The examination by Fritz of a numerous assemblage of auroral observations, enables him to extend to them also the 56-year period, as preferable to the 65-year period proposed by Olmsted, without any reference to the solar spots.

of

Sir W. Herschel, considering that the prevalence of large masses of spots on the Sun's disc was an indication that probably violent chemical action (with the extrication of an unusual amount of light and heat) was going on, was led to think that years abundant or few spots would also be noted for high temperatures and good harvests, and low temperatures and bad harvests.* Wolf, from an examination of the chronicles of Zurich from 1000 to 1800 A.D., finds decisive evidence "that years rich in solar spots are in general drier and more fruitful than those of an opposite character, while the latter are wetter and stormier than the former." Gautier, from a discussion of 62 sets of observations, extending over 11 years, and taken at various places in Europe and America, has arrived at exactly the opposite conclusion What two years could be more dissimilar than 1859 and 1860, both celebrated for profuse displays of spots-the latter, in fact, a maximum? A note of Arago's is therefore highly appropriate here" In these matters we must be careful not to generalise until we have amassed a large number of observations."

Another supposed coincidence has now to be adverted to. By carefully examining Schwabe's observations, Wolf thinks he has detected the existence of minor periods of spot prevalence, depending in some way on the periods of the Earth and Venus in their orbits. "Thus he finds a perceptibly greater degree of apparent activity to prevail annually on the average of months of September January than in the other months of the year; and again, by projecting all the results in a continuous curve, he finds in it a series of small undulations succeeding each other at an average interval of 7.65 months, or o'637 of a year. Now the periodic time of Venus (2254) reduced to a fraction of the year is o616, a coincidence certainly near enough to warrant some considerable suspicion of a physical connexion."§ It is proper to state that Wolf does not appear to have made any use of Schwabe's observations taken subsequent to 1848.||

Respecting the physical nature of the spots, much uncertainty exists. The generally received opinion, however, is that first enunciated by Professor Wilson of Glasgow, modified by Sir W. Herschel-namely, that the Sun is surrounded by two atmospheres,

* Phil. Trans. vol. xci. p. 316. 1801.

+ Op. cit.

Bibl. Univ. de Genève, vol. li. p. 56. 1844.

Sir J. Herschel, Quart. Journ. Sc. vol. i. p. 228, April 1864.
Mittheilungen, No. x.

of which the upper one is luminous (thence usually termed, after Schröter, the "photosphere "), and the under one, next the Sun's surface, is non-luminous, and that the spots are rents or apertures in these atmospheres through which we see the solid body of the Sun, otherwise known to us as the nucleus of the spots. This idea is supported by the fact that, when near either limb, the spots are narrower (foreshortened) than when seen directly in the centre of the disc. The lower stratum is assumed to receive some illumination from the photosphere, and thus to appear penumbral: to occupy, in the matter of luminosity, a medium position between the photosphere reflecting much light, and the solid matter reflecting little, or, perhaps, none at all. The temporary removal of both the strata, but more of the upper than the lower, he conceives to be effected by powerful upward atmospheric currents, origin unknown.* Above these atmospheres, it is strongly believed that a thin and gaseous envelope exists, more nearly akin to what we understand by the word atmosphere as applied to the envelope which surrounds the Earth; and this supposition finds confirmation in the fact that the margin of the Sun's disc is in general less luminous than the centre-a very obvious result on this hypothesis. Laplace gives the ratio at 30 to 48; Arago, at 40 to 41. The latter is certainly too great.

Spots on the Sun seem to have been discovered by J. Fabricius and Galileo, independently, early in 1611, and by Harriot, also independently, in December of the same year. It will readily be understood here that this observation was one of the first discoveries resulting from the invention of the telescope, though it is not improbable that they were seen before that time. Adelmus, a Benedictine monk, makes mention of a black spot being seen on the Sun on March 17, 807.† It is also stated that a similar spot was seen by a Spanish Moor named Averoës, in the year 1161.‡ An instance of a solar spot is recorded by Hakluyt. He says, that in December 1590, the good ship "Richard of Arundell" was on a voyage to the coast of Guinea, and that her log states that "on the 7 at the going downe of the sunne, we saw a great blacke spot in the sunne, and the 8 day, both at rising and setting, we saw the like, which spot to our seeming was about the bignesse of a shilling, being in 5 degrees of latitude, and still there came a great billow out of the southerboard." § The spot was also seen on the 16th.

