supply company. He is known also as a very blunt exponent of his carefully arrived at conclusions. In all new departures the public has to be educated by interested parties, which renders it necessary that as occasion requires the more sensational statements should be toned down somewhat by those who view the question more dispassionately. If those interested in supply companies will take as much trouble to educate possible customers to use power as well as light they will certainly make their paths, paths of greater pleasantness, and bring about that happy time when their machinery can be used to the greatest advantage-when they can obtain the greatest output at the least cost per unit, and hence when the commercial value of electrical apparatus can be fully exploited.

STILL THE B. A.

Mr. Walker's criticisms of the British Association are in some cases a little too severe. Surely it cannot be expected that the evening lectures, for example, should partake of the character of scientific "pap." It serves to show how very varied are the opinions regarding the association's work, for a goodly number of critics have looked upon these evening lectures as the redeeming feature at each successive meeting. As to Prof. Rücker's lecture this year, we think that the general consensus of opinion is altogether favourable. Commencing with simple optical knowledge, gradually, as it were, link by link extending the width of knowledge, experimentally showing step by step the similarity of optical and electrical phenomena, Prof. Rücker carried his audience through the whole lecture without verbal redundancy or aught but the simplest language, so clearly delivered that no one starting with the flimsiest fundamental notions could fail to follow all that was said and to comprehend the beauty of the analogies experimentally brought forward. We should never fear for the future of an association that depended for preservation upon such lectures. There would be no falling off in numbers nor in interest-rather the contrary. Prof. Rücker's lecture was worth going to Cardiff and becoming a member of the association in order to hear. Those who attend these lectures are expected to have obtained a sufficient groundwork of scientific knowledge upon which the lecturer can build. They should not be learners in the sense of starting with absolute ignorance of the subject, but should be learners in the sense of having the necessary fundamental knowledge to follow the lecturer into some special path in which few have previously trod, and which till then is more or less hidden from the multitude.

We have no objection to criticise freely the sectional arrangements, or the monetary arrangements, but we could not, if we would, find one word of fault or of disparagement to a lecture such as that of Prof. Rücker's. A condemnation may be too sweeping to be effective, and we imagine Mr. Walker will after further consideration agree that the British Associa

tion cannot be expected to ask its lecturers to confine themselves to elementary subjects solely to benefit those still in the wilderness, nor, again, must it be expected that the presidential address be such as to be understanded of the multitude. This address is either a record of general progress, of progress in some one science, or the epitomised work of a lifetime. Few have trod the whole path delineated, and the address marks at the moment the uttermost point which has been reached. History starts afresh from this point, and so the record grows. Who would have it otherwise?

HELMHOLTZ'S BIRTHDAY.

On August 31 Prof. Helmholtz celebrated his seventieth birthday at Madonna de Campiglio, where he is spending the holidays.

Campiglio is an old monastery converted into an hotel, and stands at a height of 6,000ft. in the mountains of the Austrian Tyrol. It is accessible by a 13 hours' drive from Trient on the one side, and by a mule track over the Mendola Pass on the other, and commands views of the Bocca di Brenta and other famous mountains. There is no village, and the hotel stands miles from any other habitation. Prof. Helmholtz is accompanied by Madame Helmholtz, by his sister (Baroness Schmidt Zabirow) and her husband (his Excellency Baron Schmidt Zabirow), and by his daughter (Mrs. Werner Siemens) and her husband (Mr. Werner Siemens, jun.)

At an early hour an illuminated address of congratulation, prepared by Herr Josef Hofer, the wellknown Munich artist, who is also staying at Campiglio, and signed by all the visitors, was presented, and subsequently the visitors called on Prof. and Madame Helmholtz to offer their congratulations. Prof. Helmholtz's rooms were profusely decorated with flowers. Among those who called were Prince and Princess Molfetta, Prof. Dr. von Bayer (the well-known chemist), La Duchesse di Melzi, and her daughter, the Comtesse di Melzi, Herr Josef Hofer (Munich), Herr von Dechy (Budapest), Herr von Gündel (Hamburg), Mr. and Mrs. J. E. H. Gordon (London), Canon and Mrs. Harvey (Lincoln), Mr. and Mrs. Jennett Brown

(Meran).

