132, 133

very admirable results in the production of specula 13 inches in aperture and 10 feet focal length, the perfection of which is enhanced by his practice of bestowing the same care and precision on every step of the figuring of the speculum, from the grinding, the smoothing on a bed of hones, or rather a slab of slate cut into squares, carefully brought to the same figure, and to the figuring of the polisher itself, which being thus previously rendered almost perfect, the speculum is saved the rough work of having to figure the polisher for itself on every occasion of repolishing."

In a more private communication to myself on the same subject, Sir John adds that, "Mr. De La Rue's machinery, though grounded on Mr. Lassell's rotary principle, is by no means a servile imitation of Mr. Lassell, inasmuch as several distinct improvements have been introduced tending to distribute the polishing action more equally over the whole surface of the metal. One of these improvements consists in his interposition of a plate between the supporting plate and sliding plate of Mr. Lassell's traversing slide, which, being made to revolve, causes the traversing movement of the speculum to take place, not across the same diameter of its area, but at every stroke across a different diameter; and he also obviates the irregularity of the motion of Mr. Lassell's polisher on its centre, by governing that rotation by mechanism, instead of leaving it to be determined by the excess of external over internal friction."

But it is in Celestial Photography that Mr. De La Rue has made his most important discoveries, and displayed an unfailing fertility of mechanical invention. Wisely acknowledging the growing vastness of the several departments of the same science, he has latterly, in a great measure, restricted his researches to the delineation of the various aspects of the heavenly bodies, through the medium of Photography.

It is only by acknowledging and adopting the principle of the division of labour that great results can be obtained, either in the pursuits of commercial industry or abstract science.

The days of the Admirable Crichton have long since passed

away.

Indeed Lord Bacon himself, in the Novum Organum, well observes, in anticipation of the influence of this general principle:

"Then men shall begin to find out their own powers, when all will not essay to do the same things, but each man will employ himself in the work for which he is most apt.'

Mr. De La Rue's claim to the special notice of astronomers, as a delineator of celestial objects through the medium of photography, does not rest on the absolute priority of his application of a well-known art in a new direction. It is rather based on the fact, that by methods and adaptations peculiarly his own, he has been the first to obtain automatic pictures of the Sun and Moon, sufficiently delicate in their detail to advance our knowledge regarding the physical characters of those bodies, and admitting of measurements astronomically precise.

The late Mr. Bond, of Cambridge, in the United States, in the year 1845, with the assistance of Messrs. Whipple and Bond, obtained good pictures of a Lyra and of Castor; and that, in this year, Signor De Vico made an unsuccessful attempt to photograph the nebulæ in Orion.

At about the same time, or a little later, the Rev. J. B. Reade took photographs of a Lyre at my Observatory at Hartwell, and at his own Observatory at the Vicarage of Stone.

Mr. Glaisher, writing, in 1851, as Reporter upon Philosophical Instruments in the Great Exhibition, Class X., and upon Mr. Bond's Daguerreotype of the Moon, taken in 1850, and which was placed in the Exhibition of 1851, says upon

*"Tum enim homines, vires suas nosse incipient, cum non eadem infiniti, sed alia alii præstabunt."- Liber 1, Aphor. cxiii.

134, 135

Photography," Let us now view Photography in its application to science : a process by which transient actions are rendered permanent, and which enables Nature to do her own work-or, in other words, which causes facts permanently to record themselves-is too well fitted for the purpose of science to be long overlooked; but the difficulties to be overcome in its application have been and still are great, and the results proportionably few in number. We consider, however, that the commencement of a systematic application of the photographic process to the purposes of Astronomy is indicated by the daguerreotype of the Moon by Mr. Whipple; and great indeed will be the benefit conferred upon astronomical science when we obtain permanent representations of the celestial bodies and their relative positions through the agency of light."

Enlarged copies of Mr. Bond's photographs were laid before the Royal Astronomical Society in May of the same year. At the Meeting of the British Association of Science held at Ipswich in July 1851, under the Presidency of the learned Astronomer Royal, a daguerreotype of the Moon was shown to the members of the Mathematical Section by Mr. Bond; and his Royal Highness the Prince Consort, whose loss we now deeply deplore, was present on the occasion, and inspected the daguerreotype.

On the subject of the connexion of Photography and Chemistry with Astronomy, some interesting remarks appear in the admirable Lecture on the Sun, delivered by the respected Professor Walker before the British Association of Science under the Presidency of our esteemed Member Lord Wrottesley, in 1860, at Oxford.

