres by Piazzi.

1801 Publication of Lalande's _Histoire Céleste_.

1801 Investigation by Herschel of solar emissive variability

in connection with spot-development.

1802, March 28 Discovery of Pallas by Olbers.

1802 Herschel's third Catalogue of Nebulæ.

1802 Herschel's discovery of binary stars.

1802 Marks of clustering in the Milky Way noted by Herschel.

1802 Wollaston records seven dark lines in the solar

spectrum.

1802, Nov. 9 Transit of Mercury observed by Herschel.

1804, Sept. 2 Discovery of Juno by Harding.

1804 Foundation of Optical Institute at Munich.

1805 Herschel's second determination of the solar apex.

1807, March 29 Discovery of Vesta by Olbers.

1811 Herschel's theory of the development of stars from

nebulæ.

1811, Feb. 9 Death of Maskelyne. Pond appointed to succeed him

as Astronomer-Royal.

1811, Sept. 12 Perihelion passage of great comet.

1812 Theory of electrical repulsion in comets originated

by Olbers.

1812, Sept. 15 Perihelion passage of Pons's comet.

1814 Herschel demonstrates the irregular distribution of

stars in space.

1815 Fraunhofer maps 324 dark lines in the solar spectrum.

1818 Publication of Bessel's _Fundamenta Astronomiæ_.

1819 Recognition by Encke of the first short-period comet.

1819, June 26 Passage of the earth through the tail of a comet.

1820 Foundation of the Royal Astronomical Society.

1821 Foundation of Paramatta Observatory.

1821, September First number of _Astronomische Nachrichten_.

1822, May 24 First calculated return of Encke's comet.

1822, August 25 Death of Herschel.

1823 Bessel introduces the correction of observations for

personal equation.

1823 Fraunhofer examines the spectra of fixed stars.

1824 Distance of the sun concluded by Encke to be

95-1/4 million miles.

1824 Publication of Lohrmann's Lunar Chart.

1824 Dorpat refractor mounted equatoreally.

1826 Commencement of Schwabe's observations of sun-spots.

1826, Feb. 27 Biela's discovery of a comet.

1827 Orbit of a binary star calculated by Savary.

1829 Completion of the Royal Observatory at the Cape of

Good Hope.

1829 The Königsberg heliometer mounted.

1830 Publication of Bessel's _Tabulæ Regiomontanæ_.

1832 Discovery by Brewster of "atmospheric lines" in the

solar spectrum.

1833 Magnetic observatory established at Göttingen.

1833, Nov. 12,13 Star-shower visible in North America.

1833 Completion of Sir J. Herschel's survey of the northern

heavens.

1834, Jan. 16 Sir J. Herschel's landing at the Cape.

1835, September Airy appointed Astronomer-Royal in succession to Pond.

1835, Nov. 16 Perihelion passage of Halley's comet.

1837 Solar movement determined by Argelander.

1837 Bessel's application of the heliometer to measurements

of stellar parallax.

1837 Publication of Beer and Mädler's _Der Mond_.

1837 Publication of Struve's _Mensuræ Micrometricæ_.

1837, Dec. 16 Outburst of Eta Carinæ observed by Sir J. Herschel.

1837 Thermal power of the sun measured by Herschel and

Pouillet.

1838 Parallax of 61 Cygni determined by Bessel.

1839, Jan. 9 Parallax of Alpha Centauri announced by Henderson.

1839 Completion of Pulkowa Observatory.

1839 Solidity of the earth concluded by Hopkins.

1840, March 2 Death of Olbers.

1840 First attempt to photograph the moon by J. W. Draper.

1842 Doppler enounces principle of colour-change by motion.

1842 Conclusion of Baily's experiments in weighing the Earth.

1842, July 8 Total solar eclipse. Corona and prominences observed

by Airy, Baily, Arago, and Struve.

1843, Feb. 27 Perihelion-passage of great comet.

1845, February Completion of Parsonstown reflector.

1845, April Discovery with it of spiral nebulæ.

1845, April 2 Daguerreotype of the sun taken by Foucault and Fizeau.

1845, Oct. 21 Place of Neptune assigned by Adams.

1845, Dec. 8 Discovery of Astræa by Hencke.

1845, Dec. 29 Duplication of Biela's comet observed at Yale College.

1846 Melloni's detection of heating effects from moonlight.

1846, March 17 Death of Bessel.

1846, Sept. 23 Discovery of Neptune by Galle.

1846, Oct. 10 Neptune's satellite discovered by Lassell.

1847 Publication of Sir J. Herschel's _Results of

Observations at the Cape of Good Hope_.

1847 Cyclonic theory of sun-spots stated by him.

1848 J. R. Mayer's meteoric hypothesis of solar conservation.

1848 Motion-displacements of Fraunhofer lines adverted to

by Fizeau.

1848, April 27 New Star in Ophiuchus observed by Hind.

1848, Sept. 19 Simultaneous discovery of Hyperion by Bond and Lassell.

1849 First experimental determination of the velocity of

light (Fizeau).

1850, July 17 Vega photographed at Harvard College.

1850, Nov. 15 Discovery by Bond of Saturn's dusky ring.

1851 O. Struve's first measurements of Saturn's ring-system

1851, July 28 Total solar eclipse observed in Sweden.

1851, Oct. 24 Discovery by Lassell of two inner Uranian satellites.

1851 Schwabe's discovery of sun-spot periodicity published by

Humboldt.

1852, May 6 Coincidence of magnetic and sun-spot periods announced

by Sabine.

1852, Oct. 11 Variable nebula in Taurus discovered by Hind.

1852 Lassell's two-foot reflector transported to Malta.

1853 Adams shows Laplace's explanation of the moon's

acceleration to be incomplete.

1854 Hansen infers from lunar theory a reduced value for

the distance of the sun.

1854 Helmholtz's "gravitation theory" of solar energy.

1856 Piazzi Smyth's observations on the Peak of Teneriffe.

1857 Saturn's rings shown by Clerk Maxwell to be of meteoric

formation.

1857, April 27 Double-star photography initiated at Harvard College.

1858 Solar photography begun at Kew.

1858, Sept. 30 Perihelion of Donati's comet.

1859 Spectrum analysis established by Kirchhoff and Bunsen.

1859 Carrington's discovery of the compound nature of the

sun's rotation.

1859, Sept. 1 Luminous solar outburst and magnetic storm.

1859, Oct. 19 Merope nebula discovered by Tempel.

1859, Dec. 15 Chemical constitution of the sun described by Kirchhoff.

1860, Feb. 27 Discovery by Liais of a "double comet."

1860, May 21 New star in Scorpio detected by Auwers.

1860, July 18 Total solar eclipse observed in Spain. Prominences

shown by photography to be solar appendages.

1861, June 30 The earth involved in the tail of a great comet.

1861-1862 Kirchhoff's map of the solar spectrum.

1862 Solar hydrogen-absorption recognised by Ångström.

1862, Jan. 31 Discovery by Alvan G. Clark of the companion of Sirius.

1862 Foucault determines the sun's distance by the velocity

of light.

1862 Opposition of Mars. Determination of solar parallax.

1862 Completion of _Bonner Durchmusterung_.

1863 Secchi's classification of stellar spectra.

1863 Foundation of the German Astronomical Society.

1864, March 5 Rotation period of Mars determined by Kaiser.

1864 Huggins's first results in stellar spectrum analysis.

1864, Aug. 5 Spectroscopic examination of Tempel's comet by

Donati shows it to be composed of glowing gas.

1864, Aug. 29 Discovery by Huggins of gaseous nebulæ.

1864 Value of 91,000,000 miles adopted for the sun's

distance.

1864 Croll's explanation of glacial epochs.

1864, Nov. 23 Death of Struve.

1865, Jan. 4 Spectroscopic observation by Huggins of the occultation

of Eta Piscium.

1865, Jan. 16 Faye's theory of the solar constitution.

1865 Kew results published.

1865 Zöllner argues for a high temperature in the great

planets.

1866 Identity of the orbits of the August meteors and

of comet 1862 iii. demonstrated by Schiaparelli.

1866 Delaunay explains lunar acceleration by a lengthening

of the day through tidal friction.

1866, March 4 Spectroscopic study of the sun's surface by Lockyer.

1866, March 12 New star in Corona Borealis detected by Birmingham.

1866, October Schmidt announces the disappearance of the lunar crater

Linné.

1866, Nov. 13 Meteoric shower visible in Europe.

1867 Period of November meteors determined by Adams.

1867, Aug. 29 Total solar eclipse. Minimum sun-spot type of corona

observed by Grosch at Santiago.

1867 Discovery of gaseous stars in Cygnus by Wolf and Rayet.

1868, February Principle of daylight spectroscopic visibility of

prominences started by Huggins.

1868, Aug. 18 Great Indian eclipse. Spectrum of prominences observed.

1868, Aug. 19 Janssen's first daylight view of a prominence.

1868, Oct. 26 Lockyer and Janssen independently announce their

discovery of the spectroscopic method.

1868 Doppler's principle applied by Huggins to measure

stellar radial movements.

1868 Publication of Ångström's map of the normal solar

spectrum.

1868 Spectrum of Winnecke's comet found by Huggins to agree

with that of olefiant gas.

1869, Feb. 11 Tenuity of chromospheric gases inferred by Lockyer and

Frankland.

1869, Feb. 13 Huggins observes a prominence with an "open slit."

