The Astronomer Royal: Was it constant for yourself and Mr. Carpenter on different nights? that is the point here.

Mr. Stone: All I can say is, by using this instrument, and taking the differences between the magnitudes of certain stars used as standards of comparison, Mr. Carpenter's observations and my own agreed; but by using it as an absolute measurer they disagreed as to the angle at which the stars disappeared. On different nights the angle at which the stars disappeared was entirely different, according to whether the sky was clouded or clear, and that must be so.

Mr. Knobel: I have looked very closely into Prof. Pritchard's observations, and had some correspondence with him in criticising and questioning the results. I think there has been one point not brought clearly before the Society, and that is the method adopted in making the observations. I have made many hundreds of observations of extinctions of stars with the method of limiting apertures, and I do not apprehend there is any fundamental difference between the method of extinguishing a star by limiting the aperture, or by that of employing a wedge. But I did find this one certain result: that the operation has to be performed rapidly. In extinguishing stars with a slow-moving limiting aperture contrivance, the star disappears at a smaller aperture than with a quick moving apparatus; and that is perfectly explicable. I have questioned Professor Pritchard closely on the method he adopts in making these observations, and I believe that point was never explained to the Society. The operation is performed by sliding the wedge until the star is brought down to a low brightness, and then the screw is rapidly turned, and the whole gist of it is that it should be rapidly turned to extinguish it. Now, by using a limiting aperture instrument, in which I rapidly extinguish the star, I can make the observations consistent; but, if I take a little more time over it, the observations are inconsistent, simply because the pupil of the eye dilates, and for obvious reasons. That is a point which it is extremely important to bring before the Society, because there is the possibility of systematic errors coming in here. We all know how easy it is, in turning a micrometer screw, to unconsciously turn it exactly as much at one time as at another; and I mentioned this point to Professor Pritchard as to the possibility of systematic error creeping in by the wedge being systematically pushed into the same point by the same amount of the twist of the screw, and thus extinguishing the star at the same part of the wedge in consecutive observations; and I cannot get over the point that there may be systematic errors in his method of observation.

Lord McLaren: I may mention, in confirmation of what Mr. Knobel has stated, that having made some experiments with the wedge photometer, on five or six clear nights, I found that, when the wedge was very slowly moved across the field, I was able to follow stars even of the third magnitude to the extreme end of the wedge. They were still visible as very faint points of light; but if the wedge was moved rapidly they were extinguished long before they reached the extreme end. I should not have mentioned this if it had not been confirmatory of observations made upon a much larger scale by one who has given more attention to the subject than I have done. It is evident that this is a thing which must still be investigated, to see whether it is not possible that there is considerable latitude in the estimate of the point of extinction. It seems to me that you cannot get the exact point of extinction within such narrow limits as has been supposed.

The Rev. Father Perry: Would you kindly tell the meeting why Professor Pritchard moves it so rapidly when he has brought the star to a small magnitude?

Mr. Knobel: To extinguish it. (Laughter.)

The Rev. Father Perry: To make the observations accord, I suppose.

General Tennant: Mr. Common and I spent part of a morning with Professor Pritchard, and we asked him about that, and my recollection is that the star was brought to the minimum of visibility, and I look upon that as very much more easy to be observed. But there is another matter, as to the difference between Professor Pickering's comparatively wild results, as some think, and Professor Pritchard's. I believe Professor Pritchard's excellent observations are taken by his assistants, and when he finds they are perfectly discordant, he throws away a whole night's work. My scientific grandfather, as I may call him (Colonel Emerson) taught his pupils, and his pupils taught me, that you ought never to throw away an observation if it was bona fide made, unless you have a real reason to give against it. But I am not prepared to say mere discordances, under certain circumstances, are not a sufficient reason to throw away an observation if it is presumed that there is a mistake made. I believe that a good deal of the difference in the discordances between the individual results, as given by Professor Pritchard and Professor Pickering, is, if I am right, the result of very different ways of obtaining their results. But taking results, it is marvellous to me that they are so accordant on the whole.

Lord McLaren: I should like to know what is meant by the minimum of visibility. I can understand extinction as applied

to the observation of a star, but the minimum visibility allows a large latitude for personal equation.

Mr. Ranyard: One would expect a very much greater latitude in absolute determinations, such as Professor Pritchard makes from night to night, than in the comparison of two stars on the same night, because in comparing two stars at different altitudes there would probably be less difference in the clearness of the atmosphere in the two directions than between one night and another. I must say the observation of Professor Pritchard, that moonlight does not affect the determination of the brightness of stars, is very surprising when we know how it does affect our estimate with the naked eye and the telescope, and how, on a bright moonlight night, we do not see stars of the third and fourth magnitudes.

Mr. Knobel: The star observed is Polaris, far removed from the moon.

The Astronomer-Royal: That is not a correct statement of fact, because it is not a question whether we see more or fewer stars by moonlight, but a question whether we shall extinguish the stars by the wedge photometer, or by any other means, at the same readings on a moonlight night as on a moonless night. The visibility of stars on a moonlight night is not concerned at all in this matter.