There are several references to Celestial Photography in the various volumes of the Comptes Rendus, which can only be brought to your notice in the form of notes.*

It was the sight of these very promising daguerreotypes of Mr. Bond which, in 1851, first gave the impulse to Mr. De La Rue's labours in this direction. In 1852 he availed himself of the collodion process invented by Mr. Archer in the preceding year, and succeeded in obtaining a good picture of the Moon. In 1853 Professor Phillips obtained talbotypes of the Moon at York. In 1854 lunar photographs were secured at Liverpool under the supervision of our respected member Mr. Hartnup. In 1855 the Rev. J. B. Reade, who has distinguished himself by his discoveries in photography, obtained special notice and honourable mention at the Paris Exhibition for his photograph of the Moon. Others, also, have been taken, at Rome by Signor Padre Secchi, at Brighton by Mr. Fry, and in the vicinity of London by Mr. Huggins. All these photographs possess merits of their own, and give decided promise of future and greater success.

Admiral Smyth, in the Speculum Hartwellianum, pp. 24950 and 285, speaks of Mr. Bond's labour in Celestial Photography, particularly pointing out that, in 1857, a photograph was sent to the Astronomer Royal taking in the whole field between Mizar and Alcor, with such exactitude as to show their angles of positions and distances.

Mr. De La Rue's success in obtaining photographic pictures of the Moon possessing great sharpness of definition and accuracy of detail is owing to the happy combination of a variety of 1849.-Vol. xxxviii, p. 241. "On the Observations of the Sun." By

M. Faye.

1858. Vol. xlv. p. 705 and following pages. "On the Photographs of the Eclipse of March 15, by MM. Porro and Quinet." By M. Faye. 1859. Vol. xlviii. p. 174. "Report on a Memoir addressed by M. Liais on the occasion of the Total Eclipse of 1858, September 7."

1859. Vol. xlix. "Second Memoir on the coming Eclipse of 18 July." 1860. Vol. li. p. 965. "On the State of Astronomical Photography in France." 1861. Vol. liii. p. 997. "On the Perfecting Meridional Observations of the Sun without an Observer." By M. Faye. 1862. Vol. liv. pp. 43 to 159.

On Photographs of the Sun, taken by M. Belfort during the Eclipse of the 31st December last."

causes. Possessing a large mirror of such exquisite defining power that but few existing telescopes equal it in accuracy of definition, and brought into figure by his own hands, and by peculiar machinery of his own contrivance, he was at once freed from those imperfections in the actinic image which are of necessity inherent in the very best refractors, even when corrected most accurately for chromatic dispersion.

Mr. De La Rue at first had no clock-work apparatus to govern the motion of his telescope, and, after making several successful lunar photographs with the aid of the hand-gear of the telescope, he discontinued his selenographical experiments until he had removed from Canonbury to Cranford — a change of residence which, for the interests of Astronomy, he had for some time previously in contemplation. He then furnished his telescope-his own in a double sense - with a clock-work apparatus, which from time to time has passed through numerous alterations, and which is still in course of improvement. The mechanical problem before him, as the Fellows of this Society well know, was one of extreme complexity; for not only must the motion of the clock-work be perfectly smooth and equable, but it must also be capable of acceleration and retardation, to keep pace, so to speak, with the ever-varying velocity of the Moon in the heavens-a variation compounded of its diurnal motion and its ever-changing velocity in its orbit.

Lastly, by a rare and happy combination of chemical with mechanical skill, the time necessary for the exposure of the collodion film was materially shortened. The final result is this, that images of the Moon have been repeatedly taken in the focus of the mirror, admitting of very considerable amplification, and exhibiting details on the Moon's surface sufficiently clear to admit of delineation under a microscope provided with a camera lucida, and thereby furnishing materials for a more accurate selenography than has heretofore existed.

Neither must we altogether omit that by stereoscopically combining images of the Moon taken in different phases of her librations, more particularly enlarged copies eight inches in diameter, Mr. De La Rue has brought to light details of dykes, and terraces, and furrows, and undulations on the lunar surface, of which no certain knowledge had previously existed, and which I have had the exquisite pleasure of beholding in his Observatory at Cranford.

"Man looks aloft, and, with erected eyes,
Beholds his own hereditary skies."

