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duration, which an obstinate and contradictory critic (Mr. Proctor) insisted on pointing out-such as those in Siberia, North China, and so forth—“a moment's consideration will show you, gentlemen,” said Sir George Airy, addressing the Astronomical Society, “that it would be idle to send observers to stations where not only the cold in winter is intense, but where, I am informed” (mistakenly, it turned out, but that was nothing),“ the sun is not visible in December for more than 20 per cent. of the time.” A fortnight later-such is often the strange irony of events-news reached England that America proposed to occupy three stations, Germany two, France two, and Russia no fewer than ten, in these utterly unsuitable regions. “So it is manifest, gentlemen of the Astronomical Society, that England is not called upon to occupy these places.” In one way or in another official astronomers are bound to be right.
There still remained a region-North India--which seemed especially suitable for English observers. It had been entirely overlooked in the official discussions of the transit from 1857 to 1868; not a word said about it; not a station in the region shown in any one of the official charts. What to do, under these conditions ? Fortunately, a new method of observing the transit had been suggested (by unofficial astronomers, but that is a detail), viz. by the use of photo. graphy : “ Let us then set up a photographic station in North India” (where the Halleyan and Delislean methods can be applied all the same), " and let it be generally understood among our friends that from 1857 to 1868 we had had our eye on this North Indian region, but reserved it for this photographic method, and so said nothing about either the region or the method.”
Yet again, unfortunately, the photographic method eventually adopted was not well chosen. It is readily seen that photography is theoretically a most perfect way of utilising a transit. Two observers at distant stations can take photographs at the same instant of time, or at moments readily comparable afterwards, and there on the photographic image of the sun will be shown the round black disc of Venus, nearer to the centre in one than in the other, and so telling her own distance and the sun's.. Or attempts may be made to take photographs showing Venus as she is entering on or leaving the sun's face. From what has been already learned as to the optical conditions under which her entry and exit are effected these last-mentioned photographs can be of very little use, but the others may be very valuable. English Government-astronomers decided to take photographs on both plans. There was room, however, for choice as to the method of taking mid transit photographs. The observer might
use an ordinary telescope, enlarging the focal image to make a photograph of adequate dimensions, or he might use a telescope of great focal length and photograph the larger image formed at its focus without any intermediate magnification. A number of considerations showed that the latter was the only method which could be trusted. European astronomers adopted the former method, because the only instruments they had yet used to photograph the sun were constructed on that plan. Their instruments for photographing Venus in transit were all modelled on the Kew photohelio. graph, the object-glass of which is about 3 inches in diameter, with a focal length of 50 inches, so that the focal image of the sun is rather less than half an inch in diameter. This image was enlarged by a secondary magnifier to nearly four inches. The Americans adopted the other method, attaching more importance to the circumstance that it was the only method which could be trusted, than to the difficulties which had to be surmounted in applying it. They used instruments having an aperture of five inches, a focal length of 38! feet, giving images of the sun (at the focus) rather more than four inches in diameter. Of course a telescope 40 feet long would be awkward to wield, and still more awkward to drive by clockwork so steadily that the solar image would rest unchanged in position on the photographic plate. So, as they could not conveniently turn these telescopes to the sun, they brought the sun to the telescope, using for the purpose a mirror so moved by clockwork as to send the solar rays in an unchanging direction—to wit, horizontally-into the photographing telescope, which throughout remained fixed.
Multitudes of photographs were obtained by English and continental astronomers. The latter have not published any official report on their achievements in this direction ; the former announce that “after laborious measures and calculations, it is thought best to abstain from publishing the results of the photographic measures as comparable with those deduced from telescopic view.” In short, as Professor Harkness puts it, the European photographs are useless. Consequently, at the conference held in Paris to consider how the transit of 1882 should be observed, it was agreed that "photography was a failure and should not be tried again,” though some rather remarkable achievements in celestial photography since the transit of 1874 might have suggested a less despondent tone.
The American photographs seemed likely at a first examination to fail equally. “When they were placed under the microscope only an indistinct blur could be seen." Fortunately the cause of the difficulty was soon discovered. It was found that the magnifying power employed corresponded to an attempt to enlarge the solar disc 1764 times linearly, a preposterous power to employ with an object-glass of only five inches aperture. So microscopes of less power were employed, until the magnification amounted only to 225 diameters, with which power-still a high power, be it noticed-the photographs yielded excellent results. “ The measurements made upon them seem free,” says Professor Harkness, “ from both constant and systematic errors, and the probable error of a position of Venus depending upon a single photograph is little more than half a second of arc.” This applies only to pictures showing the whole disc of Venus on the sun's face, those taken while she was advancing on his face or leaving it proved to be valueless.