Later in the day a dinner was given to Prof. Helmholtz, when his health was drunk with great enthusiasm, while the peasants' band played under the windows. Prof. Helmholtz is in the best of health and spirits, and looks nearer 60 than 70.

CORRESPONDENCE.

"One man's word is no man's word
Justice needs that both be heard.'

THE WEYMERSCH BATTERY. SIR, While thanking you for your kind notice of our battery, may I point out one inaccuracy which might possibly be misleading. You say "the battery gives a steady current of 20 amperes at 20 volts for 20 hours, the cost being given by the syndicate as just over a shilling per unit." Now, the cost per unit will necessarily fluctuate with the price of zinc, and of the materials necessary for producing the depolarising fluid. But at the present price of the above articles the cost per unit is between 1s. 5d. and

join in the game? Does it mean keeping the achievements of science also within this charmed circle? Would it not be doing more real good, advancing the interests of science more substantially if outsiders-those who pay the billwere allowed to listen to the tune. Surely it must be apparent that the object of discovery is not to be boxed up so that only a few can know of its existence, but to be spread broadcast, to bear fruit. Every scientific fact that a man or woman learns, and can apply intelligently, adds to the wealth of the world in some form or other; and the true missionary of science is he who proclaims his message in language that can be understood by everyone.

But there is also another and a very serious complaint urged against the association, and with good reason. Practical men say that its discussions and its papers are merely playing at science. They rarely advance science at all, because the papers are usually only half read, and never properly discussed. From seven to twenty papers are down to be read and discussed in the time required for one, consequently the first is sometimes read, never properly discussed, the second half read, and the rest lost. Why cannot these matters be arranged in a proper manner? Why should authors be asked to go to the trouble of preparing papers and diagrams, bringing up elaborate apparatus, and then to be met by the insulting request to read their papers to an audience of, say, three, or, if they do read them before a fairly large audience, to be constantly interrupted by reminders from the chairman that time is short?

How, also, is it possible to intelligently discuss a paper which you hear for the first time, and then perhaps in a garbled form, owing to the constant interruptions? Why cannot the example of the leading engineering institutes be followed-only as many papers be accepted as can be read and properly discussed, these papers carefully examined before being accepted and printed in advance, so that all who are interested in the subject can have an opportunity of studying the paper? Surely one paper given under these conditions would be worth a hundred given under the present arrangements.

Also, is it not possible to avoid the tactics of the election platform in discussions which take place on papers presented to the British Association? A smart personal sally may cause a laugh, but it is hardly conducive to the advancement of science, and is very undignified. Surely it should be within the power of the presidents of the sections to see that their discussions do not degenerate into personal squabbles, as unfortunately is too often the case at present. Mr. A differs from something stated in Mr. B's forthwith Mr. C states that Mr. A knows nothing of the subject. Surely it would not only be more dignified, but more effective, for Mr. C to point out where Mr. A is wrong. Also, would it not be best that there should be no favouritism shown in the selection and order of taking of the papers? Certainly give eminence the first place, but not friendship.

paper,

Further, are not three hours daily as much as human head and human eyes can endure, at any rate in one subject? Why not let some sections meet from ten till one, and others from two till five? The mornings or afternoons that were not engaged could well be devoted to the inspection of local works, etc. It often happens that a member is interested in two or more sections. The different branches of science all overlap each other, and one can never tell when one is going to receive a hint that will be useful in one's own branch, from something which appears before another branch. Geology, for instance, bears an important relation to "earth." It would be very convenient to attend, say, Section G in the morning and some other section in the afternoon; or, if there happened to be nothing on of interest in the afternoon, to go over some works or other object of interest in the neighbourhood. Possibly other members may offer other suggestions. At any rate, the writer hopes that there may be less cause for dissatisfaction at the Edinburgh meeting on the points he has named than there has been hitherto, and that the British Association, conforming to the altered conditions in which it finds. itself, may become once more a body of true missionaries, bringing home the teachings of science, not only to the cultured few, but to everybody within its reach.-Yours, etc., SYDNEY F. WALKER,.