I must now turn to a department in Celestial Photography, where Mr. De La Rue stands almost alone. I speak of Heliography. In April 1854, Sir John Herschel, in a letter to Colonel Sabine, recommended that daily photographic records of the Sun should be obtained at some observatory. Accordingly, the Royal Society placed at the disposal of the Kew Committee a sum of money to promote that object, and Mr. De La Rue was requested to administer the grant.

It becomes necessary to mention that Arago, in his elegant and popular work on Astronomy, translated by two eminent Fellows of our Society, states, that MM. Fizeau and Foucault, in 1845, obtained a photographic image of the Sun, and two spots on its disk, delineated with much apparent sharpness and accuracy; but, however this may be, it is certain that no uniformly successful method of taking images of the Sun had been devised until Mr. De La Rue took up the problem for investigation.

*

* Respecting this photograph of the Sun, the Index of the Comptes Rendus has been searched all through, under the heads of Arago, Photography, Soleil, Fizeau, Foucault, Daguerreotype, and Faye, and no mention has been found whatever of the Sun's picture in 1845; and there has not been found any reference to it, excepting the plate in the body of the original work itself.

136, 137

Yet great as had been the difficulties in obtaining a really accurate and available picture of the Moon, they sink into insignificance when compared with those which had to be overcome in the photography of the Sun; for to obtain any automatic pictures of the Sun's photosphere available for practical purposes, it was found necessary to institute a series of preliminary experiments before actual operations could be successfully commenced. At first, nothing but burnt-up and solarised pictures could be obtained by any method that had hitherto been devised, or with any the least sensitive of the media, that could be procured. Now, with the help of the Kew photoheliograph, as devised by him, and described in vol. xv. of the Monthly Notices, heliography is the easiest and simplest kind of astronomical photography. The method devised by Mr. De La Rue will enable any photographer of common average skill to take excellent heliographs. Professor Selwyn, of Cambridge, succeeds in getting good pictures of the Sun with the apparatus made for him by Mr. Dalmeyer, after the pattern of the Kew photoheliograph.

Mr. De La Rue announced at the last Meeting of the Society, that by applying the stereoscope to the examination of the Sun's disk, as he had formerly done in the case of the Moon, he had discovered that the faculæ on the surface of the Sun are to be found in the outer or highest regions of the solar photosphere.

I ought not to conclude without alluding to Mr. De La Rue's observations on the Solar Eclipse of 1860; but it must not be forgotten that one Daguerreotype picture was taken by Dr. Busch of the Solar Eclipse in 1851, and of the Solar Eclipse in 1860 four small pictures were also taken during the totality by Professor Monserrat under the direction of MM. Aguilar and Secchi at Desierto de las Palmas, in Spain.

Mr. De La Rue, during the progress of the same eclipse, took many large and exquisitely defined pictures, and secured two during the totality. I have no need to enter into details, as he has already described at several meetings of this Society the numerical results that follow from the discussion, and the comparisons of the photographs which he took on that occasion. A paper, giving the result of his labours during the expedition to Rivabellosa, has been presented to the Royal Society, and is to be considered in March of this year.

Mr. De La Rue has invented an ingenious micrometer, lately exhibited at one of our Meetings, by means of which he fully confirms the hypothesis, that the coloured protuberances belong to the Sun, and renders it almost certain that the commonly received diameters both of the Sun and Moon require a correction.

More recently still, photographic pictures of the Sun have been obtained by Mr. De La Rue, not only exhibiting its well-known mottled appearance, but showing traces of Mr. Nasmyth's "willow leaves," and by the aid of stereoscopic pictures rendering it certain that the faculæ are elevations in the Sun's photosphere.

I need not enlarge on the wonderful discoveries which have been made and the astonishing results that have been obtained by Newton and his successors in this, the most fertile and exact of all the applied mathematical sciences. Neither would it become me, an humble but zealous worshipper of science, to hazard conjectures as to the future progress of Astronomy. And yet I cannot refrain from expressing my belief that the success already achieved by our friend warrants us in entertaining the hope that before long he will be able, with the aid of stereoscopic pictures, to exhibit to us the rose-coloured prominences depicted on the sensitive plates as plainly as the faculæ have already been photographed. The depths and the successive strata of those strange interlacing outliers within the solar spots may be brought into tangible view.

*

138, 139 different planes of Saturn's rings will also come into relief, the belts of Jupiter may be manifested as portions of his dark body, and ere long the mountains and elevated continents of Mars will rise up into solidity before our delighted gaze.

