Imatges de pÓgina

In the region above the photosphere of the sun where variously coloured phenomena are seen during eclipses-a region which I have named on that account the chromosphere at Dr. Sharpey's suggestionwe find, by the method introduced some ten years ago, bright lines of different heights all round the sun, these bright lines acting as sounding lines and teaching us the depth of the various incandescent vapours overlying the photosphere. There is generally no part of the sun's circumference where they are absent, because vapour of some kind is always being thrown up. At the period of maximum sun spots their number is large, and in solar storms hundreds may be seen at once, and of marvellous brilliancy.

This is what I said about them in 18713 at Cambridge:


The heights of the various lines observed change very slowly at times in sweeping over a long arc, at others rapidly over a short one, over a region in fact where there is a prominence. The shorter lines are generally visible only where the higher lines are highest, and are most numerous where the highest lines are brightest.

Now what substances do these lines indicate ?


The longest lines are due to hydrogen, as proved by the fact that they are prolongations of the Fraunhofer lines C, F, one near G and h, in the solar spectrum. The next highest line, one in the orange, corresponds with no absorption line in the spectrum, and there can be little doubt that it represents a new element. The heights of the other lines with corresponding Fraunhofer lines, when visible in the same prominence, are almost invariably in the following order :

Magnesium or Sodium, highest;
Nickel or Barium, next below;
Iron and other lines, shortest.

The number of the lines, and their height and brilliancy, evidently depend upon some action coming from below, from the region of greater heat.

During the accumulation of these facts, however, many phenomena difficult of explanation presented themselves.

I had already (in 1869)' communicated to the Royal Society a list of these lines; and I remarked :—

I refrain from dwelling on this list at present, except to point out that taking iron as an instance, and assuming that the iron lines mapped by Angström and Kirchhoff are due to iron only, I have only been able up to the present time to detect three lines out of the total number (460) in the spectrum of the lower regions of the chromosphere-a fact full of promise as regards the results of future laboratory work. The same remark applies to magnesium and barium.

'On the Atmosphere of the Sun,' Rede Lecture, 1871. • Proc. Royal Society, No. 115, 1869.

The work, of which these were only the germs, has been carried on since 1869 with the most important results, both in America by Professor Young, and in Italy by Professor Tacchini, and has revealed the most striking anomalies; nevertheless, loyal to the old views, we have all of us continued to talk of injections of iron into the chromosphere,' ' magnesium prominences,' and the like.


The further the work was pushed, however, the greater became the difficulty; and Professor Young, at the end of an admirable memoir in which he summed up the observations accumulated during an expedition made for the purpose of solar observations to Sherman, a point 8,000 feet high in the Rocky Mountains, was compelled to point out the fact that the lines which he had seen most frequently were lines common to two or more elements. He writes: 5

Two explanations suggest themselves. The first, which seems rather the more probable, is that the metals operated upon by the observer who mapped their spectra were not absolutely pure-either the iron contained traces of calcium and titanium, or vice versa. If this supposition is excluded, then we seem to be driven to the conclusion that there is some such similarity between the molecules of the different metals as renders them susceptible of certain synchronous periods of vibrations-a resemblance, as regards the manner in which the molecules are built up out of the constituent atoms, sufficient to establish between them an important physical (and probably chemical) relationship.

If I understand Professor Young aright, his last words refer to what have been generally termed physical coincidences, that is, cases in which the common lines, being proved not to be due to impurity, are ascribed to common vibrations of dissimilar molecules.

Hitherto, then, we may say, when observations have been made of the lines visible in the sun's chromosphere, by means of the method introduced by Janssen and myself in 1868, the idea has been that we witness in solar storms the ejection of vapours of metallic elements with which we are familiar from the photosphere. Common lines have been observed, but they have been explained away.

A preliminary discussion of the vast store of observations recorded by the Italian astronomers (chief among them Professor Tacchini), Professor Young, and myself, has, however, shown me that this view is in all probability unsound. The lines observed are in almost all cases what I have described as basic lines; of these I may here refer to the following:

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b, ascribed by Ångström and Kirchhoff to iron and nickel.


to magnesium and iron.


to cobalt and iron."


to calcium and iron.


to cobalt and iron.


to nickel.

to calcium, but to strontium by myself.


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If the lines are basic, they cannot be due to impurities. Again, the number of times a line has been observed may vary from a hundred to

5 Solar Physics, p. 615.

The figures employed in this and the following instances represent the lineal wave-lengths.

five, and if we consider this frequency of apparition we have a method of testing the physical coincidence hypothesis. Speaking roughly, I find that the more frequently a line has been seen, the greater the number of elements to which it is common. For instance, the h line of hydrogen was seen a hundred times. I have photographed it in the spectra of eight different chemical elements. b, was seen forty times. I have observed it in six elements under circumstances which put impurity out of the question.

Again, most striking fact of all, when we leave out of consideration the substances visible in Sirius, the longest and strongest lines of the elements as we know them scarcely appear at all. It is the feeble line that claims all our attention, and insists upon being registered.


Hence I hold that the appeal to the sun from this special point of view is entirely in my favour, and following out the reasoning previously employed, we are driven to the conclusion that the bright lines in the solar chromosphere are chiefly lines due to the not yet isolated bases of the so-called elements.