Mr. Howlett: There is a somewhat analogous subject I was testing by means of prisms-candles of incandescent filaments in connection with electrical work, and I should like to know whether it is generally considered that the wedge should be moved rapidly or slowly. I found, practically, that when I brought the wedge to the point nearly approaching extinction I could only just see the filament. I then found, after repeated experiments, that I could see the filament longer by very slowly moving the wedge than by moving it rapidly, and I should imagine the same would be the case in the extinction of stars by Prof. Pritchard's method. With respect to the moonlight affecting the absolute point on the scale of the wedge when a star would disappear, it certainly does seem very marvellous to many of us, because if you carried out the principle logically to the bitter end, I do not see why you should not see stars by sunshine as well as by moonlight.

The President: We have had an interesting discussion, and we are all much indebted to Prof. Pritchard for his valuable paper, for which I am sure we shall all thank him. It is quite a miracle we have had this discussion to-night, because the paper only arrived while I have been sitting at this table.

The Rev. S. H. Saxby said he had hoped to have been able to

exhibit an electrical observatory lamp, but it had not arrived. It was a small incandescent lamp, supplied by a secondary battery, and arranged to hook on to the button-hole. Another thing he desired to mention to the Society: he was about to take a holiday in the High Alps, and had supplied himself with a spectroscope calculated to do good work, and ranging from A to well below the H line. He had arranged for a series of observations, at an altitude of 10,400 feet, on the Schwarzhorn, and had already sent out a six-inch equatorial, to be used at an elevation of 5,300 feet. He should feel glad if those who had paid attention to the spectroscope in high altitudes could supply him with any hints that would enable him to make his observations as an amateur of real use.

Mr. Green explained some drawings of Jupiter which he had made, and desired to call attention to some very remarkable spots following the old well-known red spots. He had made one or two remarks about them in the Observatory. The spots appeared to be following the red spot in the same latitude, and were apparently of the same kind; and if they should enlarge to anything like the proportion of the red spot, it would be interesting to know how they started. They appeared to begin in a fine dark line, and then enlarged to a claret-coloured line. The larger one, which followed the old red spot at an interval of an hour-and-a half was distinctly oval, and red, rather deeper than the old red, but not so brilliant. It was surrounded by a line of light on the equatorial side, furthest from the south pole, similar to the old red shown in his other drawings. He might say that for twenty years he had made careful drawings of Jupiter, and he hoped by degrees to know something about the question. The colour in the drawings now exhibited had been put on in the dull light of the observatory, so that in the strong light it would appear a little exaggerated.

Mr. Todd: We have made some careful drawings of Jupiter in South Australia, and I hope, during my stay in England, I shall be able to submit them to the Society for what they may be worth. The dark spots to which Mr. Green has referred we saw very distinctly at Adelaide.

Mr. Ranyard: The great red spot seems to have a whitish patch in the middle, with a reddish line round the edge. There are somewhat similar drawings made 15 years ago in America.

The President: I am happy to be able to announce that we have with us an old Fellow of the Society, and an old colleague of my own, who has only arrived in England two days ago. He has been 30 years absent from England, and is no other than Mr. Todd, Postmaster-General of South Australia, and Govern

ment Astronomer of that Colony. We shall be very glad if Mr. Todd will come forward and give us some little account of the work they are doing in South Australia, particularly in connection with astronomy. (Applause.)

Mr. Todd: When I came to this meeting this evening, I did not expect to be called upon by the President to make any remarks with regard to our small doings in South Australia. Having received an Astronomical education in your grand National Institute, the Royal Observatory at Greenwich, I did not forget my first love. I had to struggle in a young colony with a great many difficulties, and for many years, although I was styled Government Observer or Government Astronomer, I did very little else than carry on, as far as I could with the small assistance and means at my disposal, meteorological observations. But I seized on the proper time, I think, for inducing the colony to make a start towards the establishment of an observatory by availing myself of the Transit of Venus in 1874. I called the attention of the Government to the circumstance, and pointed out that South Australia, as an English colony, should certainly do something towards the advancement of astronomical science, and I made a recommendation, which, I am happy to say, the Government of the Colony and Parliament endorsed, that an equatorial should be purchased for the observatory; and I succeeded in getting that instrument into working order about one night before the Transit of Venus occurred. I was pleased to know that this, my early attempt to do some good service to astronomical science, was successful; and I believe that in the official reports the observations made by myself of egress-I was unfortunate in not being able to get the ingress, through clouds-but my observations of egress came very close to the truth when the whole of the observations over the world were taken into account. I ought to have said, however, that I had availed myself of a previous opportunity of making a start towards the foundation of an observatory. A vexed question arose between South Australia and Victoria, as to where our boundary was. It was described in the Imperial Act as the 141st meridian. Now meridians on the earth are not like those on a map or globe. They are not marked down so that we know exactly where they are, but their position can only be fixed by careful astronomical observations; and there was reason to believe that the eastern boundary line of South Australia determined and marked on the ground in 1847, was considerably to the west, and that it encroached, therefore, upon the dominions of South Australia, and the Government asked me to determine where the boundary line was. At that time it became necessary to mark the common