rings. I looked most carefully for them, expecting to see them. Once about the middle of totality I thought I saw traces of a ring about where 1474 ought to be, but a second look did not confirm the impression. The continuous spectrum seen during totality was somewhat wider than the moon's disk, but I cannot say how much, nor can I tell just when it assumed the greater width. Before totality it was quite narrow and the bright crescents projected on both sides beyond it. But I remember that in looking for the rings I expected them to be seen upon this spectrum and not to project beyond it. I did not take my eyes from the instrument during totality, my intention being to see these rings if there were any to see, and my first feeling when totality was over was one of failure because none had been visible."

Thermoscopic observations.—Mr. A. D. Anderson reports his observation with the thermospectroscope as follows: "As soon as the corona appeared the slit-tube (collimator) was pointed to it, the slit opened, the galvanometer balanced, and the lever which turns the grating moved so that the spectrum from the orange through the ultra-red traversed the face of the pile. When the extreme red was reached the lever was moved back to its original position. During totality the instrument was pointed four times to the corona, and four observations were made. The only result obtained was during the third observation when a decided deflection of the galvanometer index was noticed, in the direction indicating heat, from 54 to 575 on the scale, the index returning to its original position as the lever was moved past the point. A precisely similar and equal deflection occurred when the lever on the return stroke reached the same point." The position of the lever at the moment of the deflection was carefully marked, and after the eclipse it was found that a point in the spectrum of the second order just above G was upon the face of the pile. This would tend to indicate a heat line in the ultra-red with a wave length twice that of the coincident blue light. At the same time it must be admitted that the conclusion is doubtful, for, though it is not easy to explain what cause other than a coronal heat line could have produced the observed deflection, it is equally difficult to explain why the three other sweeps across the spectrum failed to show the same thing.

It is proper that I should here add that the American Academy of Sciences at Boston had placed at our disposal a considerable appropriation for this thermoscopic research;_thanks, however, to the industry and ingenuity of Professor Brackett in constructing our apparatus, the generosity of Mr. Edison in giving us a tasimeter, the kindness of our railroad friends in the matter of passage and freights, and the liberal scale of the

original appropriation of the Green Trustees, it was not found necessary to draw upon this fund, though we are none the less grateful for its provision.

I need not occupy much space with a discussion of the photographic work. I think we failed in obtaining the desired results, simply because the lines we hoped to find in the ultraviolet did not exist there on this occasion, though it is of course certain that lenses of more light-gathering power would have increased our chances. That there was no fault with the chemicals and the manipulations is evident from the splendid success of Mr. Calley's pictures of the corona. In twenty-five seconds he obtained a far more extensive and better photograph than the same instrument gave in 1870 at Jerez with an exposure lasting through the whole totality, more than two minutes.

I ought not to close without a word of recognition of the courtesy and helpful kindness of our Denver friends. I do not know what they could have done that they did not, to aid us in our work and make our stay among them pleasant. Princeton, September 6, 1878.

ART. XXXIII.-On a Cause for the Appearance of Bright Lines in the Solar Spectrum; by RAPHAEL MELDOLA, F.R.A.S., F.C.S., &c.*

IN July, 1877. Professor Henry Draper showed that oxygen and (probably) nitrogen are present in the sun's atmosphere, the spectral lines of these gases appearing as bright lines in the solar spectrum. The photograph accompanying Professor Draper's paper, shows that the oxygen lines are bright, although not conspicuously so, upon a less luminous background.

The discoverer of this most important fact in solar chemistry does not offer any complete explanation of the exceptional behavior of the lines of these elements, but remarks that "it may be suggested that the reason of the non-appearance of a dark line may be that the intensity of the light from a great thickness of ignited oxygen overpowers the effect of the photosphere, just as, if a person were to look at a candle flame through a yard thickness of ignited sodium vapor, he would only see bright sodium lines and no dark absorption lines. Of course such an explanation would necessitate the hypothesis that ignited gases such as oxygen give forth a relatively large proportion of solar light."

The oxygen spectrum referred to in the above-mentioned paper is the well-known "line spectrum" seen when powerful * From Phil. Mag. for July, 1878. + Nature, xvi, 364, August 30, 1877.

disruptive sparks pass through the gas. Dr. Schuster has recently succeeded in obtaining a second or "compound" spectrum of oxygen,* the fundamental lines of which he has shown with considerable certainty to be present as dark lines in the solar spectrum.

Since the publication of Professor Draper's discovery, I have given much attention to the consideration of a cause for the apparently anomalous brightness of the oxygen lines; and in the present paper I venture to advance an explanation which has recommended itself as being worthy of notice, not only because it offers a reconciliation of the known solar spectrum with the generally accepted views of the constitution of the sun's atmosphere, but likewise because it furnishes a suggestive hypothesis for the attack of many other obscure problems in solar physics.

1. I shall throughout this paper consider it to be established that the gaseous envelopes surrounding the sun succeed each other in the following order, commencing with the lowest :

(1) Photosphere; (2) Reversing layer; (3) Chromosphere; (4) Coronal atmosphere.

I also assume the truth of the hypothesis, first advanced by Johnstone Stoney,t who showed, from purely theoretical considerations, that in the sun's atmosphere the various elements must extend to heights which are, broadly speaking, inversely as their vapor densities. This view has, in my belief, been substantially confirmed by subsequent observation. Thus nitrogen and oxygen, having the respective densities 14 and 16 (H=1), would extend to a great height in the solar atmosphere, rising above sodium, calcium and magnesium, and having exterior to them the unknown substance giving the D, line (helium), hydrogen, and the element giving the coronal line "1474."