-

I may also, perhaps, be permitted to remark, that while our great national and public Observatories, indeed, I ought to indeed, I ought to say those of the civilised world as well, are day by day adding to that enduring record of the transient phenomena of the heavens, which will enable future ages to reach the final finish and last perfection in the calculation of the tables of the motions of the Moon and the planets, to eliminate any element of error, however minute, and to detect any latent disturbing force, however fecble its effect: yet it is to private Observatories and to observations made in the remoter regions of starry space, that we are chiefly to look for new discoveries. It augurs well for the future, that there is no lack in our own day of such establishments, or of accomplished observers to use them. It is almost, if not altogether, needless to bring before you the names of Admiral Smyth, or Lord Rosse, or Mr. Lassell, or Lord Wrottesley, or Mr. Dawes, or Mr. Carrington, and a host of others familiar to many of you. The elliptic motions of binary stars round their common centre of gravity, the colours of others, the discovery of new planets, the calculation of cometary orbits, the laws of change in the variable stars, the sudden burst upon the sight of some stars, and the gradual evanescence of others, will afford for many generations suitable and exhaustless subjects of sustained astronomical research. The instant splendour and gradual decay of certain stars is one of the most wonderful facts recorded in the history of Astronomy. In 1572, Cornelius Gemma observed a star in the chair of Cassiopeia, transcending Venus herself in brightness. It was Hipparchus who first, I believe, noticed the sudden appearance of a star of singular brilliancy before unknown. By this strange discovery he was urged to construct a Catalogue of Stars visible to the naked eye, "that posterity might know whether time had altered the face of the heavens."

The art of Photography is of the very highest importance in the promotion of exact science.

It stereotypes, so to speak, for the use of all time to come, the present aspect of the heavens.

As astronomical observations ranged in tables record the present positions of the heavenly bodies, so Photography registers their present aspect. It may be, that the pictures of the Sun now taken will enable future ages to test the prediction of the poet,

"The Stars shall fade away, the Sun himself

Grow dim with age, and Nature sink in years."

If, then, we take collective note of all Mr. De La Rue's long and varied labours since the 14th March, 1851, when he became one of our members-such as the perfecting of the figures of mirrors, the graphic observations of the planets, the incomparable photographs of the Moon, the invention of the photoheliograph, the observations on the solar eclipse, the invention of the new method of obtaining numerical data, the application of the stereoscope to the examination of the surface of the Moon, and afterwards to that of the Sun sure am I that the Society at large will unanimously approve of the award of their Medal made by the Council.

*If the subject of the present address were not now of necessity confined to improvements in Celestial Photography, I should here refer at some length to those exquisite and unequalled hand-drawings by Mr. De La Rue, of Saturn, Jupiter, Mars, and the Comet of 1858, which have so often delighted and informed our Society. They have embodied with micrometrical accuracy the results of years of scrupulous and skilful labour; and, as an instance of the reliable nature of the results obtained, I may mention that, by placing under the stereoscope two of Mr. De La Rue's hand-drawings of Saturn, taken at two distant periods, the inclinations of the planes of the rings alluded to in the text become unmistakeably apparent.

139, 140

It may, however, be said by some ingenious critic that Photography is only an art, which bears but indirectly on the promotion of Astronomy, and that the reward of its successful manipulation is rather the province of those societies to confer which cultivate the art of Photography, or the science of Chemistry. But I cannot admit the justice of this view. What should we now say of the early Fellows of the Royal Society, if they had relegated Newton, when he invented the telescope that bears his name, to the Company of Spectacle Makers for his meed of praise? What should we now think, had the barren honours which grace scientific discovery been denied to such mechanical inventors as Hadley, or Dollond, or Sir William Herschel, or Lord Rosse, or Lassell? With them the name of De La Rue, I feel, will hold no inferior place.

(The President, then delivering the Medal to Mr. De La Rue, addressed him in the following terms):—

Mr. De La Rue,-In compliance with a resolution of the Council, I have the pleasing duty of placing in your hands the highest tribute to merit which they have in their power to bestow. The instruments made or improved by you, the important uses to which you have applied them, and the liberality with which you have communicated the results of your discoveries to the public, all indicate, in the opinion of the Council, a mind highly cultivated, whose energy has been directed, during many years, to the attainment of scientific perfection.

But your unceasing efforts and delicate manipulation in reducing the new and wonderful art of Photography to astronomical purposes, and in rendering Chemistry a handmaid to Astronomy, supply the more immediate motive of their approbation.

