The following observations made during the transit of Mercury, yesterday, have some interest from the inferences to be drawn from them as to physical phenomena; and to devote the opportunity more wholly to this object, no measures of precision were attempted, beyond noting the times of ingress and egress. The principal instruments were the equatorial refractor of thirteen inches (used in the early part of the day with nine inches effective aperture), and a polarising solar eye-piece, which dispenses altogether with any dark glass, and presents objects in their natural colours and relative brightness.

I had the fortune, at ingress, of an unusually blue and transparent sky, and aided by this, saw with the polarising eye-piece the entire disk of Mercury outside the sun about one-half a minute before first external contact. Presumably it might have been seen even earlier, had not time been lost in searching for it, through lack of means to designate the precise position-angle, the position filar-micrometer not being adaptable to this eyepiece. After a pause to verify the reality of the phenomenon by revolving the eye-lens, etc., the chronograph key was struck at 21h. 52m. 32'458. Allegheny mean time, to record the observation. As this was really made earlier, and the disk was seen throughout its circumference, it seems clear that the coronal background is bright enough to produce this effect at least

VOL. XVI.

fifteen seconds of arc from the solar limb, and in spite of the atmospheric glare.

As a partial substitute for the filar-micrometer, there was in the field a glass reticule, ruled (by Prof. Rogers, of Harvard) in squares whose sides represented here 15" 3, and this enablednot a measurement-but a fair comparison to be made of the apparent size of the planet before and after it entered on the sun. The contrast was striking, as on a background very little brighter than itself its diameter was, if anything, greater than one of the sides of these squares, while as soon as it entered on the sun it seemed to shrink by more than one-fifth of this. First external contact was noted on the chronograph at 21h. 52m. 50 438. First internal contact was noted when the sunlight could be seen unmistakably between the disk and limb at 21h. 55m. 47 258. These entries I believe to have been made in both cases nearly two seconds late. The limb just at second contact was steady. I saw no black drop" or "ligament."

As the disk advanced on the sun it was closely scrutinised, without at any time any "bright point" or "annulus" being seen. These appearances, resting as they do on much testimony, particularly the unimpeachable evidence of Mr. Huggins, I was prepared to expect, but fruitlessly looked for with powers varying from 120 to 800 throughout the day, with the polarising eyepiece, and also by projection of the image. The phenomenon may depend for its visibility on exceptionally good definition, which Mr. Huggins* appears to have had; that here was fairly, though not unusually, good. The darkest part of the planet was the centre, the edges being decidedly less grey. The cause of this gradation came out very clearly in forming a very enlarged image for projection, being plainly due in most part to minute and rapid atmospheric tremor. In moments of best definition the surface became of a nearly uniform shade throughout.†

The planet has been almost uniformly described as looking "black in transit, but in the instrument I use (the objective of which was corrected by Mr. Alvan Clark), it certainly does not look black. The colour is decidedly less red than that of spot nuclei, being grey, slightly inclining toward a blue, like that of the spectrum between F. y G. (It may be that this bluish cast comes from the secondary spectrum of the objective). The average light from the disk in transit is very considerable,

*Monthly Notices R. A. S., vol. xxix., p. 26.

I presume that even in absolutely perfect definition there would be theoretically a slight gradation due to another cause, i.e., to the greater effect of the edge of the planet's disk of the inflection, referred to in a subsequent paragraph.

being not much less than that of some nuclei. No spots were present for comparison, but being engaged in photometric determinations of these and other parts of the solar surface, I was provided with means of comparing Mercury with tints which had previously been contrasted with sun-spots under like conditions. Absolute photometric determinations of the apparent light from Mercury in transit were attempted by projecting a greatly enlarged image (its actual diameter was three-quarters inch as projected), on a white surface in a dark camera attached to, and moving with, the equatorial. Direct measurements with a Jamin photometer were unsatisfactory. Subsequently, by another method, a trustworthy value was fixed for a minimum. It was thus found that the light actually received on the paper, apparently from the so-called "black" body of the planet, at any rate exceeded eight per cent. of that from direct sunlight, and measures taken by the thermopile and galvanometer showed that heat was coming from the same direction.

It need hardly be said that it is impossible that Mercury itself should be radiating heat and light in any such degree. Accordingly I take these numbers as representing (with some possible allowance for instrumental causes) the minimum effect we can assign to our own atmosphere in inflecting the solar radiation, a subject on which data have been hitherto desirable. It is evident, for instance, that from the facts here stated we can estimate, photometrically, the intrinsic brightness of the corona, since it was undoubtedly this, acting as a background, which enabled the planet, though itself involved to a calculable extent in atmospheric glare, to be seen before it reached the solar limb.

The observations were interrupted by haze in the afternoon, and egress was so nearly invisible that the apparent times of contact are not worth giving.

Allegheny Observatory:

May 7, 1878.

(Reprinted from the American Journal of Science and Arts, Vol. XV., June, 1878.)

ROGER BACON.

