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No ray of light which comes through such an atmosphere to the eye of a distant spectator can be inclined to the surface of the interior globe at a less angle than that made with the surface of the globe by the course of a ray, which, after traversing the atmosphere and being refracted at the boundary, issues out in the direction of a tangent to the refracting surface. Now, a ray proceeding from a point on the surface of the globe, in a direction making an angle with the surface less than this, will not pass out of the atmosphere, but will either reach a maximum distance from the point whence it proceeds, and return by a like path to the surface again, or will be reflected internally from the upper atmospheric surface or boundary.

"From these considerations it follows," says Professor Challis, "(1.) that rays proceeding from points of the sun lying beyond the surface which contains all these limiting courses cannot reach the eye of a distant spectator; (2.) that rays proceeding from points within that surface appear to come from points within the sun's periphery; (3.) that rays proceeding from any points on that surface eventually are tangential to the boundary of the atmosphere, and appear to come from the sun's periphery. Consequently "and this is the important result-" consequently, if the boundary of the atmosphere be spherical, the periphery of the sun will be an exact circle, notwithstanding any actual inequalities of the surface of the globe. Also, if we may suppose that any objects like clouds are suspended in the solar atmosphere, all the points lying on the limiting courses will be brought by the refraction to the same level—that is, to the level of the upper boundary of the atmosphere. The augmentation of the sun's apparent semidiameter, as seen from a distant point, will be just equal to the apparent height of the atmosphere." With this latter subject, however, we are not at present concerned.

The foregoing theory," in the words of its author, “fully explains why the contour of the sun is generally observed to be entirely free from inequalities." But in certain rare instances, as in that noticed above, the possibility of still seeing inequalities in the sun is evinced, showing that the solar atmosphere itself may be liable, under abnormal conditions, to disturbances and changes of level sufficient to account for their visibility.

This particular subject is so important and instructive with respect to the constitution of the sun that it becomes requisite to give in some detail the observations in which these seemingly exceptional phenomena were detected, two of which Prof. Challis has referred to.

The Rev. W. R. Dawes, a most accurate and accomplished astronomical observer, to whose solar researches we shall have repeatedly to return, employing instrumental means of the highest character, aided by a peculiar eye-piece already mentioned, thus describes the faculæ and an observation made by him of the appearance in question:-"The bright streaks or faculæ are best seen near the east and west edges of the sun's disc, where they give the impression of narrow ridges, whose sides are there presented to view. They usually lie nearly in the direction of a circle of latitude on the sun's surface, and are rarely high enough to be seen as actual projections from his limb. On one occasion, however, the 22nd of January last [1852], I

had an opportunity of observing a satisfactory confirmation of the idea, that they are ridges or heapings-up of the luminous matter; and as the requisite circumstances are extremely rare, I will advert more particularly to the observation. A large bright streak, or facula, was observed to run, as usual, nearly parallel to the sun's edge for some distance, and very near it, and then to turn rather abruptly towards the edge and pass over it. The limb was at times very well defined; and when it was most sharp and steady, the bright streak was seen to project slightly beyond the smooth outline of the limb, in the manner of a mountain ridge nearly parallel to the sun's equator.”

Mr. Dawes, also, noticed on the 22nd of October, 1859, about noon, near the south-eastern edge of the sun, an unusually large mass of faculæ. He satisfied himself that a bright streak, which formed the very edge of the sun, projected irregularly beyond the circular contour of the edge; reminding him of a ridge of low hills often seen at the enlightened limb of the moon. (Precisely the lunar appearance alluded to in p. 26, when considering the supposed necessary effect of the solar spots, if cavities, in causing inequalities of the sun's periphery.) Nearly parallel to this, and within a very small distance of it, ran another streak nearly as bright; and the space between them was observed to be darker than is the ordinary limb of the sun, but not so black as the umbra or nucleus of a spot. He supposed it to be the penumbra of a large spot just entered on the disc—that is, brought into view by the sun's rotation, or by currents in the photosphere, as made known by the researches of Mr. Carrington, or some combination of both agencies-the umbra of which was as yet hidden by the inner bright streak, either superposed upon it or also interposed in the line of sight. At two, P.M., Mr. Dawes satisfactorily saw an excessively narrow black line, a little broken in two or three places, as if by irregularities in the inner bright streak, the top or outer edge of which was projected upon it, either by superposition or interposition, as already mentioned. On October 24th a fine spot was visible, with a remarkably extensive penumbra. On the 26th the umbra had changed its form in a very extraordinary way. The successive changes to November 3 were more extraordinary than Mr. Dawes had ever before observed, and some of the forms which the umbra assumed were curiously grotesque. In the night of November 4th it passed off, and when it came round again, and up to November 21st, the date of the communication, beyond the richness of its vicinity in faculæ, there was nothing remarkable in the group into which it had resolved itself.