THE GLASGOW ELECTRICAL ENGINEER.

As in the days to come local authorities will not only have their ordinary engineers, but also a specialist as electrical engineer answering somewhat in position to that of gas engineer, or water engineer, it may be as well to watch the One of the first gentlemen who secure these positions. appointments is that at Glasgow.

At the last meeting of the Town Council of Glasgow a recommendation from the Gas and Electric Lighting Committee to appoint Mr. William Arnot, of London, as the electrical engineer to the Corporation was approved of. There were about 50 applications for the appointment, the candidates hailing from all parts of the kingdom. After the first examination of the applications a list of 18 of the most likely men was made out, then a short list of six was selected, and each of these candidates had a personal interview with the subcommittee on electric lighting, after which it was unanimously agreed to recommend the appointment of the gentleman named above. Mr. Arnot, who is an associated member of the Institution of Civil Engineers, and a full member of the Institution of Electrical Engineers, is 36 years of age, and has an excellent "record," his practical experience in connection with the electric lighting industry being alike varied and extensive, and his training in theory and practice in the university and workshop having been very thorough. He had sole charge of the erection and maintenance of the Brixton central lighting station, in which both arc and incandescent plants were put down, and in which both overhead and underground circuits were run. He ran the mains for Plymouth Pier-arc and incandescent lamps, and laid several large underground mains, including those for Buckingelectrical department of the Silvertown Works, and during that ham Palace. For over four years Mr. Arnot was employed in the time, in addition to the usual electrical installation work, he superintended the laying down of all the plant for the Cannonstreet central lighting station. In this case the plant embraced Babcock and Wilcox boilers and direct-driven engines, and dynamos for 1,000 lights, together with switchboards and fuseboards, which were of his own design. While in that employindeed, he has had a large amount of experience in estimating ment he was also engaged in the estimating for electrical plants; for all classes of work, and has put down a number of installations with gas engines and secondary batteries. His experience also includes the fitting up electrically of a number of her Majesty's warships. For some time Mr. Arnot had the management of the engineering department

of Messrs. Latimer Clark, Muirhead, and Co.'s works at Millwall, where he constructed a large number of dynamos, arc lamps, and other machinery. Then, again, it may be mentioned that he had for a length of time charge of the hightension electric lighting station at the Grosvenor Gallery, when the directors decided to adopt Mr. Ferranti's system and engaged his services. Several years ago Mr. Arnot did some excellent work in connection with the laying of electric lighting Waterproof Company, Limited, from whose manager he holds mains in the country for Callender's Bitumen, Telegraph, and a high-class testimonial. During the past 18 months he has been employed as the electrical inspector for the London County Council, and his work in that capacity at the meter-testing station is spoken of in a very generous manner by Prof. Silvanus Thompson, who is the consulting electrical engineer to the Highways Committee of the County Council. After an acquaintanceship extending over six years, Mr. Ferranti likewise bears most excellent testimony in favour of Mr. Arnot, whom we must, in closing this notice, heartily congratulate on receiving the first appointment of the kind in Scotland, and in a city where he may expect to have Sir William Thomson as 'guide, philosopher, and friend."

66

THE BRITISH ASSOCIATION AT CARDIFF. PRESIDENTIAL ADDRESS BY WILLIAM HUGGINS, Esq. (Concluded from page 232.)

