Second Meeting after the Long Vacation, December 8th, 1876. William Huggins, Esq., D.C.L., LL.D., &c., President, in the Chair.

Secretaries-Mr. Dunkin and Mr. Ranyard.

The minutes of the last Meeting were read and confirmed. It was announced that 52 presents had been received since the last meeting, and the thanks of the Society were voted to the respective donors.

Capt. James Waterhouse, Assistant Surveyor-General of
India,

was duly elected a Fellow of the Society.

Mr. Dunkin read a paper by Col. Tennant On the error in the tabular place of Venus on December 8th, 1874. Mr. Dunkin said that it would be remembered that in a late number in the Monthly Notices there was a short paper by Col. Tennant giving the errors of right ascension and north polar distance of the tabular place of Venus on the date of the late transit. It appeared that when Col. Tennant received the computations which the AstronomerRoyal has published, on comparing the Greenwich places with his own he found a difference of rather more than two seconds between the two, he had, therefore, made another computation, but can find no mistake in his former work, and yet is fully convinced that the Greenwich places are right. In writing to Mr. Dunkin he had mentioned the fact, and he thought it right to mention it in the Monthly Notices.

VOL. XV.

Mr. Dunkin said he had another paper which might be called a reclamation by Otto Struve. In a paper by Dr. Robinson reference had been made to the space-penetrating power of reflectors and refractors, and particular reference was made to a comparison between the relative powers of the 15-in. refractor at Pulkowa and Mr. Lassell's 4-ft. reflector, which M. Struve had had an opportunity of examining when it was in Malta. Dr. Robinson had spoken of the 4-ft. reflector as being decidedly superior in space-penetrating power to the Pulkowa refractor. But M. Otto Struve thought that the contrary was the case.

Mr. Lassell said: On the occasion of a visit of the late Mr. George P. Bond, of Harvard University, to this country, I conferred with him respecting the comparative powers of his 15-in. refractor (the same size as the Pulkowa telescope) and my 2-ft. reflector, and he voluntarily expressed his conviction that mine had the advantage. On my stating that I had been accustomed to consider the 2-ft. reflector as about equal to a 17-inch refractor he expressed his assent that it might be so. It will thus be inferred that my opinion totally differs from that of M. Otto Struve.

The President: I think I can confirm what Mr. Lassell has said, for on several occasions I have examined the satellites of Neptune and one or two nebulæ, first with the 2-ft. reflector at Mr. Lassell's observatory and subsequently at my own observatory with my 15-inch achromatic, which would have a greater lightgiving power than the one at Pulkowa, for the surfaces of the two lenses are united by castor oil, and the glass of both lenses is of exceeding purity and colour, there being scarcely any shade or tint of colour, and the comparison would, therefore, I think be rather in favour of my achromatic. The result of my observation is that Mr. Lassell's 2-ft. reflector is in a small degree superior to the 15-in. and consequently the 2-ft. reflector being superior in space-penetrating power to the 15-in., it seems scarcely possible to believe that the 4-ft. reflector is not also much superior to the 15-in.

Father Perry was called upon to read two papers, one On some diffraction experiments of M. Ch. André with reference to astronomical instruments and the general theory of diffraction, and the other On the phenomena exhibited by a planet in its transit across the solar disc. from observations made by M. Ch. André. Father Perry said that M. André had started with the well-known fact that the image of a brilliant point, as seen in a telescope, consists of a central disc surrounded by a succession of rings which diminish rapidly in intensity. If the intensity of any point of the image is represented by an ordinate, and the distance of the

same point from the axis of the telescope by an abscissa, a curve for the intensity may be formed and the rotation of this curve round the axis of the telescope will form what M. André calls the solid of diffraction, and he has shown that the brightness of the image of a uniform bright surface, as seen in a telescope, may be represented by the ordinates of a curved surface, the breadth of the base of which varies inversely as the diameter of the object glass or mirror. By means of this surface he had calculated the theoretical diffraction at various parts of the field of view, when a uniform bright surface was observed with a telescope of given aperture, and he had been making experiments to determine whether the black drop seen with a model of the transit of Venus corresponded with the magnitude of the theoretical diffraction as calculated by his method.