"It appeared to me very remarkable," Mr. Dawes continues, "that in so violent and extensive a disturbance of the luminous envelope, I could not find, in any part of the umbra, a really black nucleus. In spots of not one-tenth part of the area I have, in numerous instances, perceived a decidedly black portion, not always central, but comparatively well-defined, and totally devoid of light, visible through the

This distinction of umbra and penumbra arises from Mr. Dawes's prior telescopic researches on the spots, which we shall consider hereafter: it will be sufficient now to apprise the reader that he uses the former term to denote a region of a spot intermediate in darkness between the nucleus and that designated as usual by the term penumbra,

darkening glass rendered necessary by the solar illumination of our own atmosphere. The umbra of this large spot was, however, of various depths of shade in different parts. It appears probable that the upward force by which the luminous envelopes were thrown aside must, in this instance, have acted upon them very obliquely." This inference, of course, is drawn on the assumption that the black appearance of the nuclei of spots, as held by Wilson and Dr. (the late Sir William) Herschel, results from the partial exposure of the interior solar surface, beneath the mediums of the penumbræ and of the faculæ or photosphere. "To obtain ocular demonstration of the bright streaks being really elevated ridges or waves of the exterior luminous envelope is, of course, a very rare occurrence; but in the present case the evidence was as complete as could be desired," part of it being afforded by the circumstance that a bright streak or facula was precisely at the sun's edge.*

A magnificent group of spots, which, as seen near the centre of the sun's disc, was in its principal dimension about 122,000 miles-and which, therefore, could it have been placed between the Earth and the Moon, would have extended half-way-was observed from July 25th, 1862, to August 4th, when it passed off the disc, by the Rev. F. Howlett. When first noticed this group had already advanced a considerable distance upon the sun's surface. It then included a remarkable mass of brilliant photospheric matter, which measured at least 12,000 miles in length by 6,000 miles in breadth, embracing, therefore, a superficial area of not less than seventy-two millions of square miles, which is greater than that of the Pacific and Indian Oceans taken together, and not much less than double the extent of all the dry land on the globe. This lay amidst the various associated nuclei of the principal spot, and was completely insulated from the contiguous photosphere by a well-defined penumbra. On the following day, July 26th, the muclei had run into each other, and become considerably enlarged, at the expense apparently of the bright patch, which had become very much reduced in magnitude, as it was

* Among some observations on the solar spots by the (late?) Rev. J. B. Emmett, published in the Annals of Philosophy' for the year 1825, which appear to have remained unnoticed by astronomers, is the following, in which also is described a similar phenomenon to that twice observed by Mr. Dawes: "In 1818 I traced a spot which was surrounded with a fine umbra to the very edge, when there was a fine line of light beyond the spot; both the nucleus and umbra were very distinct; about half the nucleus projected beyond the umbra towards the sun's centre." Mr. Emmett here denotes by umbra what is at present understood by the penumbra of a spot.

How rare this phenomenon is, of the projection beyond the limb of any portion of a spot may additionally appear from the circumstance, that Mr. Emmett, who observed the spots for some years, and gave his attention to them especially as seen on the limb, should have noticed only one occurrence of it, as just described; while his observation of that evinces that his instrumental means, though doubtless greatly inferior to those of the present day, were yet adequate to the task. He also noticed, apparently, one instance of another rare phenomenon, the extension of the penumbra of a spot to the limb (see p. 26).

It seems probable that if the remarkable outburst of intense white light appearing to exceed in brightness that of the sun's surface, and though over a very large group of spots, apparently unconnected with and high above it, which was simultaneously observed on September 1st, 1859, by Mr. Carrington and Mr. Hodgson at their respective observatories, had taken place on the limb, it would have been seen as a bright projection beyond it. But even this would have made only a fifth or sixth case in the annals of the telescope, and we know it to have been an extraordinary one from its effect, as yet unique, upon the instruments recording the variations of terrestrial magnetism.