The spectroscopic method of determining celestial motions in the line of sight has recently become fruitful in a new but not altogether unforeseen direction, for it has, so to speak, given us a separating power far beyond that of any telescope the glassmaker and the optician could construct, and so enabled us to penetrate into mysteries hidden in stars apparently single, and altogether unsuspected of being binary systems. The spectroscope has not simply added to the list of the known binary stars, but has given to us for the first time a knowledge of a new class of stellar systems, in which the components are in some cases of nearly equal magnitude, and in close proximity, and are revolving with velocities greatly exceeding the planetary velocities of our system. The K line in the photographs of Mizar, taken at the Harvard College Observatory, was found to be double at intervals of 52 days. The spectrum was therefore not due to a single source of light, but to the combined effect of two stars moving periodically in opposite directions in the line of sight. It is obvious that if

two stars revolve round their common centre of gravity in a plane not perpendicular to the line of sight, all the lines in a spectrum common to the two stars will appear alternately single or double.

In the case of Mizar and the other stars to be mentioned, the spectroscopic observations are not as yet extended enough to furnish more than an approximate determination of the elements of their orbits.

Mizar especially, on account of its relatively long period-about 105 days-needs further observations. The two stars are moving each with a velocity of about 50 miles a second, probably in elliptical orbits, and are about 143 millions of miles apart. The stars of about equal brightness have together a mass of about 40 times as great as that of our sun.

A similar doubling of the lines showed itself in the Harvard photographs of 8 Auriga at the remarkably close interval of almost exactly two days, indicating a period of revolution of about four days. According to Vogel's later observations, each star has a velocity of nearly 70 miles a second, the distance between the stars being little more than 7 millions of miles, and the mass of the system 4.7 times that of the sun. The system is approaching us at the speed of about 16 miles a second. The telescope could never have revealed to us double stars of this order. In the case of 8 Auriga, combining Vogel's distance with Pritchard's recent determination of the star's parallax, the greatest angular separation of the stars as seen from the earth would be 1-200th part of a second of arc, and therefore very far too small for the highest powers of the largest telescopes. If we take the relation of aperture to separating power usually accepted, an object glass of about 80ft. in diameter would be needed to resolve this binary star. The spectroscope, which takes no note of distance, magnifies, so to speak, this minute angular separation 4,000 times; in other words, the doubling of the lines, which is the phenomenon that we have to observe, amounts to the easily measurable quantity of 20 seconds of arc.

There were known, indeed, variable stars of short period, which it had been suggested might be explained on the hypothesis of a dark body revolving about a bright sun in a few days, but this theory was met by the objection that no such systems of closely revolving suns were known to exist.

The Harvard photographs, of which we have been speaking, were taken with a slitless form of spectroscope, the prisms being placed, as originally by Fraunhofer, before the object-glass of the telescope. This method, though it possesses some advantages, has the serious drawback of not permitting a direct comparison of the star's spectrum with terrestrial spectra. It is obviously unsuited to a variable star like Algol, where one star only is bright, for in such a case there would be no doubling of the lines, but only a small shift to and fro of the lines of the bright star as it moved in its orbit alternately towards and from our system, which would need for its detection the fiducial positions of terrestrial lines compared directly with them.

For such observations the Potsdam spectrograph was well adapted. Prof. Vogel found that the bright star of Algol did pulsate backwards and forwards in the visual direction in a period corresponding to the known variation of its light. The explanation which had been suggested for the star's variability-that it was partially eclipsed at regular intervals of 688 hours by a dark companion large enough to cut off nearly five-sixths of its lightwas therefore the true one. The dark companion, no longer able to hide itself by its obscureness, was brought out into the light of direct observation by means of its gravitational effects.

Seventeen hours before minimum Algol is receding at the rate of about 241⁄2 miles a second, while 17 hours after minimum it is found to be approaching with a speed of about 281⁄2 miles. From these data, together with those of the variation of its light, Vogel found, on the assumption that both stars have the same density, that the companion, nearly as large as the sun, but with about one-fourth his mass, revolves with a velocity of about 55 miles a second. The bright star of about twice the size and mass moves about the common centre of gravity with the speed of about 26 miles a second. The system of the two stars, which are about 3 millions of miles apart, considered as a whole, is approaching us with a velocity of 24 miles a second. The great difference in luminosity of the stars-not less than 50 times-suggests rather that they are in different stages of condensation, and dissimilar in density.