M. André was one of the astronomers sent out with the French expedition to observe the transit of Venus, and he has recently been making experiments at the Ecole Normale in preparation for the transit of 1882. M. André's experiments are being made under the most advantageous circumstances, the splendid subterranean basement of the Laboratoire being placed at his disposal. His model of the planet moves by clock work, and the bright field which represents the sun is illuminated by a Drummond light. The conclusions he has arrived at are the following: a ligament or black drop is a necessary phenomenon under certain circumstances, and not merely accidental. With a source of light of sufficient brilliancy, the angular measures of the ligament are inversely proportionate to the object-glass. 2. It is always possible to get rid of the ligament, and reduce the phenomenon to geometrical contacts either by reducing the intensity of the source of light or augmenting the absorbing power of the dark glass. 3. Definition or imperfect focussing has but small influence on the results, the phenomenon changing little so long as a fair focus is obtained. 4. Atmospheric undulations have a considerable influence tending to give the image of the planet every description of strange form, but this difficulty may be got rid of by coating one of the surfaces of the object-glass with a thin film of silver. 5. Even when the object-glass is so small and the dark glass so thin that the ligament is still present, observations may be made very precisely. One phase of the phenomena is simultaneous for all apertures, and can be estimated to within o:75 of a second of time for interior contact at ingress and 1 sec. for egress, thus giving a probably total error of 24 sec. on the observed duration of the transit. 6. In order to secure the solar parallax to within th of a sec. it is sufficient that the error in the duration of time should not exceed five secs. of time. A

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small telescope may therefore serve to determine this parallax to within 0.005 of a second, and Halley was, therefore, not mistaken about the value of his method. 7. The above results agree with the theory of instrumental diffraction and can be rigorously deduced from it.

Father Perry said that the difficulty that had so far surrounded the observations of Venus arose from a preconceived idea that the phenomenon alone to be observed was geometrical contact which was only visible in a restricted number of cases. It would be well to determine accurately the exact thickness of the dark glass most suitable for the observations, and this might be attained experimentally at the previous transit of Mercury. Father Perry added that it would be well also to make experiments as to the tint of coloured glasses best suited for diminishing the light of the photosphere so that the light of the chromosphere might be rendered more visible, in order to give greater value to the first external contact, as M. Janssen had done. No doubt, as seen under ordinary circumstances, the first external contact was over long before anything could be seen of the limb of the planet.

Mr. Christie said that he thought that there was nothing very new in M. André's theoretical investigations as to diffraction. There was a paper by Dr. Bakhuyzen in the Astronomische Nachrichten for May, 1874, in which the whole phenomenon of the black drop was examined on the ordinary theory of diffraction. There are, however, many points of interest in M. André's investigations. His experiments as to the amount of irradiation with various apertures and degrees of illumination are of great importance. Measures of the irradiation in the case of the sun and moon and of the diameter of the diffraction discs of stars have been made long ago. The law for finding the diameter of the discs with various apertures is given in the Astronomer-Royal's tract on the theory of light, and actual measures of the diameter of the discs of stars of different magnitudes and colours have been made more recently by Mr. Knott, and these seem to correspond pretty well with the calculated diameters of the discs. But I do not think that by experiments we shall ever deduce the probable error of the actual observations of the transit, because what is seen in the actual transit is a very different thing from what is seen in the model. All the principal nations that have sent out expeditions have used models somewhat like M. André's, but when the observers saw an actual transit they were fairly non-plussed. The real difficulty they experienced, so far as I can gather, arose from the bright atmosphere of Venus, which appeared as a ring of light around the planet, and so made them think that contact was taking place before it really did.

The same thing happened in the last transit in 1769. Although I have felt it necessary to point out that others have worked on this question, I must say that M. André's results seem to be important as a continuation of former researches, because the question has not been thoroughly worked out in its details, though its principles are well understood. I do not think there is any difficulty in explaining the phenomena theoretically, but it is a practical question to find to what extent the irradiation varies with the brightness of the object.

Mr. Ranyard: I have not read M. André's paper, but as far as I understand it he has dealt chiefly with what may be called the physical irradiation, and not with the irradiation due to physiological causes, or due to the imperfections of the eye as an optical instrument. The irradiation arising from this latter cause is in the case of bright lights far greater in amount than the physical irradiation or diffraction fringe. M. André's observations were made with a model illuminated by a lime-light, but in order to determine the angular magnitudes of the black drop, as seen at a transit, it would be necessary to make experiments with the sun as a background, and even then great allowance would have to be made for the varying brightness of the sun as seen at different altitudes and in different states of the weather, as well as for the effect produced by the atmosphere at the limb of the planet, which possibly may have a disturbing effect at internal as well as external contact. As to Father Perry's remark about the body of Venus seen projected on the corona, before external contact, it seems certain, when we consider the brightness of the corona, that M. Janssen must have been deceived by seeing what everybody else observed as the illuminated line of light round the body of Venus.

Capt. Noble There is one point in M. André's resumé which possessed to me a considerable amount of interest. He says that the correction of the moon's diameter to be deduced from theory corresponds with the correction of the diameter found experimentally from occultations, and which has long been used at Greenwich. An attempt has recently been made in what I suppose I may now call our standard English book on the Moon to prove the existence of a lunar atmosphere from the discrepancy between the diameter derived from occultations and the diameter of the bright moon. It would seem from M. André's paper that the corrections to be applied may be got at within a few hundredths of a second; which, assuming his data to be correct, quite disposes of any argument for a lunar atmosphere.

Mr. Marth: I may remind you that Bessel in observing the transit of Mercury in 1832, was very intent on discovering any