1869, Aug. 7 American eclipse. Detection of bright-line coronal

spectrum.

1870 Mounting of Newall's 25-inch achromatic at Gateshead.

1870 Proctor indicates the prevalence of drifting movements

among the stars.

1870 A solar prominence photographed by Young.

1870, Dec. 22 Sicilian eclipse. Young discovers reversing layer.

1871, May 11 Death of Sir J. Herschel.

1871, June 9 Line-displacements due to solar rotation detected by

Vogel.

1871, Dec. 12 Total eclipse visible in India. Janssen observes

reflected Fraunhofer lines in spectrum of corona.

1872 Conclusion of a three years' series of observations on

lunar heat by Lord Rosse.

1872 Spectrum of Vega photographed by H. Draper.

1872 Faye's cyclonic hypothesis of sun-spots.

1872 Young's solar-spectroscopic observations at Mount

Sherman.

1872 Cornu's experiments on the velocity of light.

1872, Nov. 27 Meteoric shower connected with Biela's comet.

1873 Determination of mean density of the earth by

Cornu and Baille.

1873 Solar photographic work begun at Greenwich.

1873 Erection of 26-inch Washington refractor.

1874 Light-equation redetermined by Glasenapp.

1874 Vogel's classification of stellar spectra.

1874, Dec. 8 Transit of Venus.

1876 Publication of Neison's _The Moon_.

1876, Nov. 24 New star in Cygnus discovered by Schmidt.

1876 Spectrum of Vega photographed by Huggins. First

use of dry gelatine plates in celestial photography.

1877, May 19 Klein observes a supposed new lunar crater (Hyginus N.).

1877 Measurement by Vogel of selective absorption in solar

atmosphere.

1877, Aug. 16-17 Discovery of two satellites of Mars by Hall at

Washington.

1877, Sept. 23 Death of Leverrier.

1877 Canals of Mars discovered by Schiaparelli.

1877 Opposition of Mars observed by Gill at Ascension.

Solar parallax deduced = 8.78".

1878, January Stationary meteor-radiants described by Denning.

1878 Publication of Schmidt's

_Charte der Gebirge des Mondes_.

1878 First observations of Great Red Spot on Jupiter.

1878 Conclusion of Newcomb's researches on the lunar theory.

1878, May 6 Transit of Mercury.

1878 Foundation of Selenographical Society.

1878, July 29 Total eclipse visible in America. Vast equatoreal

extension of the corona.

1878, October Completion of Potsdam Astrophysical Observatory.

1878, Dec. 12 Lockyer's theory of celestial dissociation communicated

to the Royal Society.

1879 Michelson's experiments on the velocity of light.

1879 Publication of Gould's _Uranometria Argentina_.

1879, November Observations of the spectra of sun-spots begun at

South Kensington.

1879, Dec. 5 Abney's map of the infra-red solar spectrum presented

to the Royal Society.

1879, Dec. 18 Ultra-violet spectra of white stars described by

Huggins.

1879, Dec. 18 Communication of G. H. Darwin's researches into

the early history of the moon.

1880, Jan. 31 Discovery at Cordoba of a great southern comet.

1880 Conditions of Algol's eclipses determined by Pickering.

1880 Pickering computes mass-brightness of binary stars.

1880, Sept. 30 Draper's photograph of the Orion nebula.

1880 The bolometer invented by Langley.

1881, Jan. 20 Communication of G. H. Darwin's researches into the

effects of tidal friction on the evolution of the

solar system.

1881 Langley's observations of atmospheric absorption on

Mount Whitney.

1881, June 16 Perihelion of Tebbutt's comet.

1881, June 24 Its spectrum photographed by Huggins.

1881, June Photographs of Tebbutt's comet by Janssen and Draper.

1881, Aug. 15 Retirement of Sir George Airy. Succeeded by Christie.

1881, Aug. 22 Perihelion of Schaeberle's comet.

1881 Publication of Stone's Cape Catalogue for 1880.

1882 Struve's second measures of Saturn's ring-system.

1882 Newcomb's determination of the velocity of light.

Resulting solar parallax = 8·79".

1882 Correction by Nyrén of Struve's constant of aberration.

1882, March 7 Spectrum of Orion nebula photographed by Huggins.

1882, May 17 Total solar eclipse observed at Sohag in Egypt.

1882, May 27 Sodium-rays observed at Dunecht in spectrum of

Comet Wells.

1882, June 10 Perihelion of Comet Wells.

1882, Sept. 17 Perihelion of Great Comet. Daylight detection by Common.

Transit observed at the Cape.

1882, Sept. 18 Iron lines identified in spectrum by Copeland and

J. G. Lohse.

1882, September Photographs of comet taken at the Cape Observatory,

showing a background crowded with stars.

1882, Dec. 6 Transit of Venus.

1882 Duplication of Martian canals observed by Schiaparelli.

1882 Completion by Loewy at Paris of first equatoreal Coudé.

1882 Rigidity of the earth concluded from tidal observations

by G. H. Darwin.

1882 Experiments by Huggins on photographing the corona

without an eclipse.

1882 Publication of Holden's _Monograph of the Orion Nebula_.

1883, Jan. 30 Orion Nebula photographed by Common.

1883, May 6 Caroline Island eclipse.

1883, June 1 Great comet of 1882 observed from Cordoba at a distance

from the earth of 470 million miles.

1883 Parallaxes of nine southern stars measured by Gill

and Elkin.

1883 Catalogue of the spectra of 4,051 stars by Vogel.

1884, Jan. 25 Return to perihelion of Pons's comet.

1884 Photometric Catalogue by Pickering of 4,260 stars.

1884 Publication of Gore's Catalogue of Variable Stars.

1884 Publication of Faye's _Origine du Monde_.

1884, Oct. 4 Eclipse of the moon. Heat-phases measured by Boeddicker

at Parsonstown.

1884 Dunér's Catalogue of Stars with Banded Spectra.

1884 Backlund's researches into the movements of

Encke's comet.

1885, February Langley measures the lunar heat-spectrum.

1885 Publication of _Uranometria Nova Oxoniensis_.

1885, Aug. 17 New star in Andromeda nebula discerned by Gully.

1885, Sept. 5 Thollon's drawing of the solar spectrum presented to the

Paris Academy.

1885, Sept. 9 Solar eclipse visible in New Zealand.

1885, Nov. 16 Photographic discovery by Paul and Prosper Henry of

a nebula in the Pleiades.

1885, Nov. 27 Shower of Biela meteors.

1885 Thirty-inch achromatic mounted at Pulkowa.

1885 Publication of Rowland's photographic map of the normal

solar spectrum.

1885 Bakhuyzen's determination of the rotation period of

Mars.

1885 Stellar photographs by Paul and Prosper Henry.

1886, Jan. 26 Spectra of forty Pleiades simultaneously photographed

at Harvard College.

1886, Feb. 5 First visual observation of the Maia nebula with the

Pulkowa 30-inch refractor.

1886, March Photographs by the Henrys of the Pleiades, showing

2,326 stars with nebulæ intermixed.

1886, May Photographic investigations of stellar parallax

undertaken by Pritchard.

1886, May 6 Periodical changes in spectra of sun-spots announced

by Lockyer.

1886, June 4 An international Photographic Congress proposed by Gill.

1886, Aug. 29 Total eclipse of the sun observed at Grenada.

1886, Oct. 1 Roberts's photograph showing annular structure of

the Andromeda nebula.

1886, Dec. 8 Roberts's photograph of the Pleiades nebulosities.

1886 Solar heat-spectrum extended by Langley to below

five microns.

1886, Dec. 28 Detection by Copeland of helium-ray in spectrum of

the Orion nebula.

1886 Thirty-inch refractor mounted at Nice.

1886 Publication of Argentine General Catalogue.

1886 Completion of Auwers's reduction of Bradley's

observations.

1886 Draper Memorial photographic work begun at

Harvard College.

1886 Photographic detection at Harvard College of bright

hydrogen lines in spectra of variables (Mira Ceti

and U Orionis).

1887, Jan. 18 Discovery by Thome at Cordoba of a great comet

belonging to the same group as the comet of 1882.

1887 Publication of Lockyer's _Chemistry of the Sun_.

1887, April 16 Meeting at Paris of the International Astrophotographic

Congress.

1887 Heliometric triangulation of the Pleiades by Elkin.

1887 L. Struve's investigation of the sun's motion, and

redetermination of the constant of precession.

1887 Von Konkoly's extension to 15° S. Dec. of Vogel's

spectroscopic Catalogue.

1887 Auwers's investigation of the solar diameter.

1887 Publication of Schiaparelli's Measures of Double Stars

(1875-85).

1887, April 8 Death of Thollon at Nice.

1887, Aug. 19 Total eclipse of the sun. Shadow-path crossed Russia.

Observations marred by bad weather.

1887, November Langley's researches on the temperature of the moon.

1887, Nov. 17 Lockyer's _Researches on Meteorites_ communicated to

the Royal Society.

1887 Completion of 36-inch Lick refractor.

1888 Küstner's detection of variations in the latitude of

Berlin brought before the International Geodetic

Association.

1888 Chandler's first Catalogue of Variable Stars.

1888 Mean parallax of northern first magnitude stars

determined by Elkin.

1888 Publication of Dreyer's _New General Catalogue_ of

7,844 nebulæ.