It is obvious that if the orbit of a star with an obscure companion is inclined to the line of sight, the companion will pass above or below the bright star and produce no variation of its light. Such systems may be numerous in the heavens. In Vogel's photographs, Spica, which is not variable, by a small shifting of its lines reveals a backward and forward periodical pulsation due to orbital motion. As the pair whirl round their common centre of gravity, the bright star is sometimes advancing, at others receding. They revolve in about four days, each star moving with a velocity of about 56 miles a second in an orbit probably nearly circular, and possess a combined mass of rather more than 2 times that of the sun. Taking the most probable value for the star's parallax, the greatest angular separation of the stars would be far too small to be detected with the most powerful telescopes.

If in a close double star the fainter companion is of the whitestar type, while the bright star is solar in character, the composite spectrum would be solar with the hydrogen lines unusually strong. Such a spectrum would in itself afford some probability of a double origin, and suggest the existence of a companion star.

In the case of a true binary star the orbital motions of the pair would reveal themselves in a small periodical swaying of the hydrogen lines relatively to the solar ones.

Prof. Pickering considers that his photographs show 10 stars with composite spectra; of these, five are known to be double.

The others are: 7 Persei, Auriga, & Sagittarii, 31 Ceti, and s Capricorni. Perhaps ẞ Lyræ should be added to this list.

In his recent classical work on the rotation of the sun, Dunér not only determined the solar rotation for the equator, but for different parallels of latitude up to 75deg. The close accordance of his results shows that these observations are sufficiently accurate to be discussed with the variation of the solar rotation for different latitudes, which had been determined by the older astronomical methods from the observations of the solar spots.

Though I have already spoken incidentally of the invaluable aid which is furnished by photography in some of the applications of the spectroscope to the heavenly bodies, the new power which modern photography has put into the hands of the astronomer is so great, and has led already, within the last few years, to new acquisitions of knowledge of such vast importance, that it is fitting that a few sentences should be specially devoted to this subject. Photography is no new discovery, being about half a century old. It may excite surprise, and, indeed, possibly suggest some apathy on the part of astronomers, that though the suggestion of the application of photography to the heavenly bodies dates from the memorable occasion when, in 1839, Arago, announcing to the Académie des Sciences the great discovery of Niepce and Daguerre, spoke of the possibility of taking pictures of the sun and moon by the new process, yet that it is only within a few years that notable advances in astronomical methods and discovery have been made by its aid.

The explanation is to be found in the comparative unsuitability of the earlier photographic methods for use in the observatory. In justice to the early workers in astronomical photography, among whom Bond, De la Rue, J. W. Draper, Rutherfurd, Gould, hold a foremost place, it is needful to state clearly that the recent great successes in astronomical photography are not due to greater skill nor, to any great extent, to superior instruments, but to the very great advantages which the modern gelatine dry plate possesses for use in the observatory over the methods of Daguerre, and even over the wet collodion film on glass which, though a great advance on the silver plate, went but a little way towards putting into the hands of the astronomer a photographic surface adapted fully to his wants.

The modern silver-bromide gelatine plate, except for its grained texture, meets the needs of the astronomer at all points. It possesses extreme sensitiveness; it is always ready for use; it can be placed in any position; it can be exposed for hours; lastly, it does not need immediate development, and for this reason can be exposed again to the same object on succeeding nights, so as to make up by several instalments, as the weather may permit, the total time of exposure which is deemed necessary.

