Saturn - the altissimus planeta of the ancientsre-mains still the most distant planet respecting whose physical condition astronomers can obtain satisfactory information. The most powerful telescopes yet constructed have been turned in vain towards those two mighty orbs which circle outside the path of distant Saturn: from beyond the vast depths which .separate us from Uranus and Neptune, telescopists can obtain little intelligence respecting the physical habitudes of either planet. Nor need we be surprised at the failure of astronomers, when we consider the difficulties under which the inquiry has been conducted. In comparing the telescopic aspect of Uranus with that of Saturn (for example) we must remember that Uranus is not only twice as far from the earth but also twice as far from the sun as Saturn is. So that the features of Uranus are not merely reduced in seeming dimensions, in the proportion of about one to four, but they are less brilliantly illuminated in the same proportion. And therefore (roughly) any given portion of the surface of Uranus - say a hundred miles square near the middle of his visible disc-sends to us but about one-sixteenth part of the light which an equal and similarly-placed portion of the surface of Saturn would send to us. Now every astronomer knows how difficult it is, even with very powerful telescopes, to study the physical features of Saturn. A telescope of moderate power will show us his ring-system and some of his satellites; but to study the belts which mark his surface, the aspect of his polar regions, and in particular those delicate tints which characterise various portions of his disc, requires a telescope of great power. It will be understood, therefore, that in the case of Uranus, which receives so much less light from the sun, and is so much farther from us, even the best telescopes yet made by man must fail to reveal any features of interest. We may add also that Uranus is a much smaller planet than Saturn, though far larger than the combined volume of all the four planets, Mars, Venus, the Earth, and Mercury. If Saturn (without his rings) and Uranus were both visible together in the same telescopic field (a circumstance which may from time to time happen) the Herschelian planet would appear so small and faint that it might readily be taken for one of Saturn's moons, the ringed planet sending us altogether some sixty times as much light as Uranus.
But what the telescope had hitherto failed to accomplish, has just been achieved by means of that wonderful ally of the telescope, the spectroscope, in the able hands of the eminent astronomer and physicist, Dr. Huggins. News has been received about the constitution of the atmosphere of Uranus, and news so strange (apart from the strangeness of the mere fact that any information could be gained at all respecting a vaporous envelope so far away) as to lead us to speculate somewhat curiously respecting the conditions under which the Ura--nians, if there are any, have their being.
Before describing the results of Dr. Huggins's late study of the planet, it may be well to give a brief account of what is known respecting Uranus.
The question has been raised whether Uranus was known to the astronomers of old times. There is nothing altogether improbable in the supposition that in countries where the skies are unusually clear, the planet might have been detected by its motions. Even in our latitude Uranus can be quite readily seen on clear and moonless nights, when favourably situated. He shines at such times as a star of about the fifth magnitude-that is, somewhat more brightly than the faintest stars visible to the naked eye. In the clear skies of more southerly latitudes he would appear a sufficiently conspicuous object, though, of course, it would be wholly impossible for even the most keen-sighted observer to recognise any difference between the aspect of the planet and that of a star of equal brightness. The steadiness of the light of Saturn causes this planet to present a very marked contrast with the first magnitude stars whose lustre nearly equals his own. But although the stars of the lower orders of magnitude scintillate like the leading orbs, their scintillations are not equally distinguishable by the unaided eye. Nor is it unlikely that if Uranus were carefully watched (without telescopic aid) he would appear to scintillate slightly. Uranus would only be recognisable as a planet by his movements. There seems little reason for doubting, however, that even the motions of so faint a star might have been recognised by some of the ancient astronomers, whose chief occupation consisted in the actual study of the star groups. We might thus understand the Burmese tradition that there are eight planets, the sun, the moon, Mercury, Venus, Jupiter, and Saturn, and another named Rahu which is invisible. If Uranus was actually discovered by ancient astronomers, it seems far from unlikely that the planet was only discovered to be lost again, and perhaps within a very short time. For if anything positive had been learned respecting the revolution of this distant orb, the same tradition which recorded discovery of the planet would probably have recorded the nature of its apparent motions.
Be this as it may, we need by no means accept the opinion of Buchanan, that if the Burmese tradition relates to Uranus, Sir William Herschel must be 'stripped of his honours.' The rediscovery of a lost planet, especially of one which had remained concealed for so many centuries, must be regarded as at least as interesting as the discovery of a planet altogether unknown. Nor was there any circumstance in the actual discovery of Uranus, which would lose its interest, even though we accepted quite certainly the conclusion that the Herschelian planet was no other than old Rahu.1
Let us turn to Herschel's own narrative of his detection of Uranus. It is in many respects very instructive.
In the first place, we must note the nature of the work he was engaged upon. He had conceived the idea of measuring the distances of the stars, or at least of the nearer stars, by noting whether as the earth circles around the sun the relative positions of stars lying very close to each other seemed to vary in any degree. To this end he was searching the heavens for those objects which we now call double stars, most of which were in his day supposed to be not in reality pairs of stars that is, not physically associated together-but seen near together only because lying nearly in the same direction. The brighter star of a pair was in fact supposed to lie very much nearer than the fainter; and it was because, being so much nearer, the brighter star should be much more affected (seemingly) by the earth's motion around the sun, that Herschel hoped to learn much by studying the aspect of these unequal double stars at different seasons of the year. He hoped yet more from the study of such bright orbs as are surrounded by several very faint stars. It was a case of this kind that he was dealing with, when accident led him to the discovery of Uranus. 'On Tuesday, the 13th of March (1781),' he writes, 'between ten and eleven in the evening, while I was examining the small stars in the neighbourhood of Eta in Gemini, I perceived one that appeared visibly larger than the rest. Being struck with its uncommon magnitude, I compared it to Eta and the small stars in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet. I was then engaged in a series of observations (which I hope soon to have the opportunity of laying before the Royal Society) requiring very high powers, and I had ready at hand the several magnifiers of 227, 660, 932, 1,536, 2,010, etc, all of which I have successfully used on that occasion. The power I had on when I first saw the (supposed) comet was 227. From experience I knew that the diameters of the fixed stars are not proportionally magnified with higher powers, as those of the planets are; therefore I now put on the powers of 660 and 932, and found the diameter of the comet increased in proportion to the power, as it ought to be on a supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars presented that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded.'