May Divine Providence continue to bestow upon you health and intelligence, and every social blessing, enabling you still further to illustrate the glory of the Creator, and to promote the rational enjoyment of our fellow-creatures.

141, 142

142, 143

VOL. XXII.

Dr. LEE, President, in the Chair.

J. Norman Lockyer, Esq., Wimbledon, Rev. Edward Crofton, Heene, Sussex; and J. J. Cole, Esq., 24 Essex Street, Strand,

VII. Theoria Motus Corporum Coelestium;

the Theoria Motus to appear when the copyright interest therein has expired. The contents of the several volumes are given in No. 1348 of the Astronomische Nachrichten.

The Works are to be published by subscription, which may be either for the whole or for the separate volumes; the subscription price to be hereafter fixed, but to be at about the rate of four thalers for a volume of from fifty to sixty sheets. Subscriptions may be addressed, post-free, "An das Secretariat der Königlichen Gesellschaft der Wissenschaften zu Göttingen." On completion, the Works will be sold in the ordinary course, at the rate of six or seven thalers per volume. The impression has begun, and will be carried on so that the first six volumes may at latest appear within five years.

On the Perturbations of Uranus and the Mass of Neptune. By Truman Henry Safford, Assistant at the Observatory of Harvard College.

In the determination of the mean distance of the satellite

of Neptune, and the consequent evaluation of that planet's mass, the faintness of the satellite has proved a great difficulty; so that different observers have differed systematically inter se, although each one is quite consistent with himself.

For instance, the mass of Neptune, from O. Struve's observations of the satellite, comes out M = +0.02431 M,

I

14491 while Prof. G. P. Bond has deduced M = father's and his own observations.

I

194.00

from his

No. 5.

The perturbations of Uranus, by means of which Neptune was originally discovered, are, of course, adequate to determine its mass; and it would naturally be expected that the probable error of such a result would be much less than the difference between the numbers cited above.

Prof. Peirce, employing, I believe, the same observations as Leverrier did in his "Recherches sur les Mouvements de la Planète Herschel (dite Uranus)," has deduced (Ast. Nach., No. 637) the result that, in general, a mass of Neptune nearly

I

20000

will represent the motions of Uranus considerably nearer than a value more closely approaching to Struve's.

I was led to take up this question partly because it had been, some years ago, made the subject of a prize question by the Haarlem Society of Sciences (which, so far as I know, was not answered within the specified time), and partly because Struve's mass of Neptune seems to have been adopted by some high authorities; by Leverrier as, at least, a provisional value. The perturbations of Uranus produced by Neptune, assuming as the disturbing mass, were computed by the method of mechanical quadratures. These were then added to the perturbations of heliocentric longitude derived from Leverrier's theory, and the theory thus computed (so far as heliocentric longitude is concerned) was compared, for one date in each year, with the results from Bouvard's Tables contained in the Greenwich Planetary Reductions, and with the Nautical Almanac since 1836 inclusive, to 1858.

I 19400

Errors of these latter theories in heliocentric longitude were obtained from the Greenwich Planetary Reductions, and the Results of the Greenwich Observations, 1836-1858; and thus the errors in heliocentric longitude of the present provisional theory were obtained.

It is proper to mention here, that Bradley's observation of 1753 was omitted here, as it seemed desirable to found the firmation; also that where the error of radius vector in Bouresult on modern observations, using the ancient ones as a convard's Tables was found sensible, observations in the neighbourhood of opposition were used.

The Paris observations (1801 to 1819), the Greenwich (1831 to 1835), and the Konigsberg observations (1814 to 1846), were also used; the first series, as reduced by Leverrier, in the Annales de l'Observatoire Impériale, and the remainder after Flemming's (Ast. Nach. No. 755 et seq.), and Applebaum's (Königsberg Observations, vol. xxxii. p. 210 et seq.) reductions. It is known that the observations at these Observatories were made chiefly near opposition; and, indeed, Flemming has computed (with a few mistakes) the opposition as it is derived from observations made near it for a good many years.

reductions in the state in which they would have been if Of course, every effort has been made to put all these uniform star-places and other elements had been used in making them.

The results were found to indicate with some degree of probability, by a mere inspection, that the disturbing mass would need but little change. In fact, a preliminary solution of a set of twenty-six equations obtained by equating the residual errors of this first theory (taking the means of three years together) to functions of unknown variations of the ele