Without the assistance of photography, however greatly the resources of genius might overcome the optical and mechanical difficulties of constructing large telescopes, the astronomer would have to depend in the last resource upon his eye. Now, we cannot by the force of continued looking bring into view an object too feebly luminous to be seen at the first and keenest moment of vision. But the feeblest light which falls upon the plate is not lost, but is taken in and stored up continuously. Each hour the plate gathers up 3,600 times the light energy which it received during the first second. It is by this power of accumulation that the photographic plate may be said to increase, almost without limit, though not in separating power, the optical means at the disposal of the astronomer for the discovery or the observation of faint objects.

Two principal directions may be pointed out in which photography is of great service to the astronomer. It enables him within the comparatively short time of a single exposure to secure permanently with great exactness the relative positions of hundreds or even of thousands, of stars, or the minute features of nebulæ or other objects, or the phenomena of a passing eclipse, a task which by means of the eye and hand could only be accomplished, if done at all, after a very great expenditure of time and labour. Photography puts it in the power of the astronomer to accomplish in the short span of his own life, and so enter into their fruition, great works which otherwise must have been passed on by him as an heritage of labour to succeeding generations.

The second great service which photography renders is not simply an aid to the powers the astronomer already possesses. On the contrary, the plate, by recording light-waves which are both too small and too large to excite vision in the eye, brings him into a new region of knowledge, such as the infra-red and the ultraviolet parts of the spectrum, which must have remained for ever unknown but for artificial help.

The present year will be memorable in astronomical history for the practical beginning of the Photographic Chart and Catalogue of the Heavens, which took their origin in an international conference which met in Paris in 1887, by the invitation of M. l'Amiral Mouchez, director of the Paris Observatory.

The richness in stars down to the ninth magnitude of the photographs of the comet of 1882 taken at the Cape Observatory under the superintendence of Dr. Gill, and the remarkable star charts of the brothers Henry, which followed two years later, astonished the astronomical world. The great excellence of these photographs, which was due mainly to the superiority of the gelatine plate, suggested to these astronomers a complete map of the sky, and a little later gave birth in the minds of the Paris astronomers to the grand enterprise of an International Chart of the Heavens. The actual beginning of the work this year is in no small degree due to the great energy and tact with which the director of the Paris Observatory has conducted the initial steps through the many delicate and difficult questions which have unavoidably presented themselves in an undertaking which depends upon the harmonious working in common of many nationalities, and of no fewer than 18 observatories in all parts of the world. The three

years since 1887 have not been too long for the detailed organisation of this work, which has called for several elaborate preliminary investigations on special points in which our knowledge was insufficient, and which have been ably carried out by Profs. Vogel and Bakhuyzen, Dr. Trépied, Dr. Scheiner, Dr. Gill, the Astronomer Royal, and others. Time also was required for the construction of the new and special instruments. The decision of the conference in their final form provide for the construction of a great photographic chart of the heavens with exposures corresponding to 40 minutes' exposure at Paris, which it is expected will reach down to stars of about the fourteenth mag. nitude. As each plate is to be limited to four square degrees, and as each star, to avoid possible errors, is to appear on two plates, over 22,000 photographs will be required. For the more accurate determination of the positions of the stars, a réseau with lines at distances of 5 mm. apart is to be previously impressed by a faint light upon the plate, so that the image of the réseau will appear together with the images of the stars when the plate is developed. This great work will be divided, according to their latitudes, among 18 observatories provided with similar instruments, though not necessarily constructed by the same maker. Those in the British dominions and at Tacubaya have been constructed by Sir Howard Grubb.

Besides the plates to form the great chart, a second set of plates for a catalogue is to be taken, with a shorter exposure, which will give stars to the eleventh magnitude only. These plates, by a recent decision of the permanent committee, are to be pushed on as actively as possible, though as far as may be practicable plates for the chart are to be taken concurrently. Photographing the plates for the catalogue is but the first step in this work, and only supplies the data for the elaborate measurements which have to be made, which are, however, less laborious than would be required for a similar catalogue without the aid of photography.

