SOLAR-RADIATION RESEARCHES BY JULES CÉSAR

JANSSEN.a

[With 1 plate.]

By A. DE LA BAUME PLUVINEL.

The scientific career of Janssen was passed far more in temporary observatories built in remote parts of the globe and with equipments easily transportable than in fixed establishments provided with great instruments. Janssen was thus notably a missionary of science always ready to devote himself to new efforts in the organization and leading to success of some new expedition for science. This spirit of enterprise made him love those voyages where he was sustained with the knowledge that he was devoting himself completely to science rather than working quietly in his laboratory.

The principal missions undertaken by Janssen had for their objects the observation of phenomena observable only from some limited. portion of the earth or where he must find a sky favorable to some delicate experiment. But in either case, Janssen carried on researches which had the same goal, so his work presents a remarkable unity. We might indeed say that all his studies were made upon the selective absorption for radiation by gases. His devotion to this class. of researches was determined by Kirchoff and Bunsen's discovery of absorption spectra; indeed the first spectroscopic experiments of Janssen date from 1862, very shortly after the German physicists had published their results.

Janssen studied the absorption for solar radiation, on the one hand, by the surrounding envelopes of the sun itself, on the other, by our own terrestrial atmosphere. It was while observing these gaseous envelopes of the sun, which may be seen only during the few short moments of a total solar eclipse, that he accomplished the first part of this programme, and in undertaking his classic researches on the telluric lines, the second. We will follow Janssen through his studies in these two groups of problems.

• Translated by permission from Astrophysical Journal, September, 1908.

Janssen impatiently waited for the total solar eclipse of 1868, for it was to furnish him the opportunity of studying for the first time with the spectroscope these solar envelopes. In order to prepare himself in some manner for the observation of this important phenomenon Janssen asked that he might be sent, in 1867, to Trani, in Italy, to observe an annular eclipse of the sun. His purpose for studying the spectrum of the sun during the annular eclipse was to see if he could then detect any trace of absorption due to the solar envelopes. But he found the spectrum of the annulus the same as that from the central part of the sun's disk. During this same eclipse he unsuccessfully tried to see the corona.

But the eclipse of 1868 had in store for him the honor of making a great discovery. We know that, after having seen in his spectroscope the brilliant rays of the protuberances, which appeared during the totality, that he did not hestitate to affirm, with all the authority which his great experience in spectroscopy allowed, that he would be able to see these same rays without an eclipse. On the next day he had the joy of confirming his predictions.

We know that Sir Norman Lockyer made the same discovery independently in England, so that the names of these two scientists are associated with this so fertile application of spectrum analysis. Janssen appeared to realize from the very first day all the importance of the discovery which he had just made. Of this we find the proof in a letter which he sent to his mother and in which he said: "I read to-day from a book which, until now, was closed to all and from which but glimpses could be got during the few short moments of a total eclipse."

The results obtained from the eclipse of 1868 were too enticing for Janssen to stop on the path which he had just opened for astronomers, and so he decided to go to Algeria for the total eclipse. of December, 1870. He was unfortunately prevented from observing this eclipse on account of bad weather, but it furnished remarkable evidence of his devotion to his work. Apparently imprisoned in Paris by the siege, Janssen did not fear to risk the dangers of a balloon voyage in order to break through the lines of the enemy. This act of courage did more to make him popular than his beautiful discovery about the prominences and, for the public which realized the real dangers of that time in daring a voyage through the air, this audacious way of escaping from the beleagured capital gave a measure of the devotion to science of which Janssen was capable. Janssen retained from this aerial trip, made under such perilous conditions, a sincere love for aerial navigation. Many occasions came for proving his interest in this science in giving to aeronauts valuable counsel, in consenting to preside over various

aeronautical meetings, and in giving bountiful hospitality, at Meudon, to the International Congress of Aerostation.

In the year after the eclipse of 1870 there came another, visible this time in India and Java. Janssen was careful not to lose this new opportunity for the examination of the solar envelopes. A careful study of the meteorological aspects of the various places where the eclipse might be visible made him adopt a station in India, in the Neelgheries; the outcome showed the justice of his choice, for it would have been scarcely possible to observe an eclipse under more favorable conditions. This time Janssen confined his attention principally to the corona. He noted in the spectrum of this, not only the green ray, whose presence was already known, but also dark lines, indicating that a part of the light of the corona is reflected sunlight, and tending to prove that the coronal envelope is not exclusively gaseous but composed partly of liquid or solid particles. In 1875 we again find Janssen observing an eclipse, this time near the island of Malacca, on the return voyage from a trip to Japan. Then, in 1883, with no fear for the fatigue of a difficult voyage, he went to the island of Carolina, in the middle of the Pacific Ocean, in order to observe a total solar eclipse remarkable for the duration of totality. Thanks to the silver-bromide gelatine dry plates, which had then just been invented, he was able to photograph the phenomena of this eclipse under very varied conditions, and brought back data of the greatest interest on the extent of the solar corona. Before terminating this work Janssen wished to observe for a last time these beautiful phenomena, which for him had always had such a fascination. So, in 1905, despite his advanced age, he went to Spain to have the pleasure of seeing an eclipse, more as a curious human being than as an astronomer.

