The annual inspection of the Observatory by the official visitors took place on Saturday, June 1, when, in addition to the Members of the Board, a number of scientific men availed themselves of the invitation of the Chairman of the meeting, to make a visit to this most interesting Establishment and inspect the instruments with which so much valuable work is accomplished. The Observatory was opened at two instead of three o'clock, as on former occasions, so that ample time was secured for careful examination of the buildings and their contents. The Report of the Astronomer Royal goes minutely into detail of the work performed, and the state of the reductions and printing, and describes the present condition of the buildings and instruments. No alterations of any importance have been made in these respects, the principal addition having been that of a new normal sidereal clock, by Dent, to distribute sidereal time throughout the Observatory and make the seconds punctures on the chronograph, which, being erected in the magnetic basement, and thus kept at a uniform temperature, has rendered the clock rates almost constant; the only fluctuations observed now being due to changes in barometric pressure, which can readily be estimated

VOL. X.

and allowed for. The objects of most interest to the visitors, both official and unofficial, appeared to be the instruments and observatories prepared for the Transit of Venus in 1874, which were to be seen dotted about the grounds. The observatories are of wood, and generally hexagonal in form. Those for the Altazimuths have revolving roofs, but for the Equatoreals portions of the roof can be removed as required. The stations, it may be remembered, are five in number, and for each a transit, an Altazimuth and a 6-inch Equatoreal have to be provided. Three first-class clocks, besides extra telescopes and clocks, will be wanted for each station. Most of these instruments are ready and were exhibited. The beautiful Altazimuths of Messrs. Simms, and the Transits by Messrs. Cooke excited universal admiration. The Equatoreals, which, as a rule, are not new, are not quite complete, but among them is the celebrated one formerly belonging to Admiral Smyth, and afterwards to Dr. Lee, and now adapted for the station at Alexandria. Many of those present recognised an old friend in this instrument to which they had been indebted for many delightful views of the heavenly bodies when at Hartwell. We were glad to find from the Report that the merits of the assistants have been acknowledged by some increase in their salaries granted by the Admiralty. The Observatory may well be proud of the devotion of these gentleman to their duties and the manner in which the work is performed, and this recognition is only what they have long deserved. The Astronomer Royal also hints that he is preparing a sketch of the history of the Observatory during his term of office which will partake of a biographical form. We need scarcely say that, looking to the vast improvements in instruments and other changes which have been introduced since 1836, this account of his labours will possess the greatest interest to all astronomers.

Perhaps the most important part of the Report is the notice in which the Astronomer Royal draws attention to the probable necessity of taking up a different class of observations in addition to those for which the Observatory is famed. He suggests that those phenomena which can be observed at convenient times, with moderate instruments, and without continuous expense, such as measures of double stars, may well be left to private observers; but that where large telescopes, constant routine and consequent expense are required, a public observatory should be employed. The scrutiny of nebula and planets and solar spectroscopy may possibly fall within this category; but he has no doubt that a continued series of observations of the solar spots should be carried on, and he recommends that, with this object

Meeting of the Royal Astronomical Society. 159

in view, another assistant should be added to the strength of the establishment. With these and other like investigations supplementing the magnetic and meteorological observations already constantly made, Greenwich would become pro tanto a physical observatory, and as such might, in future, produce results comparable in value with its meridional work.

These remarks, no doubt, occupied the earnest attention of the Board during their protracted sitting, and will, doubtless, bear good fruit.

The visitors and their friends dined together as usual in the evening.

ROYAL ASTRONOMICAL SOCIETY.

Session 1871-72.

Eighth Meeting, June 14th, 1872.

Professor Arthur Cayley, F.R.S., President, in the Chair. Secretaries-E. Dunkin, Esq., and R. A. Proctor, Esq. The minutes of the last meeting were read and confirmed. Thirty-nine presents were announced and the thanks of the Society voted to the respective donors.

Rev. W. Falconer,

E. Sheppard, Esq.,

W. Mathieu Williams, Esq.,

Rev. Thos. Tordiffe,

H. W. Jackson, Esq., and

Rev. F. W. H. Courtier,

were balloted for, and duly elected Fellows of the Society. The following papers were announced and partly read: Discovery of Planet 121: by Professor Watson.

This small planet was discovered on May 12, in the United States.

On New Tables of Uranus: by Professor Simon Newcombe. This was an extract of a letter to the Astronomer Royal asking for the last Greenwich observations of the planet, and stating that the author intended to conclude his labours with the use of the places obtained at the opposition of 1871-2, and hoped, after twelve years' work, very shortly to bring the matter to a close. So far as he had at present compared the results with observation, his tables gave the places within a fraction of a second of time and a second of arc, and the theory gave no indication of the existence of any trans-Neptunian planet.