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still more on the morning of the 27th, and by three o'clock in the afternoon it was nearly obliterated, a slight hazy "bridge," or linear portion crossing a nucleus alone remaining visible, and which in a feeble diffused form was still faintly to be traced on the 28th and 29th, but seemed to have entirely vanished on the 31st of the month. On that day and August 1st the great nucleus was grandly fringed by projections from the contiguous penumbral matter, often with sharp tooth-like terminations. Two days afterwards, on the 3rd of August, the group, and especially the principal nucleus, was seen to be remarkably foreshortened, in consequence of its approach to the apparent margin of the sun, and to be about to be divided again, apparently into two portions, by a sharply-defined and fresh bridge," a feature which had characterised this spot during its earlier stages, though then springing from a different side of the nucleus. At half-past six on the morning of August 4th, the nucleus and bridge," and many of the subordinate details, though still very distinct, had become extremely attenuated. Three hours afterwards the nucleus was reduced to a mere line (the reader will be reminded by this of the nucleal black line seen by Mr. Dawes in the observations just cited), clearly distinguishable, however, from its adjoining penumbra. When observed again, about six in the afternoon, the nucleus had completely disappeared; portions of its penumbra, however, as well as other patches of penumbræ adjoining, still remaining visible at this point of time. "And now it was," Mr. Howlett records, "that I am confident I could plainly perceive a small notch in the sun's margin, precisely over the place where the great nucleus must have been existing; and it struck me that it was not caused so much by any deficieney in the circular limb of the sun itself, but rather by reason of abnormally heaped-up masses of the contiguous photosphere, which had perhaps been swept away, as it were, from the area occupied by the nucleus and penumbra, in like manner as a slight fall of snow might have been swept away from off a gravel path, and thus have been thrown up a little on each side. The portion of penumbra, in fact, which still remained visible appeared to me to form the bottom of a shallow valley, seen obliquely over one of the elevations by which it was bounded, the right hand or south bank being the higher of the two."

Mr. Howlett's observations are, in a great degree, supported by those of Prof. Challis himself, who saw the same remarkable group of spots on the 26th of July with the Northumberland telescope of the Cambridge Observatory. What chiefly struck him "was a 'bridge' stretching across the principal nucleus, remarkably bright, and not bounded by a penumbra. Í judged it to be brighter," he says, "than the general surface of the sun.' This is the object described in the preceding observations of Mr. Howlett as a remarkable mass of brilliant photospheric matter, seen on the day before, at which time it had not decidedly assumed the form of a "bridge.'

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The phenomenon of the notch, as observed by Mr. Howlett, Prof. Challis remarks, would appear to be similar to that seen by Mr. Dawes. These, however, are the only instances of inequality of contour in the sun with which the Cambridge astronomer is

acquainted, that noticed by Mr. Dawes in 1852, being the first cited above, having apparently escaped his attention.* In them, as observed by all three astronomers, the appearances of the spots, penumbræ, and faculæ are of an unusual character, including in one of them, as observed on successive days by Mr. Howlett and Prof. Challis, the peculiarity of small nuclei situated on the edges of large patches of penumbra, which, like the corresponding appearance witnessed by Mr. Dawes, seems to indicate great obliquity of direction in the ascending torrents, and to manifest unusual activity. We must regard them, therefore, as indicative of an unusual amount of disturbance in and below the photosphere, and extending to a height and depth in the sun having some proportion to the enormous magnitude of the areas of surface disturbed.

An instructive observation made since Prof. Challis's paper was written supports this and some previous reasoning as well as his deductions. On the 24th of May, 1863, Captain W. Noble, a vigilant recorder of passing celestial occurrences, saw a remarkable appearance presented by a facula situated between a spot and the limb of the sun. "It was that of a most extraordinary elevation above the general level of the sun's apparent surface, the facula exhibiting quite a dark line" (in this respect agreeing with the second observation of Mr. Dawes and that of Mr. Howlett) on the edge which as the sun revolved followed the spot; "presenting a perfectly stereoscopic effect." We have here, first, a confirmation of the inference that the faculæ are the summitridges of physical objects having great altitude; and, secondly, an illustration or example of the optical effect ascribed to the peculiar refraction of the sun's atmosphere. We have seen that the dimensions of the faculæ, including their altitude above the general level of the photosphere, and their activities, are equally enormous; and yet one of them, at first concealed from the observer by forming a part of the hemisphere of the sun directed from the earth, became visible by the sun's rotation, having been brought through, having in fact been made to form a part of, the visible contour of the limb, without having caused any inequality in it, or impairing its circularity of outline.

But here another and a new consideration arises. If such stupendous billows of photospheric matter can form a part of the edge of the disc, without occasioning any deviation from the geometrical perfection of its form, how enormous, and how violent-even with reference to the scale on which solar phenomena obviously proceed-must be those actions occasionally occurring in the sun (or, perhaps, only occasionally observed) which, under the peculiar nature of its atmosphere as to refraction even, can still make themselves known by local elevations of the photosphere, and perhaps disturbances of equilibrium in the atmosphere itself, and thus actually causing irregularities on the visible margin of the sun, as in the instances recited!

According to Prof. Challis, the photosphere is virtually brought up, by the peculiar refraction he has explained, to the boundary or spherical surface of the sun's atmosphere, visibly represented in pro

According to Sir John Herschel, "the great spot of December, 1719, is stated to have been seen as a notch in the limb of the sun."

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