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"Laplace communicated his discovery to the Academie des Sciences of Paris on the 19th November 1787. He thus states the cause of the secular acceleration :- The secular equation of the Moon is due to the action of the Sun on this satellite, combined with the secular variation of the excentricity of the terrestrial orbit.'”

The President then described in detail how the change in the excentricity of the Earth's orbit, which is a continuous variation of long periods, produces the effect in question, the value of which, as ascertained by Laplace, was stated, and an account given of the subsequent researches of Plana and Damoiseau, which agreed with the former value of between 10' and u" per century. Professor Hausen has recently raised this value to 12:18. Mr. De la Rue then proceeded to give an elaborate and exhaustive summary of the investigations undertaken by Mr. Adams on this subject, resulting in his reducing the value to about half that found by the other mathematicians we have named, and of the controversy with Pontécoulant which ensued, and in which Mr. Adams was supported by M. Delaunay and Mr. Main, and his result at last admitted to be correct. The value thus obtained does not suffice to reconcile observation and theory with respect to the ancient eclipses, which Hausen's value does, and hence the necessity for looking for some other cause than the attraction of the Sun and planets for about half the acceleration—a want which has been attempted to be met by M. Delaunay in the manner mentioned in the Report.

" That the tidal wave tends to cause a retardation of the Earth's rotation has been pointed out by Dr. J. R. Mayer, of Heilbronn, in his Celestial Physics, 1848 ; and this action of the Moon has been made familiar to the English public by Dr. Tyndall's Lectures on Heat. Other papers on the same subject have been published by different authors; but while Mayer and others have seen that the tidal wave must act as a sort of brake tending to retard the rotation of the Earth, this illustrious French mathematician is the first to attempt to prove that the effect is an appreciable quality, and such as could account for that part of the lunar acceleration which is not produced by the secular change in the excentricity of the Earth's orbit.”

Mr. De la Rue proceeded to show how the result of a retardation of the Earth's rotation would effect other elements of the planetary system, and then gave a full account of the investigations of Mr. Adams as to the Lunar Parallax, and some references to his labours on Biela's Comet, the correction of the Tables of Saturn, the mass of Uranus, and the orbit of y Virginis, and concluded thus:

“ Laplace has said that every difficulty which has arisen in

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explaining the inequalities of planetary movements has ultimately served to establish on a firmer basis that most brilliant discovery of Newton-the law of universal gravitation. Among those ardent and illustrious mathematicians who have contributed towards the clearing away of these difficulties, there is none who stands higher than the recipient of the medal this day, whose name is and ever will be associated with that grand investigation of the perturbations of .Uranus, by an unknown planet, with which he began his career. It does not come within the purport of my present address to enter upon the connexion of Professor Adams with the discovery of the planet Neptune.

“ It will suffice to say that those most competent to judge of the merits of this celebrated investigation have always been greatly impressed by the power and comprehensiveness with which, from beginning to end, he grasps all the bearings of this difficult sub ject, as well as by the clearness, directness of aim and precision, which characterise his whole treatise.

And now, Mr. Adams, it only remains for me to congratulate you most heartily on the full confirmation which your theory of the Secular Acceleration of the Moon's mean motion has received at the hands of so many distinguished analysts, and in presenting you with this medal to express a hope that your

health

may long permit of the exercise of that great intellect with which you

have been endowed, and which you have so highly and so successfully cultivated."

Mr. Burr moved that the Report be received, and, with the President's address, be printed and circulated in the usual manner. He remarked that the Report offered matter of great congratulation as to the position of the Society; it contained most interesting obituary notices of the eminent members deceased, and an excellent résumé of the progress of astronomical science; while the address did justice to the labours of one of the brightest ornaments of astronomy here or elsewhere. The Society was singularly fortunate in having such members, and equally so in possessing officers qualified to describe their labours so admirably. The motion was seconded by Mr. Chambers, and carried unanimously.

The President, in resigning the chair, said :“Before retiring from the high and honourable position to which, for two successive years, you have been pleased to call me, and resigning into the able hands of my successor the responsible duties of the Presidency, I desire emphatically to assure you how fully I have always appreciated the generous support on all occasions accorded me by my colleagues in the Council, and by yourselves in this room. I have never occupied this chair without feeling that I was in the presence of considerate friends, cordially sympathising and cooperating with me in my continual, however imperfect, endeavours to make our meetings both instructive and agreeable, and ever ready to condone with their indulgent kindness my inevitable shortcomings. To each and to all I tender

my grateful thanks. “During my tenure of office one or two innovations have been introduced, to which I now refer, simply for the purpose of stating that they were purely personal, and in no way binding on any future occupant of the President's chair. One of these is, the report of the vivâ voce discussions at our meetings; the other is, the annual réunion of our own members and the members of other scientific bodies, as well as of other gentlemen of eminent position in political or social life. It was a matter of great gratification to me to meet so many distinguished friends and cultivators of science, and I believe that such assemblages are productive of good ; but at the same time I wish it to be understood that I had no desire to establish a precedent.

“And now let me add, that my two years of office have been to me a source of the highest and purest pleasure.”

Scrutineers having been appointed, the ballot for officers for the ensuing year was taken, when the following gentlemen were duly elected :

President, Rev. Charles Pritchard, M.A., F.R.S.
Vice-Presidents.

Treasurer.
Rev. Professor Challis, M.A., S. C. Whitbread, Esq., F.R.S.
F.R.S.

Secretaries.
Warren De la Rue, Esq., F.R.S. Richard Hodgson, Esq.
J. R. Hind, Esq., F.R.S.