If this be granted, then we can make a gigantic stride forward, for it is clear that the solar phenomena in their totality are in all probability due to dissociation at the photospheric level, and ass0ciation at higher levels. In this way the vertical currents in the solar atmosphere, both ascending and descending, intense absorption in sun-spots, their association with the faculæ, and the apparently continuous spectrum of the corona and its structure, find an easy solution.

We are yet as far as ever from a demonstration of the cause of the variation in the temperature of the sun; but the excess of socalled calcium with minimum sun-spots, and excess of so-called bydrogen with maximum sun-spots, follow naturally from the hypothesis, and afford indications that the temperature of the hottest region in the sun closely approximates to that of the reversing layer in stars of the type of Sirius and a Lyræ.

This result explains the various thicknesses of the lines of the so-called elements in the solar spectrum, and gives a chain of perfect continuity of phenomena between those substances which do not yet exist in the sun, and those which exist alone in the hottest stars; for while these bright line phenomena arise in the hottest region of the sun, the solar spectrum is a record of all vibrations of all the substances which exist from the coldest region, the outer part of the corona, down to the photosphere. The coldest, most complex substances have their record written most faintly, because they exist in a region where the pressure is least.





'IN the critical study of art, range is not necessary to penetration, and labour should be directed less in width than in depth.' 'The most modest work may contain occasion for long processes of analysis.' "Very great laws may be illustrated in a very small compass.' These are reflections of an accomplished artist, M. Eugène Fromentin, whom France has lately lost, and whose essays on the Dutch and Flemish schools are among the most valuable things ever written about painting by a painter. Such reflections cannot but come up in the mind of the student when he enters upon studies so formidably various and extensive as those to which the two great winter exhibitions just now invite him. Between Burlington House and the Grosvenor Gallery, there are at this moment on public view in London works of the old masters to the number of two hundred and fiftyeight pictures, three hundred and twenty miniature portraits, and twelve hundred and sixty-four sketches and studies; counting doubtful examples, and extending the title of old masters to certain classics of our own and neighbouring schools who have died in recent times; but not counting the English contemporary school of water-colours. These comprehensive collections include examples of the handiwork of almost every man or group of men who have practised the arts with much repute in Europe from the thirteenth century to the nineteenth. It seems ungracious to the Royal Academy and to Sir Coutts Lindsay, who have brought together all these treasures for our benefit, to speak of the regrets which mingle with our gratitude for their exertions; but since regrets there are, it is better to have them out. And it is in every one's mouth, surely not without justice, that to exhibit so much in one season is to kill the goose with the golden eggs. England, rich as she is, cannot contain materials for repeating such exhibitions as these; and if it were possible to repeat them, to do so would not be in all respects desirable. They are magnificent, they are full of pleasure and instruction; but their contents are much too numerous, and very much too miscellaneous. To master or even properly to enjoy them in the given time is impossible. If this

complaint were only true of the two galleries taken together, it might be replied that to take them together is unfair, since they have nothing to do with one another, and since the combined result of the Royal Academy and the Grosvenor Exhibitions is due neither to cooperation, nor yet (it may be hoped) to competition, but to accident. The complaint, however, is true of each exhibition separately. Excess, a fault though a generous fault, is characteristic of both alike.

Granting that the Royal Academy have done wisely in adding to their annual show of pictures new elements in the shape of engravings, drawings, miniatures, and the rest, they would, I think, have done more wisely still if they had determined to draw the elements to be so added from a comparatively narrow range in each year. Thus, the noble series of studies by the three great Italians, Lionardo, Michelangelo, and Raphael, which have been borrowed from Windsor and Oxford, with supplements from Chatsworth and other private collections, would by themselves have furnished forth in this department an ample exhibition for one season. The collections of miniature portraits lent by the Queen, the Duke of Buccleuch, Lord Wharncliffe, Messrs. A. Morrison, Whitehead, Joseph, and others, these again offer a whole range of most attractive studies on a scale, and as it were at a focus, of their own; studies in history and biography, in physiognomy, character, and beauty, and in the successive variations of an exquisite skill of which our own generation has lost the use. These might well have occupied a second year, either by themselves or perhaps in connection with that renowned series of portrait drawings by Holbein from Windsor-for history, physiognomy, and power of hand together the most instructive in the world-which is this year too prodigally thrown in with all the rest. Lastly, the miscellaneous drawings of many schools, Venetian, Florentine, Roman, Bolognese, German, Flemish, Dutch, French, and English, which now hang in Galleries VII. and X. at Burlington House, might with advantage have been reserved to form part of future exhibitions destined to illustrate, not all these groups at once in the history of art, but at most some two or three groups at a time. For there is very much more to be learned at a time, and to better purpose, by the study of twenty works of one school or one hand, than by the study of twenty works of as many different schools or hands. In the latter case, one example does not explain, carry on, or furnish the comment to another; the apprehension is hurried through a series of changes and contrasts, and has not time to understand or accommodate itself to the peculiarities of one artist, his choice of subject and mode of presentment, his gifts, habits, and predilections, before it has to go on and try to understand and accommodate itself to those of another. The study is thus made, in the phrase of M. Fromentin, a study in width, indeed, but not in depth; it almost necessarily becomes disconnected, desultory, superficial; whereas, when a number of kindred

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