1888 Vogel's first spectrographic determinations of stellar

radial motion.

1888 Carbon absorption recognised in solar spectrum by

Trowbridge and Hutchins.

1888, Jan. 28 Total eclipse of the moon. Heat-phases measured at

Parsonstown.

1888, Feb. 5 Remarkable photograph of the Orion nebula spectrum

taken at Tulse Hill.

1888, June 1 Activity of the Lick Observatory begun.

1888 Completion of Dr. Common's 5-foot reflector.

1888 Heliometric measures of Iris for solar parallax at the

Cape, Newhaven (U.S.A.), and Leipsic.

1888 Loewy describes a comparative method of determining

constant of aberration.

1888 Presentation of the Dunecht instrumental outfit to

the nation by Lord Crawford. Copeland succeeds

Piazzi Smyth as Astronomer-Royal for Scotland.

1888, Sept. 12 Death of R. A. Proctor.

1889 Photograph of the Orion nebula taken by W. H. Pickering,

showing it to be the nucleus of a vast spiral.

1889 Discovery at a Harvard College of the first-known

spectroscopic doubles, Zeta Ursæ Majoris and

Beta Aurigæ.

1889 Eclipses of Algol demonstrated spectrographically by

Vogel.

1889 Completion of photographic work for the Southern

Durchmusterung.

1889 Boeddicker's drawing of the Milky Way.

1889 Draper Memorial photographs of southern star-spectra

taken in Peru.

1889 Pernter's experiments on scintillation from the

Sonnblick.

1889 H. Struve's researches on Saturn's satellites.

1889 Harkness's investigation of the masses of Mercury,

Venus, and the Earth.

1889 Heliometric measures of Victoria and Sappho at the Cape.

1889, Jan. 1 Total solar eclipse visible in California.

1889, Feb. 7 Foundation of the Astronomical Society of the Pacific.

1889, March Investigation by Sir William and Lady Huggins of

the spectrum of the Orion nebula.

1889, July-Aug. First photographs of the Milky Way taken by Barnard.

1889, August 2 Observation by Barnard of four companions to Brooks's

comet.

1889, Nov. 1 Passage of Japetus behind Saturn's dusky ring observed

by Barnard.

1889, December Schiaparelli announces synchronous rotation and

revolution of Mercury.

1889, Dec. 22 Total eclipse of the sun visible in Guiana. Death of

Father Perry, December 27.

1889 Spectrum of Uranus investigated visually by Keeler,

photographically by Huggins.

1890 Long-exposure photographs of ring-nebula in Lyra.

1890 Determinations of the solar translation by L. Boss

and O. Stumpe.

1890 Schiaparelli finds for Venus an identical period of

rotation and revolution.

1890 Publication of Thollon's map of the solar spectrum.

1890 Bigelow's mathematical theory of coronal structures.

1890 Foundation of the British Astronomical Association.

1890 Measurements by Keeler at Lick of nebular radial

movements.

1890 Janssen's ascent of Mont Blanc, by which he ascertained

the purely terrestrial origin of the oxygen-absorption

in the solar spectrum.

1890 Newcomb's discussion of the transits of Venus of

1761 and 1769.

1890 Spiral structure of Magellanic Clouds displayed in

photographs taken by H. C. Russell of Sydney.

1890 Publication of the Draper Catalogue of Stellar Spectra.

1890, April 24 Spica announced by Vogel to be a spectroscopic binary.

1890, June Gore's Catalogue of computed Binaries.

1890, November Study by Sir William and Lady Huggins of the

spectra of Wolf and Rayet's stars in Cygnus.

1890, November Discovery by Barnard of a close nebulous companion

to Merope in the Pleiades.

1890, November McClean Spectrographs of the High and Low Sun.

1891 Capture-theory of comets developed by Callandreau,

Tisserand, and Newton.

1891 Dunér's spectroscopic researches on the sun's rotation.

1891 Preponderance of Sirian stars in the Milky Way

concluded by Pickering, Gill, and Kapteyn.

1891 Detection by Mrs. Fleming of spectral variations

corresponding to light-changes in Beta Lyræ.

1891 Establishment of the Harvard College Station at

Arequipa in Peru (height 8,000 feet).

1891 Variations of latitude investigated by Chandler.

1891 Prominence-photography set on foot by Hale at

Chicago and Deslandres at Paris.

1891 Schmidt's Theory of Refraction in the Sun.

1891, April Meeting at Paris of the Permanent Committee for the

Photographic Charting of the Heavens.

1891, May 9 Transit of Mercury.

1891, Aug. 19 Presidential Address by Huggins at the Cardiff Meeting

of the British Association.

1891, Dec. 10 Nova Aurigæ photographed at Harvard College.

1891, Dec. 20 Photographic maximum of Nova Aurigæ.

1891, Dec. 22 First photographic discovery of a minor planet by

Max Wolf at Heidelberg.

1892 Commencement of international photographic charting

work.

1892 Photographic determination by Scheiner of 833 stars

in the Hercules Cluster (M 13).

1892 Publication of Vogel's spectrographic determinations

for fifty-one stars.

1892 Publication of Pritchard's photographic parallaxes.

1892, Jan. 2 Death of Sir George Airy.

1892, Jan. 21 Death of Professor Adams.

1892, Feb. 1 Announcement by Anderson of the outburst of a new

star in Auriga.

1892, Feb. 5 Appearance of the largest sun-spot ever photographed

at Greenwich.

1892, March Photograph of Argo nebula taken by Gill in twelve hours.

1892, March 6 Discovery of a bright comet by Swift.

1892, June 29 Death of Admiral Mouchez. Succeeded by Tisserand

as director of the National Observatory, Paris.

1892, Aug. 4 Favourable Opposition of Mars.

1892, Aug. 17 Rediscovery at Lick of Nova Aurigæ.

1892, Sept. 9 Discovery by Barnard of Jupiter's inner satellite.

1892, Oct. 12 First photographic discovery of a comet by Barnard.

1892, Nov. 6 Discovery of Holmes's comet.

1892, Nov. 23 Shower of Andromede meteors visible in America.

1892 Poynting's Determination of the Earth's Mean Density.

1892 Dunér's Investigation of the System of upsilon Cygni.

1892 Photographic investigation by Deslandres of the

spectra of prominences.

1892 Photographs of the sun with faculæ and chromospheric

surroundings taken by Hale with a single exposure.

1892 Investigation by T. J. J. See of the ancient colour of

Sirius.

1892 Publication of T. J. J. See's Thesis on the Evolution

of Binary Systems.

1892 Chandler's theory of Algol's inequalities.

1892 Nebula in Cygnus photographically discovered by

Max Wolf.

1893, Jan. 28 Kapteyn's investigation of the structure of the stellar

universe.

1893, March 10 Gill announces his results from the Opposition of

Victoria, among them a solar parallax = 8.809".

1893, April 16 Total solar eclipse observed in South America and

West Africa.

1893 Publication of Kruger's _Catalog der Farbigen Sterne_.

1893 Conclusion of Boys's series of Experiments on the

Density of the Earth.

1893 Publication of _Cordoba Durchmusterung_, vol. i.

1893 Fabry shows comets to be dependents of the Solar System.

1893 Publication of Easton's _Voie Lactée_.

1893 Campbell detects bright H Alpha in Gamma Argûs and

Alcyone.

1893 Nova Normæ photographed July 10; discovered on

plates, October 26.

1893, May 28 Death of Professor Pritchard.

1893, July 27 Installation of 28-inch refractor at the Royal

Observatory, Greenwich.

1893, December Exterior nebulosities of Pleiades photographed by

Barnard.

1893, Dec. 6 Death of Rudolf Wolf.

1894, January Sun-spot maximum.

1894 Publication of Potsdam _Photometric Durchmusterung_,

part i.

1894 Publication of Roberts's _Celestial Photographs_,

vol. i.

1894 Wilson and Gray's determination of the sun's

temperature.

1894 Barnard's micrometric measures of asteroids.

1894 McClean's gift of an astrophysical outfit to the Cape

Observatory.

1894 Establishment of the Lowell Observatory at Flagstaff,

Arizona.

1894 Taylor's triple achromatic objective described.

1894, April 3 Discovery of Gale's Comet.

1894 Sampson's investigation of the sun's rotation.

1894, Oct. 20 Favourable opposition of Mars.

1894, Nov. 11 Transit of Mercury.

1894, December Howlett impugns the Wilsonian theory of sun-spots.

1894, Dec. 14 Death of A. Cowper Ranyard.

1895 Publication of Newcomb's _Astronomical Constants_.

1895 Bailey's Photometric Catalogue of 7,922 Southern Stars.

1895 Bailey's photographic discovery of variable star

clusters.

1895 Publication of E. W. Brown's _Lunar Theory_.

1895 Tisserand's theory of the inequalities of Algol.

1895 Stratonoff's determination of the sun's rotation from

photographs of faculæ.

1895 Binary character of Eta Aquilæ spectroscopically

recognised by Bélopolsky.

1895 Presentation of the Crossley reflector to the Lick

Observatory.

1895, March 23 Great nebula in Ophiuchus discovered photographically

by Barnard.

1895, March 25 Ramsay's capture of Helium.

1895, April Constitution of Saturn's rings spectrographically

demonstrated by Keeler.

1895 Binary character of Delta Cephei spectroscopically

detected by Bélopolsky.

1895, June 11 Death of Daniel Kirkwood.