We have now seen what Janssen accomplished in the investigation of the gaseous surroundings of the sun by the application of the spectroscope to the study of solar eclipses. We will now pass in review his work upon the absorption of our own atmosphere for the radiation from the sun.

The first of these spectroscopic studies relates to the black bands which appear in the sun's spectrum as the sun nears the horizon. These had already been noted by Sir David Brewster, but the latter recognized neither the real structure of these bands nor the cause of their appearance. By observations made in Rome from 1862 to, 1863 Janssen found that these bands were resolvable into lines, and proved that their origin must be attributed to the selective absorption produced upon the solar rays by the gases of our own atmosphere. Later it was found that the oxygen in the air produced the bands A, a, and B, which appear in the solar spectrum. But the oxygen of our atmosphere might not be the only oxygen giving

birth to these lines, for its existence in the envelopes of the sun could as well play in the production in the phenomena. With respect to the theory of the sun, it is of the greatest importance to know whether oxygen coexists with hydrogen in the solar envelopes. Janssen, indeed, attributed to the question of this existence of oxygen in the sun a capital importance, and so he searched by all the methods available to find out whether the bands, A, a, and B, had their origin in both solar and terrestrial absorption or were produced solely by our atmosphere. In order to solve this problem he produced these absorption bands in his own laboratory in such a manner as to see whether a column of oxygen equivalent to the oxygen contained in the air would produce bands of the same intensity as those we observe in the solar spectrum. The same train of thought caused him to observe the spectrum of a luminous source so far distant that the air intervening between it and his apparatus would produce an absorption equivalent to that of all the air between us and the sun and at different heights of the sun above the horizon. Then we find him making, in a sense, the reverse experiment-diminishing sufficiently the action of the interposed air until the bands under examination are no longer visible.

The study of gaseous spectra was carried on very extensively by Janssen. He examined at his spectroscope the light from a source producing a continuous spectrum after this light had traversed tubes containing gases under various pressures and temperatures. His laboratory, situated in the old stables of the chateau at Meudon, allowed him to use tubes reaching a length of 60 meters. His researches related chiefly to oxygen. By varying the pressure of the gas in the tube he could at will make the absorption lines of oxygen disappear, especially the line B. His experiments showed that certain absorption lines-B, for instance-always appeared when the product of the length of the tube by the pressure of the gas reached a certain value.

The experiments upon the absorption lines of oxygen led Janssen to a remarkable phenomenon which required repetition with the perfected means at the disposal of modern physics. He discovered that, besides the telluric rays, the absorption spectrum of oxygen showed, under certain conditions, a system of bands difficult to resolve into lines, and whose production was ruled by an entirely different law than that stated above for the telluric lines. These bands appear when the product of the length of the tube by the square of the pressure reaches a certain value. This law had a remarkable confirmation when Olszewski studied the spectrum of liquid oxygen. It was found that the bands of Janssen appeared when the layer of liquid oxygen reached a thickness which the law of the square of the pressure would require. Janssen confirmed his law in yet another

manner. He calculated that when the height of the sun above the horizon is less than 4 degrees the thickness of the layer of air traversed by the solar rays would be sufficient to produce these lines. And so he went to the Desert of Sahara in order to observe the sun under these conditions and noted the presence of these bands when the sun had reached precisely this altitude of 4 degrees.

This remarkable law must have an importance for the theory of molecular physics which has not yet been sufficiently appreciated.

In his researches at the laboratory of Meudon, Janssen was not content with trying the effect of the variation of the length and pressure of his column of gas traversed by the light; he also raised the gas to high temperatures in order to approach somewhat the conditions as they exist in the sun. By electrical means, very remarkable for the time when they were devised, Janssen was able to raise his gas to a temperature of 900° C. No new phenomenon appeared at that temperature, but the visibility of the absorption bands was somewhat increased.

The absorption produced by water vapor also was studied at Meudon. Already at the beginning of his spectroscopic studies in 1867, Janssen had observed the absorption spectrum of water vapor by causing the luminous rays to pass through a tube 37 meters long filled with this vapor. This remarkable experiment, made at the gas manufactory of Vilette, allowed Janssen to recognize the principal lines due to the absorption of water vapor. This research was taken up under better conditions and with better apparatus at the laboratory at Meudon in 1887.

The object of this study of the spectrum of water vapor was to find out whether it exists in the atmospheres of the planets. This question, which is of capital importance to astronomers, always very greatly interested Janssen. In 1867 on Mount Etna, and then in 1869 on the Himalaya Mountains, Janssen observed the spectrum of Mars to see whether he could detect in its spectrum the principal lines due to the presence of water vapor. To that end he compared the spectrum of Mars and the moon when these two bodies were at the same altitude above the horizon. Janssen concluded from his observations that the spectrum of Mars gave plain evidence of the presence of the vapor of water in the atmosphere of that planet and he considered this evidence sufficiently decisive to maintain these conclusions when, in 1895, Campbell announced that the great instruments of the Lick Observatory would not show to him the trace of any water vapor on Mars. And now, very recently, Mr. Slipher, of the Lowell Observatory, has obtained photographs in which the lines due to water vapor appear more intense in the spectrum of Mars than in that of the moon. This seems to support Janssen's conclusions.

45745°-SM 1909-17