How, then, is the transit of 1882 to be utilised by astronomers? As both Delisle's method and Halley's have proved to be of very little value, one would say they would not be applied at all, or only as subsidiary methods, which may just as well be employed by observers appointed to apply better methods, since it costs nothing to throw these observations in. But as the photographic method had failed in their hands, European astronomers had no choice. “ Under the merciless pressure of necessity," as Professor Harkness puts it, “they decided to try the contact methods once more.” Luckily Airy's old mistake about the suitability of Halley's method for the approaching transit was corrected in time, or we should undoubtedly have had expeditions to the antarctic seas to occupy the stations which he lauded in 1868. It is a matter of little moment whether the nation has been saved this expense and whatever discredit might have resulted from so strange a blunder, through my showing the uselessness of these stations in 1882, or their utility (if the Admiralty were right about the stations being accessible) in 1874. Suffice it, the expense and discredit have been saved. All the stations selected by British and Continental astronomers are such as accord with what I indicated in 1869. But --and this is a very great drawback, and I think a great misfortunephotography is not to be applied.
The astronomers of the United States have taken a very different view of the matter. They knew that the probable error of a contact observation (that is, one for timing Venus as she just enters fully on or is about to leave the solar disc) is considerable, the phase observed always doubtful, the chance of failure through a passing cloud considerable. The photographic method cannot be defeated by passing clouds, is not liable to uncertain interpretation, seems to be free from systematic errors, and, though the measurement from a single negative may be affected by a considerable error, yet
so many can be taken that the mean probable error will be greatly reduced. As Professor Harkness well sums up the matter," in 1761 Halley thought that by the application of this method to the transit of 1761 the sun's distance could certainly be determined within the five-hundredth part of its whole amount. Since then, three transits have come and gone, and the contact methods have failed to give half that accuracy. From the photographic method as developed by the United States Transit of Venus Commission we hope better things, and perhaps fifty years hence its results may be regarded as the most valuable of the present transit season.” For my own part I have very little doubt that this will be the case. I know something of the zeal, the skill, the ingenuity, with which Americans attack problems of the sort; and from the exceptional success achieved by them on the photographic method in 1874, and the vast progress which photography has made in the interval, I feel well assured views secured between th. 55m. 575. and 8h. 12m. gs. (Greenwich time) on December 6, 1882, will be material records of the transit of Venus more valuable by far than any mere telescopic observations, however skilful or experienced the astronomers by whom they may be made. It is assuredly to be hoped that something more satisfactory than the contact observations of December 9, 1874, will on this occasion be secured. For, after the transit of December 6, 1882, none will occur for 105 years and a half.
RICHARD A. PROCTOR.
SUGAR-FIELDS VERSUS LAVA
N oft-quoted nature-proverb tells us that “The darkest hour
is next the dawning," and in the history of various countries it is noteworthy how often the time of deepest commercial depression has been followed by some discovery of hidden wealth, which has produced a reaction completely changing the aspect of affairs, and infusing new life into a lethargic community.
Such was the discovery of the diamond-fields in South Africa-an event which occurred when every species of industry was at a standstill, and there appeared no possibility of restoring any sort of energy to the disheartened colonists. Suddenly, from every corner of the earth, came the great multitude, all intent on this royal road to fortune, but creating a demand for all manner of produce, and then followed a season of prosperity which even the near approach of war itself has failed to destroy. Again, in Australia.
The tide of fortune in Victoria was at the very lowest ebb when the discovery of gold brought a sudden influx of treasure-seekers, giving a new impetus to everything. The tide turned, and from that date the flood of progress has rolled onward unchecked.
A similar timely reaction seems to have set in in the little island kingdom of Hawaii, since the discovery of the miraculous effect produced on its arid, volcanic soil by the simple process of irrigation, or, I should rather say, since means have been found to make the discovery of practical use—a boon more precious than the finding of gold or gems, inasmuch as broad tracks of land, which heretofore have been considered altogether worthless, are now yielding bounteous crops in return for the simple gift of a little water; and so the isles, which were in danger of being altogether depopulated by the lamentably rapid decrease of the natives, now offer a fair field to the best class of fortune-seekers—the steady agriculturalists, who, of all men, are most likely to become permanent colonists.
That the doom of extinction does overshadow the Hawaiian race does, alas ! appear only too probable, for, as you pass from isle to