1895, July 7 Death of F. W. G. Spörer.

1895, October Nova Carinæ spectrographically discovered by

Mrs. Fleming.

1895, Dec. 12 Nova Centauri spectrographically discovered by

Mrs. Fleming.

1895, Dec. 28 Death of John Russell Hind.

1896 Gill's Report on the Geodetic Survey of South Africa.

1896 Appearance of Loewy's Photographic Atlas of the Moon,

part i.

1896, January Fessenden's electrostatic theory of comets.

1896 Chandler's Third Catalogue of Variable Stars.

1896 Publication of Lick Observatory Photographic Atlas

of the Moon, part i.

1896, February Effects of pressure on wave-length described by

Humphreys and Mohler.

1896, April 5 Opening of new Scottish Royal Observatory on Blackford

Hill, Edinburgh.

1896, April Pickering's photometric determinations of light curves

of variable stars.

1896 One of the stars of Castor spectroscopically resolved

into two by Bélopolsky.

1896, May Third Astrographic Chart Conference at Paris.

1896, Aug. 9 Total eclipse of the sun visible in Novaya Zemlya.

Reversing layer photographed by Shackleton.

1896, Aug. 30 Death of Hubert A. Newton.

1896, Sept. 18 Death of Hippolyte Fizeau.

1896, Oct. 20 Death of F. Tisserand. Succeeded by Maurice Loewy.

1896, Nov. 13 Detection by Schaeberle of Procyon's missing satellite.

1896, Nov. 26 Death of Benjamin Apthorp Gould.

1896, November Second series of hydrogen-lines discovered by Pickering

in stellar spectra.

1896, December Zeeman's discovery of spectral modifications through

magnetic influence.

1896, December Oxygen-absorption identified in the sun by Runge and

Paschen.

1896 Study of lunar formations by Loewy and Puiseux.

1896 Mounting of the Mills spectrograph at the

Lick Observatory.

1897 Installation at Greenwich of the Thompson 26-inch

photographic refractor.

1897 Publication of Miss Maury's Discussion of the

Photographed Spectra of 681 Stars.

1897 Callandreau's researches on cometary disaggregation.

1897 Braun's determination of the earth's mean density.

1897 Tenuity of calcium vapour in chromosphere demonstrated

spectroscopically by Sir William and Lady Huggins.

1897 Completion at the Cape Observatory of McClean's

spectrographic survey of the heavens.

1897 Twenty-one Wolf-Rayet stars found by Mrs. Fleming

in Magellanic Cloud.

1897 Percival Lowell's _New Observations on the Planet

Mercury_ presented to the American Academy.

1897, April 8 McClean recognises oxygen-absorption in helium stars.

1897, May 9 Death of E. J. Stone, Radcliffe Observer.

1897, June 10 Death of Alvan G. Clark.

1897, June 18 Spectrum of a meteor photographed at Arequipa.

1897, Oct. 21 Inauguration of the Yerkes Observatory.

1897 Rabourdin's photographs of nebulæ with the Meudon

reflector.

1897 Dr. See's discoveries of Southern double stars with the

Lowell 24-inch refractor.

1898, Jan. 22 Total eclipse of the sun visible in India.

1898, February Binary character of Zeta Geminorum ascertained

spectroscopically by Bélopolsky.

1898 Star with proper motion of nearly 9" discovered by

Innes and Kapteyn from the Cape Durchmusterung plates.

1898, March 8 Nova Sagittarii photographed on Draper Memorial plates.

1898, June 20 Opening of Grand-ducal Observatory at Königsstuhl,

Heidelberg.

1898 Keeler succeeds Holden as Director of the Lick

Observatory.

1898 Bruno Peter's results in stellar parallax.

1898 Lewis Swift's discoveries of nebulæ at Echo Mountain,

California.

1898 Hale's photographic investigation of carbon stars.

1898, Aug. 14 Discovery of Eros by Witt.

1898 Flint's investigations of stellar parallax by meridian

differences.

1898 Easton's spiral theory of the Milky Way.

1898 Seeliger's research on star distribution.

1898, October Multiple hydrogen-bands observed by Campbell in

Mira Ceti.

1898, November Orbit of a Leonid meteor photographically determined

by Elkin.

1899 Publication of Potsdam _Photometric Durchmusterung_,

part ii.

1899 Innes's _Reference Catalogue of Southern Double Stars_.

1899 Keeler's photographs of nebulæ with the Crossley

reflector and generalization of their spiral

character.

1899, January Spectrum of Andromeda nebula photographed by Scheiner.

1899, April Photographic discovery of Nova Aquilæ by Mrs. Fleming.

1899, Aug. 26 Installation of 31-inch photographic refractor at

Potsdam.

1899 Campbell's detection of Polaris as spectroscopically

triple.

1899, October Duplicate discovery by Campbell and Newall of

Capella as a spectroscopic binary.

1899, Nov. 15 Failure of the Leonids. Deflection of the stream

predicted by Johnstone Stoney and Downing.

1899, December Publication of Sir William and Lady Huggins's _Atlas

of Representative Stellar Spectra_.

1899 Thirty-two-inch photographic refractor mounted at

Meudon.

1899 Issue of first volume of Potsdam measures of

international catalogue plates.

1900, Jan. 27 Kapteyn's determination of the apex of solar motion.

1900 Chase's measures for parallax of swiftly-moving stars.

1900 Publication of Gill's _Researches on Stellar Parallax_.

1900 Kapteyn proposes a method for a stellar parallax

Durchmusterung, and gives specimen results for

248 stars.

1900 Burnham's general catalogue of 1,290 double stars.

1900 Publication of the concluding volume of the _Cape

Photographic Durchmusterung_.

1900, May 28 Spanish-American total eclipse of the sun.

1900, July International Conference at Paris. Co-operation

arranged of fifty-eight observatories in measures of

Eros for solar parallax.

1900 Horizontal refractor, of 50 inches aperture, 197 feet

focus, installed in Paris Exhibition.

1900, Aug. 12 Death of Professor Keeler. Succeeded by Campbell

in direction of Lick Observatory.

1900, November Opposition of Eros.

1900 Publication of Roberts's _Celestial Photographs_,

vol. ii.

1900 Complete publication of Langley's researches on the

infra-red spectrum.

1900 Printing begun of Paris section of International

Photographic Catalogue.

1901, Feb. 22 Nova Persei discovered by Anderson.

1901, February Variability of Eros announced by Oppolzer.

1901, April 23 Apparition of a great comet at the Cape.

1901 Publication of Pickering's _Photometric Durchmusterung_.

1901 Miss Cannon's discussion of the spectra of 1,122

Southern stars.

1901 Kapteyn's investigation of mean stellar parallax.

1901 Campbell's determination of the sun's velocity.

1901 Porter's research on the solar motion in space.

1901 Bigelow's magnetic theory of the solar corona.

1901 Hussey's measurements of the Pulkowa double stars.

1901 Radial velocities of the components of Delta Equulei

measured at Lick.

1901, April 16 Death of Henry A. Rowland.

1901, June Nebular spectrum derived from Nova Persei.

1901, Aug. 23 Nebula near Nova Persei photographed by Max Wolf.

1901, Sept. 20 The same exhibited in spiral form on a plate taken by

Ritchey at the Yerkes Observatory.

1901, Nov. 8 Photograph taken by Perrine with the Crossley

reflector showed nebula in course of rapid change.

1901, Sept. 19 Unveiling of the McClean "Victoria" telescope at the

Royal Observatory, Cape of Good Hope.

1901 Sun-spot minimum.

TABLE II.

CHEMICAL ELEMENTS IN THE SUN (ROWLAND, 1891).

Arranged according to the number of their representative Lines in the

Solar Spectrum.

Iron (2000+). Neodymium. Cadmium.

Nickel. Lanthanum. Rhodium.

Titanium. Yttrium. Erbium.

Manganese. Niobium. Zinc.

Chromium. Molybdenum. Copper (2).

Cobalt. Palladium. Silver (2).

Carbon (200+). Magnesium (20+). Glucinum (2).

Vanadium. Sodium (11). Germanium.

Zirconium. Silicon. Tin.

Cerium. Strontium. Lead (1).

Calcium (75+). Barium. Potassium (1).

Scandium. Aluminium (4).

_Doubtful Elements._--Iridium, osmium, platinum, ruthenium, tantalum,

thorium, tungsten, uranium.

_Not in Solar Spectrum._--Antimony, arsenic, bismuth, boron, nitrogen

(vacuum tube), cæsium, gold, iridium, mercury, phosphorus, rubidium,

selenium, sulphur, thallium, praseodymium.

Oxygen was added to the solar ingredients by Runge and Paschen in 1896,

gallium by Hartley and Ramage in 1899. Lithium may be admitted

provisionally, and the chromospheric constituent helium takes rank,

since 1895, as a chemical element.

TABLE III.

EPOCHS OF SUN-SPOT MAXIMUM AND MINIMUM FROM 1610 TO 1901.