The natural purity of the Sun seems to have been an article of * In Lardner's Handbook of Ast. p. 166 et seq., these speculative subjects are discussed at considerable length and with much perspicuity. † Bede; Polydore Virgil.

Commentary on the Almagest, quoted by Copernicus De Revol. orb. cel. lib. x. The Principal Navigations, Voiages, Traffiques, and Discoveries of the English Nation, &c., vol. ii. p. 131. London, 1599.

faith with the ancients, on no account to be called in question'; so that we find that when Scheiner reported what he had seen to his superior (he was a Jesuit at Ingolstadt), the idea was treated as a delusion. "I have read,” replied the superior, "Aristotle's writings from end to end many times, and I can assure you that I have nowhere found anything in them similar to what you mention. Go, my son, tranquillize yourself; be assured that what you take for spots in the Sun are the fault of the glasses or of your own eyes." Scheiner in the end, though permitted to publish his opinions,* was obliged to do so anonymously, so great were the difficulties with which he had to contend in consequence of his peculiar position.

In addition to the spots, streaks of light may frequently be remarked upon the surface of the Sun towards the equatorial margin of the disc. These are termed faculæ,† and are generally found near the spots, or where the spots have previously existed or afterwards appeared. They are of irregular form, and may be likened somewhat to certain kinds of coral, and are more luminous than the solar surface surrounding them. They are considered by Secchi not to be brighter than the centre of the Sun. They are elevations or ridges in the photosphere, as is proved by Dawes having seen one project above the limb in turning the (apparent) corner into the invisible hemisphere. Sir W. Herschel saw a facula on December 27, 1799, 2′ 46′′ or 74,368 miles long.§ They are first alluded to by Galileo in his third

letter to Welser.||

The surface of the Sun is frequently found to be covered with irregular specks of light, presenting a mottled appearance not unlike that of the skin of an orange. This phenomenon prevails

most in the regions to which the spots belong, but its cause is unknown. Short, the optician, seems to have first noticed it during the eclipse of July 14, 1748 (o.s.). The term luculi¶ has been applied to the constituent specks.

on the

Nasmyth has put forth some remarkable observations** structure of the Sun's surface, which he finds to be formed all

* Rosa Ursina, &c. Alluding to this enormous book, Delambre says: "There are few books so diffuse and so void of facts. It contains 784 pages; there is not matter in it for 50 pages."-Hist. Ast. Mod. vol. i. p. 690. Either printing must have been cheap or authors very rich in those days.

† Latin, facula, a torch.

Month. Not. R.A.S. vol. xx. p. 56.

Phil. Trans. vol. xci. p. 293. 1801.

Istoria e Dimostrazioni intorno alle Macchie Solari, p. 131. Rome, 1613.

Latin, luculus, a little light.

** See the discoverer's own statement in Month. Not. R.A.S. vol. xxiv.

p. 66.

over of long narrow filaments resembling willow leaves. His testimony is supported by some, and rejected by others.

The only other observation which it appears necessary to cite, before concluding this article, is by Ballot of Utrecht. He thinks he has established, by means of thermometric observations made at Haerlem, Zwanenbourg, and Dantzic, during a great number of years, the fact that at each period of 27.7 days (that of the Sun's axial rotation), there is in these localities a small elevation of temperature, while at the intermediate epochs a depression of temperature is noticeable.

Sir W. Herschel supposed that one of the hemispheres of the Sun is by its physical constitution less adapted to emit light and heat than the opposite one, but the grounds of this conclusion are not known.

Spectrum analysis has taken a sudden start within the last two or three years, specially from assertions made that it will enable us to ascertain something about the physical constitution of the Sun. The subject is too purely a physical one, and also in too infantine a state to require notice in these pages at present, though the time may come.

CELESTIAL PHOTOGRAPHY.
(Continued from our last.)