"The life of Roger Bacon almost covers the thirteenth century. From Oxford he passed to the University of Paris, where his whole heritage was spent in costly studies and experiments. From my youth up,' he writes, I have laboured at the sciences and tongues. I have sought the friendship of all men among the Latins who had any reputation for knowledge.

I have caused youths to be instructed in languages, geometry, arithmetic, the construction of tables and instruments, and many needful things besides.' 'Without mathematical instruments no science can be mastered,' he complains afterwards, and these instruments are not to be found among the Latins, and could not be made for two or three hundred pounds. Besides, better tables are indispensably necessary, tables on which the motions of the heavens are certified from the beginning to the end of the world without daily labour, but these tables are worth a king's ransom, and could not be made without a vast expense. I have often attempted the composition of such tables, but could not finish them through failure of means, and the folly of those whom I had to employ.' . . It is probably of himself that he speaks, when he tells us that 'the science of optics has not hitherto been lectured on at Paris or elsewhere among the Latins, save twice at Oxford.' It was a science on which he had laboured for 10 years. After 40 years of incessant study, Bacon found himself, in his own words, unheard, forgotten, buried.' He seems at one time to have been wealthy, but his wealth was gone. 'During the 20 years that I have specially laboured in the attainment of wisdom, abandoning the path of common men, I have spent on these pursuits more than £2,000, not to mention the cost of books, experiments, instruments, tables, the acquisition of languages, and the like. Add to all this the sacrifices I have made to procure the friendship of the wise, and to obtain wellinstructed assistants.' The Opus Majus' is alike won

derful in plan and detail. The development of his scheme is on the largest scale; he gathers together the whole knowledge of his time in every branch of science which it possessed, and as he passes them in review he suggests improvements in nearly all. His labours, both here and in his after works, in the field of grammar and philology, his perseverance in insisting on the necessity of correct texts, of an accurate knowledge of languages, of an exact interpretation, are hardly less remarkable than his scientific investigations. But from grammar he passes to mathematics, from mathematics to experimental philosophy. Under the name of mathematics was included all the physical science of the time. The neglect of it for nearly 30 or 40 years,' pleads Bacon passionately, 'hath nearly destroyed the entire studies of Latin Christendom. For he who knows not mathematics cannot know any other sciences; and, what is more, he cannot discover his own ignorance or find its proper remedies.' Geography, chronology, arithmetic, music, are brought into something of scientific form, and the same rapid examination is devoted to the question of climate, to hydrography, geography,

and astrology. The subject of optics, his own especial study, is treated with greater fulness; he enters into the question of the anatomy of the eye, besides discussing the problems which lie more strictly within the province of optical science. In a word, the 'Greater Work,' to borrow the phrase of Dr. Whewell, is 'at once the Encyclopædia and the Novum Organum of the thirteenth century.' Such a work was its own great reward. From the world around Roger Bacon could look for, and found, small recognition. Unheard, forgotten, buried,' the old man died as he had lived, and it has been reserved for later ages to roll away the obscurity that had gathered round his memory, and to place first in the great roll of modern science the name of Roger Bacon."-Green's "Short History of the English People," pp. 133-136.

REVIEWS.

Annual Report of the Director of Harvard College Observatory. Presented to the Visiting Committee, November 26th, 1877. By Professor Edward C. Pickering.

Annals of the Astronomical Observatory of Harvard College. Vol. X. Observations made with the Meridian Circle during the years 1871 and 1872, under the direction of the late Joseph Winlock, A. M., Phillips Professor of Astronomy and Director of the Observatory. By William A. Rogers, A.M., Assistant Professor of Astronomy in the Observatory. pp. lxxxix. and 239. Cambridge: Press of John Wilson and Son. 1877.

This observatory possesses two instruments of the largest size and finest quality, an equatorial of 15 inches aperture, and a meridian circle whose telescope has an aperture of 8 inches. Professor Pickering states that all the work heretofore done with the last mentioned has been dependent on a standard catalogue of stars observed at the Pulkova observatory, which is provisionally accepted as correct. One of the greatest defects of modern astronomical observations is the systematic error thus engrafted from one catalogue into another. It is accordingly proposed as soon as possible to prepare a catalogue wholly independent of all previous observations. The report proceeds to particulars of Books and MS. and Publications. During the past year Vol. X. of the Annals has been published and distributed, but a large amount of material is not yet printed. Under the head of Scientific Work, we find that 400 measurements have been made of the brightness of the outer satellite of Mars, and about 70 of the inner satellite. Two or three wholly independent methods have been employed, and will give the true brightness with considerable precision. It is believed that no accurate measures of the brightness of these bodies have been made elsewhere. The satellites of Jupiter and Saturn have been similarly measured. Over a hundred measures have been obtained of Hyperion, the faintest of the satellites of Saturn. To simply see this satellite is regarded as a severe test to any but the very largest telescopes. Several asteroids have been compared in brightness with Mars and Saturn. These comparisons will furnish the first satisfactory means of estimating the actual size of the smaller planets. The two