+----------+----------++----------+----------+----------+----------+

| Minima. | Maxima. || Minima. | Maxima. | Minima. | Maxima. |

+----------+----------++----------+----------+----------+----------+

| 1610.8 | 1615.5 || 1712.0 | 1718.2 | 1810.6 | 1816.4 |

| 1619.0 | 1626.0 || 1723.5 | 1727.5 | 1823.3 | 1829.9 |

| 1634.0 | 1639.5 || 1734.0 | 1738.7 | 1833.9 | 1837.2 |

| 1645.0 | 1649.0 || 1745.0 | 1750.3 | 1843.5 | 1848.1 |

| 1655.0 | 1660.0 || 1755.2 | 1761.5 | 1856.0 | 1860.1 |

| 1666.0 | 1675.0 || 1766.5 | 1769.7 | 1867.2 | 1870.6 |

| 1679.5 | 1685.0 || 1775.5 | 1778.4 | 1878.9 | 1884.0 |

| 1689.5 | 1693.0 || 1784.7 | 1788.1 | 1890.2 | 1894.0 |

| 1698.9 | 1705.5 || 1798.3 | 1804.2 | 1901.9 | |

+----------+----------++----------+----------+----------+----------+

TABLE IV.

MOVEMENTS OF SUN AND STARS.

1. Translation of Solar System.

+--------------------+-------------------------+-------+

| Apex of Movement. | Authority. | Date. |

+--------------------+-------------------------+-------+

| R. A. Dec. | | |

| | | |

| 277° 30' + 35° | Newcomb | 1898 |

| 273° 36' + 29° 30' | Kapteyn | 1901 |

| 279° + 46° | Porter | 1901 |

| 275° + 45° | Boss | 1901 |

| 277° 30' + 20° | Campbell (from stellar | 1902 |

| | spectroscopic measures) | |

+--------------------+-------------------------+-------+

| Velocity = 12·4 miles per second (Campbell). |

+------------------------------------------------------+

2. Stellar Velocities.

+---------------------+------------+-------------+---------------------+

| Name of Star. | Rate. | Direction. | Remarks. |

| | Miles per | | |

| | Sec. | | |

+---------------------+------------+-------------+---------------------+

| Delta Leporis | 58 | Receding | Campbell, 1901 |

| Eta Cephei | 54 | Approaching | " 1899 |

| Theta Canis Majoris | 60 | Receding | " 1901 |

| Iota Pegasi | 47 | Approaching | " " |

| Mu Sagittarii | 47 | Approaching | " " |

| Eta Andromedæ | 52 | Approaching | " " |

| Zeta Herculis | 44 | Approaching | Bélopolsky, 1893 |

| 61 Cygni | 34 | Approaching | " " |

| Mu Cassiopeiæ | 60 | Approaching | Campbell, 1901 |

| 1830 Groombridge | 59 | Approaching | " " |

| Arcturus | 4.3 | Approaching | Keeler, 1890 |

| Arcturus | 278 | Tangential | Accepting Elkin's |

| | | | parallax of 0·024" |

| 1830 Groombridge | 150 | Tangential | Parallax = 0·14" |

| Mu Cassiopeiæ | 113 | Tangential | Parallax = 0·10" |

| | | | (Peter) |

| Z. C. 5^h 243 | 82 | Tangential | Parallax = 0·312" |

| | | | (Gill) |

| Lacaille, 2,957 | 78 | Tangential | Parallax = 0·064" |

| | | | (Gill) |

| Lacaille, 9,352 | 73 | Tangential | Parallax = 0·283" |

| | | | (Gill) |

| o_2, Eridani | 72 | Tangential | Parallax = 0·166" |

| | | | (Gill) |

| Eta Eridani | 61 | Tangential | Parallax = 0·149" |

| | | | (Gill) |

+---------------------+------------+-------------+---------------------+

TABLE V.

LIST OF GREAT TELESCOPES.