E. J. Stone, Esq., M.A.
Rev. R. Main, M.A., F.R S.
Foreign Secretary, Admiral R. H. Manners.

Council.
Professor Adams, M.A., F.R.S. Rev. F. Howlett.
G. B. Airy, Esq., M.A., F.R.S. Wm. Huggins, Esq.
R. C. Carrington, Esq., F.R.S. John Lee, Esq., LL.D., F.R.S.
Professor Cayley, M.A., F.R.S. Captain Wm. Noble.
Thomas Cooke, Esq.

J. N. Lockyer, Esq. James Glaisher, Esq., F.R.S. Major-General Shortrede.

The thanks of the Society to the President and other retiring officers were, on the motion of Professor Selwyn, carried by acclamation, after which the meeting adjourned.

CORRESPONDENCE.

N.B.We do not hold ourselves answerable for any opinions

expressed by our correspondents.

BINOCULAR VISION.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-- In the excellent and lucid letter of my friend Mr. Huggins, in the February number of the Register, he recommends that such vision should be obtained “with achromatic telescopes

“by the employment of a modified form of Mr. Wenham's prism.”

Now, with the utmost deference to Mr. Huggins's admitted eminence as an observer, I would venture to suggest that he seems quite to disregard the totally different manner in which images are formed in the microscope and in the telescope. In the former instrument, from the proximity of the object to the object-glass and great divergence of the rays, the images transmitted by different parts of the glass differ very materially (a fact of which anyone may satisfy himself by alternately covering the right and left halves of an object-glass while viewing an opaque object), and the presentation of these dissimilar images to the two eyes, by the aid of Mr. Wenham's invention, and their coalescence when viewed simultaneously by both eyes, gives us the well-known stereoscopic effect.

In the case of the telescope, however, we are dealing with rays which are practically parallel, and consequently can only, by the adoption of Mr. Wenham's arrangement, get two identical images of the object viewed. Of what advantage this can be, I leave others to determine.

It certainly seems to me that our best chance of obtaining sufficient dissimilarity in the two images to afford us the effect of solidity, would be found in the adoption of the late Andrew Ross's method of combining two telescopes, although even in this case the effect can be but a quasi stereoscopic one; a consideration which impresses me with the belief that all our future great celestial discoveries will be made in the same way as our past oneswith one eye only.

I am, Sir, your obedient servant, Forest Lodge, Maresfield, Uckfield:

WILLIAM NOBLE. Feb. 3, 1866.

APPARENT SIZE OF CELESTIAL OBJECTS.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-A. C., in the Astronomical Register for February, inquires if there exists any standard by which the apparent size of objects seen by the telescope may be described. Mr. Webb remarks (Celestial Objects for Common Telescopes, pp. 13, 14)—"When people have been told that a telescope magnines 200 or 300 times, they are often disappointed at not seeing the object larger. In viewing Jupiter in opposition with a power of only 100, they will not believe that he appears between two and three times as large as the Moon to the naked eye; yet it is demonstrably so. There may be various causes for this illusion--want of practice; of sky-room, so to speak; of a standard of comparison. A similar disappointment is frequently felt in the first impression of very large buildings; St. Peter's, at Rome, is a well

a

known instance. If an obstinate doubt remains, it may be dissipated for ever when a large planet is near enough to the Moon to admit of both being seen at once-the planet through the telescope, the moon with the naked eye.”

On May 19, 1864, Jupiter and the Moon being near each other, having a power of 175 on the telescope, I viewed both together as above directed; and though my eye has had some practice, I was rather struck by seeing how much the larger Jupiter appeared.

I should think the best way of exhibiting drawings or lantern views of the planets would be to place with them a circle, drawn to scale, to represent the relative size of the Moon seen by the naked eye, mentioning at the same time the aperture and magnifying power of the telescope by which the drawings were made.

I have done this for my own amusement with Venus, whose thin delicate, pearly-white crescent, when she is near the Sun, appears surprisingly great even with low powers.

In exhibiting drawings to unpractised persons, they should be told what Sir W. Herschel wrote long ago about his portraits of Saturn: “ The outlines and all the other features of the engravings of Saturn are far more distinct than we can see them in the telescope at one view; but it is the very intention of a copper-plate to collect together all that has been successfully discovered by repeated and occasional perfect glimpses, and to re-present it united and distinctly to our inspection. Indeed, by looking at the drawings contained in books of astronomy, this will be found to be the case with them all.”

• Moreover," adds the humorous Dr. Kitchener, from whom the above extract is taken, “to have as good a view of Saturn as that given by Sir W. H., the planet must be in the same position; you must have as good an eye, as good a telescope, in as good adjustment,* in as good temper; the stand of the telescope must be good, and it must be placed on good firm ground, in a good situation, on a good clear evening.”

Yours sincerely,

G. J. W.

66

:

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—In answer to your correspondent “C. A.," whose letter appeared in your February number, allow me to remark that I have been frequently surprised at the exaggerated ideas of friends with regard to the size of celestial objects, whether viewed by the naked eye or through the telescope.

Your correspondent inquires, "Does any standard exist ?” In microscopic examinations comparison is made with an object placed at ten inches from the

eye,

that distance being considered the shortest at which average eyesight can work with advantage; and Mr. Dawes, in a letter quoted in your February number, confirms this distance with reference to the telescope. Now at this distance from the eye, the disc of the moon covers a space of about the yth of an inch, which will supply the standard required. Let us proceed to apply this scale to the measurement of the telescopic image of Jupiter or Saturn. Divide an inch into 12 parts on a piece of cardboard or any convenient substance, hold it up close to the instrument at a distance of ten inches from the eye, and while one eye observes the planet, with the other read the scale, which by a slight movement of the head may be brought into contact, and the numbers of divisions counted without any difficulty: it may then be said to look 2, or 4 times larger than the moon, according to

* A very useful term in optics, which, like nervous in physic, serves to account for all kinds of unaccountable derangements and non-performancos in optical machineries.

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