2. Two suppositions can be made concerning the sun's temperature. In the first place, it may be assumed that the temperature is so enormously elevated that no chemical compound is anywhere capable of existing in his atmosphere; in other words, dissociation may be considered to be complete. In the next place, it may be supposed that the temperature falls off sufficiently at some region of the outer portion of the sun's atmosphere for certain chemical combinations to take place.

3. Let us first assume that the temperature of the sun is so great that there is perfect dissociation throughout his whole atmosphere. Under these circumstances free oxygen would exist in the presence of electro positive elements; and, in accordance with Stoney's hypothesis, both this element and nitrogen (if present) would extend to a considerable height in

* Nature, xvii, 148, December 20, 1877.

Proc. Roy. Soc., xvi, p. 2 and xvii, p. 1; Phil. Mag., August, 1868; Monthly Notices Roy. Astr. Soc., Dec., 1867; Lockyer, Phil. Trans., 1873, clxiii, 265.

the sun's atmosphere, rising as a necessary consequence, into regions which are cooler than that stratum which is cool enough to reverse the spectral lines of those metals having the smallest molecular mass, viz: Na, Ca and Mg.* Professor Draper's suggestion that the enormous thickness of incandescent oxygen may overpower the light of the photosphere, can only hold good, when considered in connection with this hypothesis, if the temperature of the upper portions of the oxygen atmosphere does not differ to any great extent from that of the lower and hotter portions. When, however, we bear in mind the comparatively low vapor density of oxygen, and consider at the same time to what an enormous height the hydrogen atmosphere extends, it appears probable that the height reached by oxygen would be such that the temperature of the upper portions of this gas would be considerably lower than that of the subjacent layers; so that any excess of radiation over that of the photosphere given out by the hottest portions of the incandescent oxygen would be obliterated by the absorption of the cooler portions above.

[The same reasoning can be applied if we suppose that the temperature of the oxygen falls off at some particular level; so that above this boundary the state of molecular aggregation of the gas corresponds to Dr. Schuster's "compound line" spectrum, while below this boundary the greater heat of the gas resolves its molecules into the atoms giving the ordinary line spectrum. The effect of this state of affairs is practically the same as would be brought about by annihilating a certain portion of the upper oxygen layers, since the two different molecular states of the gas give totally dissimilar spectra. We are thus reduced to an oxygen atmosphere of smaller extent, and the foregoing reasoning obtains.]

Ångström suggested that the non-appearance of the lines of oxygen and nitrogen in the solar spectrum might be accounted for by supposing that, at the temperature of the sun, the specific absorptive power of these gases may be insufficient to reverse their spectra. This view, however, equally fails to account for the brightness of the lines in question.

4. Let us now make the not improbable assumption that the temperature of the sun's nucleus, photosphere, and reversing layer is so great that dissociation is perfect throughout these

*Stoney has shown (Proc. Roy. Soc., xvii, p. 14) that a gas or vapor, even when present in only small quantity, will nevertheless extend to nearly its full height in the solar atmosphere.

He remarks (Recherches sur le spectre Solaire, Upsal, 1869, p. 37) that it is "très-probable que la température élevée du soleil ne suffit pas pour produire les raies brillantes de l'oxygène et de l'azote, et que par conséquent, même en supposant que ces corps existent actuellement dans le soleil, ils ne doivent pourtant pas occasioner de raies obscures dans le spectre solaire." He further suggests that oxygen and nitrogen may exist in the corona.

regions, but that somewhere in the higher regions, or above the chromosphere,* the temperature falls off sufficiently for some kinds of chemical combination to take place—say, in the present instance, for oxygen to combine with hydrogen. Under these circumstances we should have, concentric with, and exterior to, the chromosphere, a zone of combustion where oxygen and hydrogen, already at a very elevated temperature, enter into combination and become thereby raised to a state of more vivid incandescence.† All elements which, by virtue of their small vapor density, extended into the region of combustion, would be raised to incandescence by contact with the flaming gases, if not actually taking part in the combustion. Thus, according to the present hypothesis, we would not expect to find in the solar spectrum the bright lines of elements having a high vapor density.

5. The possibility of combination taking place in the higher regions of the sun's atmosphere is admitted by Stoney, who states that "gases in the solar atmosphere which are kept asunder by the temperature of its lower strata may be able to combine in the cooler regions above." Such combination, although arising from the cooling down of gases previously at a temperature of dissociation, would nevertheless be attended with the evolution of heat, and would possess the character of true combustion. Professor Draper also remarks, in the paper before referred to,§ that "diffused and reflected light of the outer corona could be caused by such bodies (oxygen compounds) cooled below the self-luminous point."

6. The following considerations appear to give support to the view that oxygen extends into regions sufficiently reduced in temperature for combustion to take place:

The region which is called the chromosphere is distinguishable as such through what may be called an optical accident: it is that zone of incandescent hydrogen which is rendered visible by the telespectroscope; the true boundary of the hy drogen atmosphere lies far above the visible chromosphere; and from this latter zone outward the temperature falls off rapidly.

*It is generally admitted that the true height of the chromosphere is considerably greater than that seen by means of the telespectroscope, since the amount of dispersion necessary to weaken the scattered light of our atmosphere must weaken and shorten the hydrogen lines by which the chromosphere is revealed.

It is well known that the oxyhydrogen flame does not show the lines of either of the burning gases. In the sun, however, the conditions are probably very different. The combining gases may be largely diluted with other inactive gases. Furthermore the pressure, as shown by the researches of Frankland and Lockyer (Proc. Roy. Soc., xvii, p. 288), is apparently far less in the upper regions of the chromosphere than in our own atmosphere. Both these causes would conspire to raise the point of ignition of the gases in question, so that a much higher temperature would be necessary to bring about combination than if they were undiluted and under greater pressure.

Proc. Roy. Soc., xvii.

§ Loc. cit., p. 366.