1. Reflectors--A. Metallic Specula.

+-------------+-----------+------------+-------------+------------------+

| Locality. |Aperture in|Focal Length| Constructor.| Remarks. |

| | Inches. | in Feet. | | |

+-------------+-----------+------------+-------------+------------------+

|Birr Castle, | | | Third Earl | |

|Parsonstown, | 72 | 54 | of Rosse, |Newtonian. |

|Ireland | | | 1845 | |

+-------------+-----------+------------+-------------+------------------+

|Melbourne | 48 | 28 | T. Grubb, |Cassegrain. |

|Observatory | | | 1870 | |

+-------------+-----------+------------+-------------+------------------+

| | | | Third Earl |Newtonian. |

|Birr Castle | 36 | -- | of Rosse, |Remounted |

| | | | 1839 |equatoreally 1876.|

+-------------+-----------+------------+-------------+------------------+

| | | | William |Newtonian. |

|Royal | | | Lassell, |Presented |

|Observatory | 24 | 20 | 1846 |by the Missess |

|Greenwich | | | |Lassell to the |

| | | | |Royal Observatory |

+-------------+-----------+------------+-------------+------------------+

| B. Silvered Glass Mirrors. |

+-------------+-----------+------------+-------------+------------------+

|Ealing, near | 60 | 27 |A. A. Common,|Newtonian. |

|London | | | 1891 | |

+-------------+-----------+------------+-------------+------------------+

| | | |G. W. Richey,|Can be employed |

|Yerkes | 60 | 25 | 1902 |at choice as a |

|Observatory | | | |Coudé or a |

| | | | |Cassegrain. |

+-------------+-----------+------------+-------------+------------------+

|National | 48 | -- |Martin, 1875 |Newtonian. |

|Observatory, | | | |Remodelled for |

|Paris | | | |spectrographic |

| | | | |work by |

| | | | |Deslandres in |

| | | | |1892. |

+-------------+-----------+------------+-------------+------------------+

|Meudon | 39 | 9·7 | | |

|Observatory | | | | |

+-------------+-----------+------------+-------------+------------------+

|Lick | 36 | 17·5 |Calver, 1879 |Mounted by |

|Observatory | | | |Common at |

| | | | |Ealing in 1879. |

| | | | |Sold by him to |

| | | | |Crossley, 1885. |

| | | | |Presented by |

| | | | |Crossley to the |

| | | | |Lick |

| | | | |Observatory, 1895.|

+-------------+-----------+------------+-------------+------------------+

|Toulouse | 32·5 | 16·2 | Brothers | |

|Observatory | | | Henry | |

+-------------+-----------+------------+-------------+------------------+

|Marseilles | 31·5 | -- |Foucault | |

|Observatory | | | | |

+-------------+-----------+------------+-------------+------------------+

|Royal | 30 | -- |Common, 1897 |Cassegrain. |

|Observatory, | | | |Mounted as a |

|Greenwich | | | |counterpoise |

| | | | |to the |

| | | | |Thompson |

| | | | |equatoreal. |

+-------------+-----------+------------+-------------+------------------+

|Westgate- | | | Common, |The property |

|on-Sea | 30 | -- | 1889 |of Sir Norman |

| | | | |Lockyer. |

+-------------+-----------+------------+-------------+------------------+

|Harvard | | | H. Draper, |Mounted for |

|College | 28 | -- | 1870 |spectrographic |

|Observatory | | | |work,1887. |

+-------------+-----------+------------+-------------+------------------+

|Royal | | | T. Grubb, | |

|Observatory, | 24 | -- | 1872 | |

|Edinburgh | | | | |

+-------------+-----------+------------+-------------+------------------+

|Daramona, | | | Sir H. |Remounted 1891. |

|Ireland | 24 | 10·5 | Grubb, |Owned by Mr. W. E.|

| | | | 1881 |Wilson. |

+-------------+-----------+------------+-------------+------------------+

| | | | |Can be used as a |

|Yerkes | 23·5 | 7·7 | Ritchey, |Cassegrain, with |

|Observatory | | | 1901 |an equivalent |

| | | | |focal length of |

| | | | |38 feet. |

+-------------+-----------+------------+-------------+------------------+

|Harvard | | | | |

|College | 20 | -- | Common, | |

|Observatory | | | 1890 | |

+-------------+-----------+------------+-------------+------------------+

|Crowborough, | 20 | 8·2 | Sir H. |Mounted with a |

|Sussex | | | Grubb, |7-inch |

| | | | 1885 |refractor. |

+-------------+-----------+------------+-------------+------------------+

| 2. Refractors. |

+-------------+-----------+------------+-------------+------------------+

|Palais de | | | Gautier, |Mounted as a |

|l'Optique, | 49·2 | 197 | 1900 |siderostat in |

|Paris | | | |connection with |

| | | | |a plane mirror 79 |

| | | | |inches across. |

+-------------+-----------+------------+-------------+------------------+

|Yerkes | 40 | 62 | Alvan G. | |

|Observatory | | | Clark, 1897 | |

+-------------+-----------+------------+-------------+------------------+

| | | | |For photographic |

|Lick | | | A. Clark and|purposes a |

|Observatory | 36 | 57·8 | Sons, 1888 |correcting lens is|

| | | | |available, of 33 |

| | | | |inches aperture, |

| | | | |47·8 feet focus. |

+-------------+-----------+------------+-------------+------------------+

| | | | |Mounted with a |

|Meudon | 32·5 | 55·2 | Henrys and |photographic |

|Observatory | | |Gautier, 1891|refractor of 24·4 |

| | | | |inches aperture. |

+-------------+-----------+------------+-------------+------------------+

| | | | |Photographic. |

|Astrophysical| | |Steinheil and|Mounted with a |

|Observatory, | 31·5 | 39·4 |Repsold, 1899|visual refractor |

|Potsdam | | | |20 inches in |

| | | | |aperture. |

+-------------+-----------+------------+-------------+------------------+

|Bischoffsheim| | | |Visual. Mounted |

|Observatory, | 30·3 | 52·6 | Henrys and |on Mont Gros, |

|Nice | | |Gautier, 1886|1,100 feet above |

| | | | |sea level. |

+-------------+-----------+------------+-------------+------------------+

|Imperial | | | A. Clark and|Visual. Mounted |

|Observatory, | 30 | 42 | Sons, 1885 |by Repshold. |

|Pulkowa | | | | |

+-------------+-----------+------------+-------------+------------------+

|National | | | | |

|Observatory, | 28·9 | -- | Martin | |

|Paris | | | | |

+-------------+-----------+------------+-------------+------------------+

|Royal | | | Sir H. |Visual and |

|Observatory, | 28 | 28 | Grubb, |photographic. |

|Greenwich | | | 1894 |Mounted by |

| | | | |Ransome and Simms.|

+-------------+-----------+------------+-------------+------------------+

| University | | |Sir H. Grubb,| Visual. |

| Observatory,| 27 | 34 | 1881 | |

| Vienna | | | | |

+-------------+-----------+------------+-------------+------------------+

| Royal | | |Sir H. Grubb,| The Thompson |

| Observatory,| 26 | 26 | 1897 | photographic |

| Greenwich | | | | equatoreal. |

+-------------+-----------+------------+-------------+------------------+

| Naval | | | A. Clark and| |

| Observatory,| 26 | 29 | Sons, 1873 | |

| Washington | | | | |

+-------------+-----------+------------+-------------+------------------+

| Leander | | | A. Clark and| |

| McCormick | 26 | 32·5 | Sons, 1881 | |

| Observatory,| | | | |

| Virginia | | | | |

+-------------+-----------+------------+-------------+------------------+

| Cambridge | | | T. Cooke and| Presented to the |

| University | 25 | -- | Sons, 1870 | University in |

| Observatory | | | | 1889 by |

| | | | | Mr. R. S. Newall.|

+-------------+-----------+------------+-------------+------------------+

| Meudon | | | Henrys and | Photographic. |

| Observatory | 24·4 | 52·2 | Gautier, | Mounted with a |

| | | | 1891 | visual 32.5 |

| | | | | refractor. |

+-------------+-----------+------------+-------------+------------------+

| Harvard | | | A. Clark and| Photographic |

| College | 24 | 11·3 | Sons, 1893 | doublet. The gift|

| Observatory | | | | of Miss Bruce. |

| | | | | Transfered in |

| | | | | 1896 to Arequipa,|

| | | | | Peru. |

+-------------+-----------+------------+-------------+------------------+

| Royal | | |Sir H. Grubb,| Photographic. |

| Observatory,| | | 1898 | The gift or Mr. |

| Cape of | 24 | 22·6 | | McClean. Mounted |

| Good Hope | | | | with an 18-inch |

| | | | | visual refractor.|

+-------------+-----------+------------+-------------+------------------+

| Lowell | | | Alvan G. | Visual. First |

| Observatory,| | | Clark, 1896 | mounted near the |

| Flagstaff, | 24 | 31 | | city of Mexico. |

| Arizona | | | | Installed at |

| | | | | Flagstaff, 1897. |

+-------------+-----------+------------+-------------+------------------+

| National | | | Henrys and | Visual and |

| Observatory,| 23·6 | 59 | Gautier, | photographic. |

| Paris | | | 1891 | Mounted as an |

| | | | | equatoreal Coudé.|

+-------------+-----------+------------+-------------+------------------+

| Halsted | | | A. Clark and| |

| Observatory,| 23 | 32 | Sons, 1883 | |

| Princeton, | | | | |

| N.J. | | | | |

+-------------+-----------+------------+-------------+------------------+

| City | | | | Mounted as a |

| Observatory,| 22 | 30 | | visual |

| Edinburgh | | | | equatoreal on |

| | | | | the Calton Hill, |

| | | | | 1898. |

+-------------+-----------+------------+-------------+------------------+

| Etna | | | Merz, 1897 | |

| Observatory | 21·8 | -- | | |

+-------------+-----------+------------+-------------+------------------+

| Buckingham | | | Buckingham | |

| Observatory | 21·2 | -- | and Wragge | |

+-------------+-----------+------------+-------------+------------------+

| Porro | | | Porro | |

| Observatory,| 20·5 | -- | | |

| Turin | | | | |

+-------------+-----------+------------+-------------+------------------+

| Chamberlin | | | Alvan G. | Visual. |

| Observatory,| 20 | 28 | Clark and | with a reversible|

| Colorado | | | Saegmüller, | crown lens for |

| | | | 1894 | photography. |

+-------------+-----------+------------+-------------+------------------+

| Manila | | | Merz and | Visual. |

| Observatory | 20 | -- | Saegmüller, | Provided with a |

| | | | 1894 | photographic |

| | | | | correcting lens. |

+-------------+-----------+------------+-------------+------------------+

| Strasburg | 20·5 | 23 |Merz and | |

| Observatory | | |Repsold, 1880| |

+-------------+-----------+------------+-------------+------------------+

| Brera | | | Merz and | |

| Observatory,| 19·1 | 23 | Repsold | |

| Milan | | | | |

+-------------+-----------+------------+-------------+------------------+

| Dearborn | | | A. Clark and| Mounted 1864. |

| Observatory,| 18·5 | 27 | Sons, 1862 | |

| Illinois | | | | |

+-------------+-----------+------------+-------------+------------------+

| National | | | Henrys and | Coudé Mount. |

| Observatory,| 18·1 | 29·5 | Gautier, | Visual. |

| La Plata | | | 1890 | |

+-------------+-----------+------------+-------------+------------------+

| Lowell | | | Brashear, | Mounted with a |

| Observatory,| 18 | 26·3 | 1894 | 12-inch Clark |

| Flagstaff, | | | | refractor as |

| Arizona | | | | counterpoise. |

+-------------+-----------+------------+-------------+------------------+

| Van der Zee | | | Fitz | Dismounted. |

| Observatory,| | | | |

| Buffalo, | 18 | -- | | |

| N.Y. | | | | |

+-------------+-----------+------------+-------------+------------------+

|Bischoffsheim| | | Henrys and | Coudé Mount. |

| Observatory,| 16·5 | 26·2 | Gautier, | Visual. |

| Nice | | | 1889 | |