Already Dr. Gill has nearly brought to conclusion, with the assistance of Prof. Kapteyn, a preliminary photographic survey of the southern heavens.

With an exposure sufficiently long for the faintest stars to impress themselves upon the plate, the accumulating action still goes on for the brighter stars, producing a great enlargement of their images from optical and photographic causes. The question has occupied the attention of many astronomers whether it is possible to find a law connecting the diameters of these more or less over-exposed images with the relative brightness of the stars themselves. The answer will come out undoubtedly in the affirmative, though at present the empirical formula which have been suggested for this purpose differ from each other. Captain Abney proposes to measure the total photographic action, including density as well as size, by the obstruction which the stellar image offers to light.

A further question follows as to the relation which the photographic magnitudes of stars bear to those determined by eye. Visual magnitudes are the physiological expression of the eyes integration of that part of the star's light which extends from the red to the blue. Photographic magnitudes represent the plate's integration of another part of the star's light, namely, from a little below where the power of the eye leaves off in the blue, to where the light is cut off by the glass, or is greatly reduced by want of proper corrections when a refracting telescope is used. It is obvious that the two records are taken by different methods in dissimilar units of different parts of the star's light. In the case of certain coloured stars the photographic brightness is very different from the visual brightness; but in all stars changes, especially of a temporary character, may occur in the photographic or the visual region, unaccompanied by a similar change in the other part of the spectrum. For these reasons it would seem desirable that the two sets of magnitudes should be tabulated independently, and be regarded as supplementary of each other.

The determination of the distances of the fixed stars from the small apparent shift of their positions when viewed from widelyseparated positions of the earth in its orbit is one of the most refined operations of the observatory. The great precision with which this minute angular quantity, a fraction of a second only, has to be measured, is so delicate an operation with the ordinary micrometer-though, indeed, it was with this instrument that the classical observations of Sir Robert Ball were made- that a special instrument, in which the measures are made by moving the two halves of a divided object-glass, known as a heliometer, has been pressed into this service, and quite recently, in the skilful hands of Dr. Gill and Dr. Elkin, has largely increased our knowledge in this direction.

It is obvious that photography might be here of great service, if we could rely upon measurements of photographs of the same stars taken at suitable intervals of time. Prof. Pritchard, to whom is due the honour of having opened this path, aided by his assistants, has proved by elaborate investigations that measures for parallax may be safely made upon photographic plates, with, of course, the advantages of leisure and repetition; and he has already by this method determined the parallax for 21 stars with an accuracy not inferior to that of values previously obtained by purely astronomical methods.

The remarkable success of astronomical photography, which depend upon the plate's power of accumulation of a very feeble light acting continuously through an exposure of several hours, are worthy to be regarded as a new revelation. The first chapter opened when, in 1880, Dr. Henry Draper obtained a picture of the nebula of Orion; but a more important advance was made in 1883, when Dr. Common, by his photographs, brought to our knowledge details and extensions of this nebula hitherto unknown. A further disclosure took place in 1885, when the brothers Henry showed for the first time in great detail the spiral nebulosity issuing from the

bright star Maia of the Pleiades, and shortly afterwards nebulous streams about the other stars of this group. In 1886 Mr. Roberts, by means of a photograph, to which three hours' exposure had been given, showed the whole background of this group to be nebulous. In the following year Mr. Roberts more than doubled for us the great extension of the nebula region which surrounds the trapezium in the constellation of Orion. By his photographs of the great nebula in Andromeda, he has shown the true significance of the dark canals which have been seen by the eye. They are in reality spaces between successive rings of bright matter, which appeared nearly straight owing to the inclination in which they lie relatively to us. These bright rings surround an undefined central luminous mass. I have already spoken of this photograph. Some recent photographs by Mr. Russell show that the great rift in the Milky Way in Argus, which to the eye is void of stars, is in reality uniformly covered with them. Also quite recently Mr. George Hale has photographed the prominences by means of a grating, making use of the lines H and K.