NEARLY a quarter of a century has elapsed since the moon was first photographed in America, and our friends on that side of the Atlantic have not been idle in the interval. To an American gentleman we are indebted for the best pictures of our satellite yet produced, and it is difficult to conceive that anything superior can ever be obtained; and yet with the fact before us that Mr. De la Rue's are better than any others taken in this country, so it may prove that even the marvellous pictures by Mr. Rutherford may be surpassed.

Mr. Rutherford appears, from a paper in the American Journal of Science for May of the present year, to have begun his work in lunar photography in 1858 with an equatorial of eleven-and-a-quarter inches aperture, and fourteen feet focal length, and corrected in the usual way for the visual focus only. The actinic focus was found to be seven-tenths of an inch longer than the visual. The instrument gave pictures of the moon, and of the stars down to the fifth magnitude, satisfactory when compared with what had previously been done, but did not satisfy Mr. Rutherford, who, after trying to correct for the photographic ray by working with combinations of lenses inserted in the tube between the object glass and sensitive plate, commenced some experiments in 1861 with a silvered mirror of thirteen inches diameter, which was mounted in a frame and strapped to the tube of the refractor. Mr. Rutherford enumerates several objections to the reflector for this kind of work, but admits the advantage of the coincidence of foci. The reflector was abandoned and a refractor specially constructed of the same size as the first one, and nearly of the same focal length, but corrected only for the chemical ray. This glass was completed in December last, but

it was not until March 6th of the present year that a sufficiently clear atmosphere occurred, and on that night the negative was taken from which the prints were made, and through the kindness of Dr. Roscoe I now have the pleasure of showing them to you.

I have entered somewhat minutely into particulars of what has been done in this branch of our favourite art, in order that we may have before us a kind of summary or index of the work done up to the present time, so that those who desire further information may at once refer to the authorities quoted. It may be asked why it was thought necessary to draw up this paper, as Mr. De la Rue and others have said almost all that is necessary to enable anyone to take up the subject and to pursue it successfully.

It is true there are very elaborate papers, and from their perusal I have derived much useful information; but at the same time it must be confessed their very elaborateness deterred me, for a long time after I possessed the necessary apparatus, from commencing the experiments which have since afforded me so much enjoyment.

Every writer on this subject speaks of the difficulties encountered from optical, instrumental, and atmospheric causes; and to this may be attributed the fact that we have so few of our amateur astronomers giving their attention to the subject. Another reason may be that comparatively few of those who possess telescopes may have the necessary photographic knowledge; but surely some friend having this knowledge might be found who would be willing to spare a few hours occasionally to assist in taking negatives of the stars, planets, or of the moon. The reason, then, why this subject is brought before your notice this evening is, that it is believed that the apparatus I use is, in some particulars, more simple than any heretofore described, and as it can be used with any kind of telescope, a greater number of amateurs than are now engaged in it may be induced to follow this fascinating branch of photography.

It will have been noticed that when particulars of the apparatus have been given the writer has spoken of a small camera, which has been fixed at the eye-piece end of the telescope. As to how this was effected I have seen no description, and as no camera box is required I need not enter into any supposition as to the mode in which it may be done. Before deciding what was necessary to be done, it occurred to me that the telescope tube itself is the camera, and all that was required was the means of fixing the dark frame or plate-holder. If the telescope be pointed to the moon, the eye-piece removed, and a piece of ground glass held between the eye and the aperture, the image will be seen on the glass, and we require then the means of holding the sensitive plate steadily near the same place. All that is needed is a brass tube about four or five inches long, of the size exactly fitting the tube of the telescope in the place of the eye-piece. In some cases the sliding tube of the eye-piece may be unscrewed and used for this purpose. At one end of this tube a thread is cut and is made to screw into a piece of metal plate (in the centre of which is a circular aperture of the same size as the tube), of the same dimensions as the dark frame. Attached to the plateholder are clips accurately fitting the brass plate, but so that the frame will easily slide off and on without disturbing the telescope. This is all the additional apparatus required to enable photographs of the moon or any other celestial object to be made. A separate frame for the ground glass is not necessary: it must be cut to fit the dark frame, and while in use can be held by slight springs fixed inside the frame at the sides.

The accompanying photo-lithographed copy of a drawing shows the arrangement of the apparatus when in its place for taking a negative, and renders further description unnecessary. The method for ascertaining the