+-------------+-----------+------------+-------------+------------------+

| University | | | Henrys and | Coudé Mount. |

| Observatory,| 16·5 | 29·5 | Gautier, | Visual. |

| Vienna | | | 1890 | |

+-------------+-----------+------------+-------------+------------------+

| Jesuit | | | Henrys and | Photographic. |

| Observatory,| 16·5 | 22·5 | Gautier, | Mounted with a |

| Zi-ka-Wei | | | 1897 | visual refractor |

| | | | |of equal aperture.|

+-------------+-----------+------------+-------------+------------------+

| Goodsell | | | Brashear, | |

| Observatory,| 16·2 | -- | 1891 | |

| Northfield, | | | | |

| Minnesota. | | | | |

+-------------+-----------+------------+-------------+------------------+

| Warner | | | A. Clark and| |

| Observatory,| 16 | 22 | Sons, 1891 | |

| Rochester, | | | | |

| N.Y. | | | | |

+-------------+-----------+------------+-------------+------------------+

| Grand-Ducal | | | Brashear and| A twin

| Observatory,| 16 | 6·6 | Grubb, 1900 | photographic |

| Königsstuhl,| | | |doublet. The gift |

| Heidelberg | | | | of Miss Bruce. |

| | | | | Mounted with a |

| | | | | visual 10-inch |

| | | | | refractor by |

| | | | | Pauly. |

+-------------+-----------+------------+-------------+------------------+

| Meudon | | | | |

| Observatory | 15·7 | 5·3 | | |

+-------------+-----------+------------+-------------+------------------+

| Washburn | | | A. Clark and| |

| Observatory,| 15·6 | 20·3 | Sons, 1879 | |

| Wisconsin | | | | |

+-------------+-----------+------------+-------------+------------------+

| Teramo | | | T. Cooke and| Formerly the |

| Observatory,| 15·5 | -- | Sons, 1885 | property of |

| Italy | | | | Mr. Wigglesworth.|

+-------------+-----------+------------+-------------+------------------+

| Royal | | | T. Grubb, | Presented by |

| Observatory,| 15·1 | -- | 1872 | Lord Crawford. |

| Edinburgh | | | | |

+-------------+-----------+------------+-------------+------------------+

| Madrid | | | Merz | |

| Observatory | 15 | -- | | |

+-------------+-----------+------------+-------------+------------------+

| Tulse Hill | | |Sir H. Grubb,| Lent by the |

| Observatory | 15 | 15 | 1870 | Royal Society to |

| | | | | Sir William |

| | | | | Huggins. Mounted |

| | | | | with an 18-inch |

| | | | | Cassegrain |

| | | | | reflector. |

+-------------+-----------+------------+-------------+------------------+

| National | | | Lerebours | |

| Observatory,| 15 | 29 | | |

| Paris | | | | |

+-------------+-----------+------------+-------------+------------------+

| Harvard | | | Merz, 1847 | |

| College | 15 | 22 | | |

| Observatory | | | | |

+-------------+-----------+------------+-------------+------------------+

| National | | | | |

| Observatory,| 15 | -- | | |

| Rio de | | | | |

| Janeiro | | | | |

+-------------+-----------+------------+-------------+------------------+

| Tacubaya | | |Sir H. Grubb,| |

| Observatory,| 15 | 15 | 1880 | |

| Mexico | | | | |

+-------------+-----------+------------+-------------+------------------+

| Stonyhurst | | |Sir H. Grubb,| |

| College | 15 | 15 | 1893 | |

| Observatory | | | | |

+-------------+-----------+------------+-------------+------------------+

| Brera | | | | |

| Observatory,| 15 | -- | | |

| Milan | | | | |

+-------------+-----------+------------+-------------+------------------+

| University | | |Sir H. Grubb,| Visual. |

| of | 15 | 15 | 1893 | Mounted with a |

| Mississippi | | | | photographic |

| | | | | 9-inch refractor.|

+-------------+-----------+------------+-------------+------------------+

| Imperial | | | Merz and | |

| Observatory,| 15 | 22·5 | Mahler, 1840| |

| Pulkowa | | | | |

+-------------+-----------+------------+-------------+------------------+

| Maidenhead | | |Sir H. Grubb,| The property of |

| Observatory | 15 | -- | 1893 | Mr. Dunn. |

| | | | | Mounted with a |

| | | | | twin photographic|

| | | | | reflector. |

+-------------+-----------+------------+-------------+------------------+

| Odessa | | | Merz, 1881 | |

| Observatory | 14·9 | -- | | |

+-------------+-----------+------------+-------------+------------------+

|Bischoffsheim| | | Henrys and | |

| Observatory,| 14·9 | 23 | Gautier | |

| Nice | | | | |

+-------------+-----------+------------+-------------+------------------+

| Brussels | | | Merz and | |

| Observatory | 14·9 | 20 | Cooke, 1877 | |

+-------------+-----------+------------+-------------+------------------+

| Observatory | | | Merz and | |

| of Bordeaux | 14·9 | 22·4 |Gautier, 1880| |

+-------------+-----------+------------+-------------+------------------+

| Observatory | | | Merz and | |

| of Lisbon | 14·9 | -- | Mahler | |

+-------------+-----------+------------+-------------+------------------+

TABLE VI.

List of Observatories employed in the Construction of the Photographic

Chart and Catalogue of the Heavens.

All are provided with 13-inch photographic, coupled with 11-inch visual

refractors:

----------------------------------------------------------

| Name of Observatory. | Constructors of Instruments.|

| |-------------------------------|

| | Optical Part.|Mechanical Part.|

|------------------------|--------------|----------------|

|Paris | Henrys | Gautier |

|Algiers | " | " |

|Bordeaux | " | " |

|Toulouse | " | " |

|San Fernando (Spain) | " | " |

|Vatican | " | " |

|Cordoba | " | " |

|Montevideo | " | " |

|Perth, Western Australia| " | " |

|Helsingfors | " | Repsold |

|Potsdam | Steinheil | " |

|Catania | " | Salmoiraghi |

|Greenwich | Sir H. Grubb | Sir H. Grubb |

|Oxford | " | " |

|The Cape | " | " |

|Melbourne | " | " |

|Sydney | " | " |

|Tacubaya (Mexico) | " | " |

----------------------------------------------------------

INDEX

Abbe, Cleveland, corona of 1878, 176, 177

Aberdour, Lord, solar chromosphere, 68

Aberration, discovered by Bradley, 3, 15;

cause of, 31, 231;

investigations of, 241, 438

Abney, daylight coronal photographs, 179;

infra-red photography, 210, 223, 438

Absorption, terrestrial atmospheric, 134, 211, 214-216, 225;

solar, 134-136, 172, 213, 221, 222, 225, 277;

correlative with emission, 135, 136, 140

Adams, discovery of Neptune, 79-82;

lunar acceleration, 271;

orbit of November meteors, 331

Aerolites, falls of, 339, 340

Airy, solar translation, 39;

observations during eclipses, 62, 64, 70;

Astronomer-Royal, 79;

search for Neptune, 80, 81;

corona of 1851, 175;

solar parallax, 227, 236;

transit of Venus, 233;

Mercurian halo, 235;

lunar atmosphere, 264

Aitken, double star discoveries, 419

Albedo, of Mercury, 246;

of Venus, 255;

of Mars, 283;

of minor planets, 288;

of Jupiter, 290;

of Saturn, 303;

of Uranus, 304

Alexander, spiral nebulæ, 118;

observation during eclipse, 245

Algol, variability of light, 10, 390;

eclipses, 390;

nature of system, 391

Altitude and azimuth instrument, 120 _note_, 121

Amici, comet of 1843, 103

Anderson, discovery of Nova Aurigæ, 396;

of Nova Persei, 400

Andrews, conditions of liquefaction, 151

Ångström, C. J., _Optical Researches_, 138;

spark spectrum, 139;

nature of photosphere, 152;

solar spectroscopy, 210, 212;

hydrogen in sun, 211;

temperature of stars, 375

Ångström, K., infra-red solar spectrum, 210;

solar constant, 225

Arago, eclipse of 1842, 62, 64, 65;

prominences, 69;

polarization in comets, 103;

magnetic relations of auroræ, 130;

nature of photosphere, 151;

meteor-systems, 329

Arai, photographs of corona of 1887, 185

Arcturus, spectrum, 373, 383;

radial movement, 387

Argelander, Bonn Durchmusterung, 32, 423;

solar motion, 39;

centre of Milky Way, 40;

comet of 1811, 100

Aristotle, description of a comet, 350

Arrhenius, light-pressure theory of comets, 348

Asten, movements of Encke's comet, 94

Asteroids, so designated by Herschel, 75

Astronomical circles, 121, 122

Astronomical physics, 7, 141, 142

Astronomical Society founded, 6;

Herschel its first President, 14

Astronomy, classification, 1;

popularity and progress, 5;

in United States, 6;

in Germany, 28;

practical reform, 32;

of the invisible, 42;

physical, 141

Atmosphere, solar, 94, 182, 192, 221, 225;

of Venus, 236, 239, 253, 254;

of Mercury, 246-248;

of the moon, 263, 264;

of Mars, 276;

of minor planets, 288

Auroræ, periodicity, 129, 162;

excited by meteors, 335

Auwers, reduction of Bradley's observations, 39;

system of Procyon, 42;

opposition of Victoria, 238;

solar parallax, 240;

new star in Scorpio, 395

Babinet, nebular hypothesis, 314

Backlund, movements of Encke's comet, 94, 360

Baden-Powell, Sir George, eclipse expedition, 188

Bailey, nebulosity round Pleiades, 411;

stellar photometric observations, 421;

discovery of variable clusters, 436

Baily, early life and career, 59-61;

observations of eclipses, 61-64;

density of the earth, 60, 261

Baily's Beads, 61, 62

Bakhuyzen, rotation of Mars, 275

Ball, Sir Robert, parallaxes of stars, 36 _note_, 416;

contacts in transits, 239

Balmer's Law, 198, 383

Barnard, micrometrical measures of Neptune, 84;

of minor planets, 288;

of Saturn's rings, 301;

photographs of solar corona, 186, 190;

transit of Mercury, 245;

halo round Venus, 254;

surface of Mars, 280;

ellipticity of Jupiter's first satellite, 292;

of Uranus, 304;

discovery of inner Jovian satellite, 293, 434;

red spot on Jupiter, 296;

eclipse of Japetus, 300;

attendants on comet of 1882, 363;

on Brooks's comet, 366, 367;

Swift's comet, 368;

photographic discovery of a comet, 369;

observations of Nova Aurigæ, 398, 399;

Hind's variable nebula, 403;

exterior Pleiades nebulosities, 411;

galactic stars, 423;

photographs of Milky Way, 424, 425;

cluster variables, 433;

horizontal telescope, 438

Bartlett, photograph of a partial eclipse, 166

Basic lines, 206, 207

Baxendell, meteors of 1866, 331

Becker, drawings of solar spectrum, 211

Beckett, Sir E. (Lord Grimthorpe), value of solar parallax, 232

Beer and Mädler, surveys of lunar surface, 265, 267;

studies of Mars, 275

Bélopolsky, coronal photographs, 185;

theory of corona, 191;

rotation of Venus, 252;

of Jupiter, 297;

spectroscopic determinations of Saturn's rings, 300;

spectrum of Gamma Cassiopeiæ, 378;

system of Castor, 389, 391;

detection of variable stars as spectroscopic binaries, 391

Berberich, mass of asteroids, 287;

orbit of Holmes's comet, 337

Berkowski, daguerrotype of eclipsed sun, 166

Bessel, biographical sketch, 28-30;

reduction of Bradley's observations, 32;

parallax of 61 Cygni, 36;

disturbed motion of Sirius and Procyon, 41;

trans-Uranian planet, 79;

Halley's comet, 102;

theory of instrumental errors, 122;

personal equation, 123;

rotation of Mercury, 246;

lunar atmosphere, 263;

cometary emanations, 325, 345;

multiple tails, 347;

comet of 1807, 352

Betelgeux, remoteness, 37, 417;

spectrum, 373, 381, 383, 384;

radial movement, 387

Bianchini, rotation of Venus, 250

Biela, discovery of a comet, 95

Bigelow, magnetic and solar disturbances, 161;

theory of corona, 191

Bigourdan, eclipse of 1893, 187;

velocity of comet of 1882, 364

Bird's quadrants, 4, 112, 121

Birmingham, colours of stars, 375 _note_;

discovery of T Coronæ, 393

Birt, rotation of a sun-spot, 144;