Mr. Dunkin said that the Tables of Neptune now in use gave

the places with the same degree of accuracy. He also added that M. Leverrier had published an important paper in the Comptes Rendus on the theory of the planets Jupiter, Saturn, Uranus, and Neptune, which, at the request of the Astronomer Royal, he (Mr. D.) was translating for the Monthly Notices.

Occultations of Stars by the Moon, and Phenomena of Jupiter's Satellites by the Rev. Mr. Main.

:

These observations were from fifty to sixty in number, and Mr. Dunkin observed that they showed the Tables of Damoiseau represented the phenomena of Jupiter's Satellites better than was usually admitted. As to eclipses of the 1st satellite the errors were almost nil; with the 2nd satellite, a few seconds; as to the 3rd, about a minute; and with the 4th, about three minutes. The other phenomena were not so exact; one transit egress was twenty-three minutes wrong, but the observation was a very bad one, and made under exceptional circumstances of weather, and the view being obstructed.

On the Aurora of February 4, 1872: by the Rev. Mr. Slatter. These observations, compared with others gave, the author considered, a result showing the height of the phenomenon was 118 miles. On the Radiant Points of Meteors: by Mr. Greg.

This was a very valuable table of the radiant points and epochs of the groups known, collated from the various observations, which agreed better than might be expected. From the researches of Schiaparelli great interest attached to the subject.

Mr. Glaisher bore testimony to the great value of this Table, with which he was well acquainted, and found it of much service where any discrepancies existed as to the radiating points of groups, by limiting the area of search.

Addition to Paper on the Errors of Vlacq's Tables of Logarithms: by Mr. J. W. L. Glaisher.

The author explained the object of this paper as being to give further corrections which had been omitted in Vega's errata.

On Photographic Irradiation in Over-exposed Plates: by Lord Lindsay.

The most cursory observer of eclipse photographs must have noticed the cccurrence of a halo of light extending over the dark limb of the moon, and having a different extent where there are prominences to where there are none. This is similar to the nebulous haze or border seen in over-exposed pictures of other objects. The effect had been referred to an extension of the chemical action continuing after the light had been cut off, but, whatever the cause, the author proposed to call it Photographic Irradiation, and had performed an extensive series of experiments to ascertain, if possible, the circumstances under

which it was produced, and the best means of preventing it. The halo in these pictures consisted of two parts, an inner luminous border and an outer less defined fringe. The boundary between the two is difficult to fix, as they fade gradually into one another. The edge of the moon and outer limb is, of course, better defined. Lord Lindsay then detailed the apparatus and method used in order to test the production of and get rid of these appearances. A plate of blackened zinc, with a triangle cut out of it, and a wire crossing the space, was placed in front of a gas flame, and a negative taken in a portrait camera on a plain white glass with one minute's exposure. This formed an excellent standard picture. Other pictures, with the same focus, were allowed from ten to twenty minutes, and these all had extensive fringes or halos surrounding the triangle. Other plates were then tried, including white glass with one side ground, and with both sides ground. These gave results very little better. A backing of white or black paper was tried, but unless kept adherent by moisture made little difference. A backing of black asphalte varnish, giving a very dead black, was a great improvement; and non-transparent plates, such as slate and ebonite, hardly showed any halo in ten minute's exposure. The black varnish over half a plate and the other plain showed the great advantage of this mode of backing. One of the best materials was a yellow glass used for photographic dark-room windows, and called yellow pot metal. This, when ground on both sides and varnished, produced pictures in which it could hardly be seen whether it had been exposed for one or ten minutes. An incandescent platinum wire, heated by ten cells of Grove's Battery, was also tried as the source of light. In ten seconds the halo was hardly visible; in ten minutes very markedly produced. In order to test whether the effect was produced by chemical or optical causes, a small ivory rule was held against the collodion plate by capillary attraction, when the picture was found to be cut clean off by the limb of the rule. Apertures in black paper were printed by contact in daylight in the usual manner, but the action was found not at all to extend under the edges of the holes. The author was about to repeat this experiment with a silver plate and various apertures, and also to use dry collodion plates. He thought such plates with a dark back would do very well. These experiments seemed to show that by proper precautions the greater part of these defects in photographic pictures might be eliminated, but there would remain a very fine band of light at the limb of the moon which rendered the image different in size to that seen on the ground glass, and the extension of this halo was greatest where the light was the strongest.