The heavens are richly but very irregularly inwrought with stars. The brighter stars cluster into well-known groups upon a background formed of an enlacement of streams and convoluted windings and intertwined spirals of fainter stars, which becomes richer and more intricate in the irregularly rifted zone of the Milky Way.

We, who form part of the emblazonry, can only see the design distorted and confused; here crowded, there scattered, at another place superposed. The groupings due to our position are mixed up with those which are real.

Can we suppose that each luminous point has no relation to the others near it than the accidental neighbourship of grains of sand upon the shore, or of particles of the wind-blown dust of the desert? Surely every star from Sirius and Vega down to each grain of the light dust of the Milky Way has its present place in the heavenly pattern from the slow evolving of its past. We see a system of systems, for the broad features of clusters and streams and spiral windings which mark the general design are reproduced in every part. The whole is in motion, each point shifting its position by miles every second, though from the august magnitude of their distances from us and from each other, it is only by the accumulated movements of years or of generations that some small changes of relative position reveal themselves.

The deciphering of this wonderfully intricate constitution of the heavens will be undoubtedly one of the chief astronomical works of the coming century. The primary task of the sun's motion in space, together with the motions of the brighter stars, has been already put well within our reach by the spectroscopic method of the measurement of star-motions in the line of sight.

:

From other directions information is accumulating from photographs of clusters and parts of the Milky Way, by Roberts, in this country; Barnard, at the Lick Observatory, and Russell, at Sydney, from the counting of stars, and the detection of their configurations, by Holden and by Backhouse; from the mapping of the Milky Way by eye, at Parsonstown; from photographs of the spectra of stars, by Pickering, at Harvard, and in Peru; and from the exact portraiture of the heavens in the great international star chart which begins this year.

I have but touched some only of the problems of the newer side of astronomy. There are many others which would claim our attention if time permitted. The researches of the Earl of Rosse on lunar radiation, and the work on the same subject and on the sun, by Langley. Observations of lunar heat with an instrument of his own invention, by Mr. Boys; and observations of the variation of the moon's heat with its phase, by Mr. Frank Very. The discovery of the ultra violet part of the hydrogen spectrum, not in the laboratory, but from the stars. The confirmation of this spectrum by terrestrial hydrogen in part, by H. W. Vogel, and in its all but complete form by Cornu, who found similar series in the ultra-violet spectra of aluminium and thallium. The discovery of a simple formula for the hydrogen series by Balmer. The important question as to the numerical spectral relationship of different substances, especially in connection with their chemical properties; and the further question as to the origin of the harmonic and other relations between the lines and the groupings of lines of spectra ; on these points contributions during the past year have been made by Rudolf v. Kövesligethy, Ames, Hartley, Deslandres, Rydberg, Grünwald, Kayser and Runge, Johnstone Stoney, and others. The remarkable employment of interference phenomena by Prof. Michelson for the determination of the size, and distribution of light within them, of the images of objects which when viewed in a telescope subtend an angle less than that subtended by the lightwave at a distance equal to the diameter of the objective. A method applicable not alone to celestial objects, but also to spectral lines, and other questions of molecular physics.

Along the older lines there has not been less activity; by newer methods, by the aid of larger or more accurately constructed instruments, by greater refinement of analysis, knowledge has been increased, especially in precision and minute exactness.

Astronomy, the oldest of the sciences, has more than renewed her youth. At no time in the past has she been so bright with unbounded aspirations and hopes. Never were her temples so numerous, nor the crowd of her votaries so great. The British Astronomical Association formed within the year numbers already about 600 members. Happy is the lot of those who are still on the eastern side of life's meridian!

Already, alas! the original founders of the newer methods are falling out-Kirchhoff, Angström, D'Arrest, Seechi, Draper, Becquerel; but their places are more than filled. The pace of the race is gaining, but the goal is not, and never will be in sight.

Since the time of Newton our knowledge of the phenomena of nature has wonderfully increased, but man asks, perhaps more