Selenographical Society, 266

Bischoffsheim, Coudé telescope, 436

Black Ligament, 235

Bode, popular writings, 5;

solar constitution, 57;

missing planet, 72, 73

Bode's Law, 71, 83, 286

Boeddicker, heat-phases during lunar eclipses, 269, 270;

drawings of Jupiter, 296;

of the Milky Way, 424

Boehm, solar observations, 146, 148

Boguslawski, centre of sidereal revolutions, 41;

observation of Halley's comet, 102

Bolometer, principle of construction, 222

Bond, G. P., his father's successor, 86;

light of Jupiter, 289;

Saturn's rings, 298;

Donati's comet, 324, 325;

Andromeda nebula, 409;

double-star photography, 409

Bond, W. C., observation of Neptune's satellite, 84;

discovery of Hyperion, 85;

of Saturn's dusky ring, 86;

resolution of nebulæ, 119;

celestial photography, 153, 409;

satellite-transit on Jupiter, 291

Borda, repeating circle, 121

Boss, solar translation, 40;

observations on comets, 352, 356

Bossert, proper motions of stars, 415

Bouguer, solar atmospheric absorption, 221

Boulliaud, period of Mira, 10

Bouvard, tables of Uranus, 78;

Encke's comet, 90

Boys, radio-micrometer, 220;

density of the earth, 261

Bradley, discoveries of aberration and nutation, 3;

solar translation, 10;

star-distances, 10, 16;

observation

on Castor, 17;

instruments, 28, 120;

observations reduced by Bessel and Auwers, 32, 39

Brahe, Tycho, star of 1572, 24

Brandes, observations of meteors, 327, 334;

Braun, prominence photography, 197;

density of the earth, 261

Brayley, meteoric origin of planets, 311

Brédikhine, theory of cometary appendages, 100, 348;

repulsive forces, 346, 347;

chemical differences, 347, 348;

formative types, 351, 352, 355, 363, 369;

structure of chromosphere, 199;

red spot on Jupiter, 294;

Andromede meteors, 337;

stationary radiants, 341;

spectrum of Coggia's comet, 343

Bremiker, star maps, 81

Brenner, rotation of Venus, 252

Brester, _Théorie du Soleil_, 152

Brewster, diffraction theory of corona, 67;

telluric lines in solar spectrum, 134;

absorption spectra, 136

Brinkley, ostensible stellar parallaxes, 33

Brisbane, establishment of Paramatta Observatory, 6, 90

Brooks, fragment of 1882 comet, 363;

cometary discoveries, 365, 366

Brünnow, stellar parallaxes, 113, 416

Bruno, Giordano, motion of stars, 9

Buffham, rotation of Uranus, 303

Buffon, internal heat of Jupiter, 289

Bunsen, discovery of spectrum analysis, 132

Burchell, magnitude of Eta Carinæ, 48

Burnham, stellar orbits, 46;

coronal photographs, 186;

measures of Nova Aurigæ, 399;

of planetary nebulæ, 404;

discoveries of double stars, 418, 430, 433, 435;

catalogue, 419;

system of 61 Cygni, 419

Burton, canals of Mars, 279;

rotation of Jupiter's satellites, 292

Calandrelli, stellar parallaxes, 33

Callandreau, capture theory of comets, 98

Campani, Saturn's dusky ring, 86

Campbell, Lieutenant, polarisation of corona, 170

Campbell, Professor, stellar radial velocities, 39, 406, 434;

flash spectrum, 189;

spectroscopic observations of Saturn's rings, 300;

Wolf-Rayet stars, 380;

spectroscopic binaries, 389;

Nova Aurigæ, 398;

translation of solar system, 406;

stellar diffraction-spectra, 440

Canals of Mars, 278-280

Cannon, Miss A. J., spectrographic researches, 386

Canopus, remoteness, 37, 417;

spectrum, 416

Capella, spectrum, 373, 383, 384;

a spectroscopic binary, 389

Carbon, material of photosphere, 152;

absorption by, in sun, 212;

in stars, 374

Carbonelle, origin of meteorites, 340

Carinæ, Eta, light variation, 48, 49;

spectrum, 379

Carrington, astronomical career, 144, 145;

sun-spot observations, 146;

solar rotation, 147;

spot-distribution, 148;

luminous outburst on sun, 159, 160;

Jovian and sun-spot periods, 163;

origin of comets, 370

Cassini, Domenico, discoveries of Saturnian satellites, 84;

of division in ring, 85;

solar rotation period, 146;

solar parallax, 228;

rotation of Venus, 250;

of Mars, 274;

of Jupiter, 290, 295;

satellite of Venus, 256;

satellite-transit on Jupiter, 291

Cassini, J. J., stellar proper motions, 10;

sun-spots on limb, 54;

theory of corona, 66;

rotation of Venus, 250;

structure of Saturn's rings, 299

Castor, system of, 18, 389

Cavendish experiment, 60, 261

Ceres, discovery, 73, 74;

diameter, 75, 288

Chacornac, observation of sun-spot, 156;

star-maps, 284, 413;

variable nebula, 403

Challis, search for Neptune, 81, 82;

duplication of Biela's comet, 96

Charlois, discoveries of minor planets, 283

Charroppin, coronal photographs, 186

Chase, photographic discovery of a comet, 338;

stellar parallaxes, 416

Chladni, origin of meteors, 327, 332

Christie, Mercurian halo, 245

Chromosphere, early indications, 68;

distinct recognition, 69, 70, 167;

depth, 174, 175, 200;

metallic injections, 195;

eruptive character, 199;

spectrum, 200

Clark, Alvan, large refractors, 114, 429, 430, 436

Clark, Alvan G., discovery of Sirian companion, 42, 430;

40-inch refractor, 433

Clarke, Colonel, figure of the earth, 262

Clarke, F. W., celestial dissociation, 206

Clausen, period of 1843 comet, 105;

cometary systems, 362

Clerihew, secondary tail of 1843 comet, 103

Clusters, variable stars in, 436

Coggia, discovery of a comet, 343

Comet, Halley's, return in 1759, 4, 88;

orbit computed by Bessel, 29;

capture by Neptune, 98, 365;

return in 1835, 101-103, 345;

type of tail, 346, 352;

of 1843, 7, 103-105;

type of tail, 346, 352;

relationships, 349-351;

Newton's, 88, 364;

Encke's, 90;

changes of volume, 92;

of brightness, 95;

acceleration, 93, 94;

capture by Mercury, 99;

Winnecke's, 94, 342;

Biela's, 95-97, 333;

Brorsen's, 97;

Vico's, 97, 367;

Faye's, 98;

of 1811, 99-101, 346;

of 1807, 100, 347, 352;

of 1819, 101, 103;

Lexell's, 106, 367;

Tewfik, 178, 358, 362, 369;

Donati's, 323-325, 347, 348;

of 1861, 326, 327, 346;

Perseid, 327, 332;

Leonid, 327, 332, 333, 343;

Klinkerfues's, 335;

Holmes's, 337, 343, 369;

Coggia's, 343, 346, 347;

of 1901, 343;

of 1880, 349, 351;

Aristotle's, 350;

Tebbutt's, 352-355;

Schaeberle's, 355, 356;

Wells's, 356, 357;

of September, 1882, 358-361, 362-364;

Thome's, 361;

Pons-Brooks, 365, 366;

Sawerthal's, 366;

Brooks's, of 1889, 366, 367;

Swift's, 368

Cometary tails, repulsive action upon, 100, 103, 104, 346-348;

coruscations in, 105;

three types, 346-348, 355, 363;

multiple, 347, 348, 351, 352, 355, 363, 368

Comets, subject to gravitation, 88;

of short period, 92, 93;

translucency, 95, 105, 106, 353;

small masses, 96, 106;

capture by planets, 98, 306, 367;

changes of volume, 102, 365, 369;

polarisation of light, 103, 354, 355;

refractive inertness, 106, 353;

relations to meteor-systems, 327, 332-336;

disintegration, 333, 339, 362, 363;

spectra, 342-344, 354, 355, 362-364;

luminous by electricity, 344, 355, 357;

systems, 353, 355, 357, 362, 365;

origin, 369-371

Common, reflectors for eclipse photography, 187;

Jupiter's inner satellite, 293;

detection of great comet near the sun, 358;

its five nuclei, 362;

photographs of Andromeda nebula, 395;

of Orion nebula and Jupiter, 407, 408;

great reflectors, 412, 429;

cluster variables, 436

Common, Miss, drawing of eclipsed sun, 187

Comstock, lunar atmosphere, 264

Comte, celestial chemistry, 140;

astronomy, 142

Cooke, 25-inch refractor, 430

Copeland, comets of 1843 and 1880, 349;

spectrum of comet of 1882, 364;

of Gamma Cassiopeiæ, 378;

of Nova Andromedæ, 395; of Orion nebula, 407;

discoveries of gaseous stars, 379;

Nova Aurigæ, 396, 398

Copernicus, stellar parallax, 16

Cornu, telluric lines in solar spectrum, 202;

velocities in prominences, 205;

ultra-violet solar spectrum, 210, 215;

velocity of light, 232 _note_, 241;

spectrum of hydrogen, 383;

of Nova Cygni, 393

Cornu and Bailie, density of the earth, 261