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Janssen, Pierre Jules César

Janssen, Pierre Jules César

(b. Paris, France, 22 February 1824; d. Meudon, France, 23 December 1907)

physical astronomy, spectroscopy, photography.

Janssen was born into a cultivated family. His father was a musician of Belgian descent, and his maternal grandfather was the architect Paul-Guillaume Le Moyne. An accident in his early childhood left him permanently lame. He was thus kept at home and never attended school. Financial difficulties obliged him to go to work at an early age. While working for a bank from 1840 to 1848, he devoted himself to completing his education and earned the baccalaureate at the age of twenty-five.

Janssen attended the University of Paris, receiving his licence ès sciences in 1852. He then obtained a post as substitute teacher in a lycëe. In 1857 he was sent on an official mission to study the position of the magnetic equator in Peru. He contracted a severe case of dysentery and had to return to France, where he agreed to become a tutor for the Schneider family, who owned iron and steel mills in Le Creusot.

Janssen’s first scientific work was a study of the absorption of radiant heat in the mediums of the eye (“Sur I’absorption de la chaleur rayonnante obscure dans les milieux de I’oeil,” in Annales se physiqueet de chimie, 3rd series, 60 , 71-93). He showed that the mediums are transparent only for visual rays and that the focalization of the thermal radiation has no harmful effect on the retina because ninetenths of the radiation is absorbed. This carefully executed work earned him a doctorate of science in 1860. The work actually had no real scientific significance; the conclusion could be expected because the absorbing mediums, being aqueous, have precisely the properties of water. Yet it is of interest because of the way in which Janssen was led to undertake it. In it he wrote;

. . . having often had the opportunity to be present during the tapping of blast furnaces, I noticed that the radiation from the bath of molten metal . . . in no way affects the eyes; thus one can follow without fatigue the various phases of the operation if one takes the precaution of protecting the face with a mask that exposes only the eyes. This absorption by the mediums of the eye having appeared to me to be an important physiological fact, I proposed to verify and measure it by precise experiments.

Throughout his life, when a phenomenon aroused Janssen’s curiosity, he immediately studied it. In the case of the blast furnace he perfected a delicate experimental device that permitted, in particular, the measurement of weak radiations by suitable adaptation of Melloni’s thermopile; and in less than six months he carried out the essential portion of his study.

In October 1859, in a celebrated report, Kirchhoff demonstrated the presence of terrestrial elements in the constitution of the sun. Janssen immediately realized that the study of the radiation from the Le Creusot furnaces would never allow him to make discoveries so splendid as that of solar radiation, and he decided to direct his career toward physical astronomy.

In Paris, where in 1862 he had come to work with E. Follin of the Faculty of Medicine on the construction of an ophthalmoscope, Janssen mounted a small observatory on the flat roof of the house that his wife owned north of Montmartre. Here he began work on a problem posed by Brewster in 1833, that of the nature of certain dark bands in the solar spectrum, bands irregular in presence and most noticeable at sunrise and sunset. For this purpose Janssen constructed a spectroscope possessing a high dispersive power and furnished with a device for regulating the Iuminous intensity. He was able to establish in 1862 that these spectral bands resolve into rays and that their presence is permanent. On a mission to Italy in the following year he demonstrated with precision that the intensity of therays varies in the course of the day as a function of the density of the terrestrial atmosphere traversed. The terrestrial origin of the phenomenon was demonstrated-thus Janssen proposed for them the name “telluric rays.”

In 1864 Janssen moved to the Bernese Alps, at an altitude of 2,700 meters, to verify that the intensity of the telluric rays is lessened in the mountains. The weakening exceeded that expected on the basis of the decrease in density of atmosphere traversed, and Janssen attributed it partly to the dryness of the air. In order to verify his supposition he studied, at Geneva, the telluric rays of the spectrum of an artificial source (in this case a wood fire) situated twenty-one kilometer aways on the shores of Lake Leman. His results confirmed the effect of humidity on the intensity of the rays. Janssen then turned to the direct study of the absorption rays of water vapor, carrying out his experiment in a gasworks near his home and using an iron tube thirty-seven meters long to hold the water vapor under a pressure of seven atmospheres. The source, illuminating gas, which ordinarily yields a continuous spectrum, furnished a spectroscopic image displaying most of the telluric rays.

Janssen had stated at the beginning of his research that the existence of telluric rays entailed the possibility of making a chemical analysis of the atmospheres of the planets. His determination of the water vapor spectrum was a major step in this direction. As early as 1867 he was able to announce the presence of water vapor in the atmosphere of Mars.

In the following year Janssen made another important contribution to knowledge of solar structure. In order to observe the total eclipse of 18 August 1868, which was visible in India, Janssen went to the city of Guntur, near the Bay of Bengal. His aim was to study the solar prominences. Keeping the slit of the spectroscope on the lunar limb, he was able to observe highly luminous spectra while the sun was in eclipse. Visual observation in a finder showed that these spectra came from two great prominences. Janssen measured the position of the brightest rays: they corresponded to rays C and F of the solar spectrum, which are produced by hydrogen.

The brightness of the rays led Janssen to suspect the possibility of observing the prominences even when there was no eclipse. The next day he resumed his observation of the solar limb, admitting only the red portion of the spectrum. He ascertained first that a bright line appeared in the exact extension of a dark line of the solar spectrum, the C ray. Exploring the contour of the sun, Janssen observed the variations in the intensity of the line and the modifications in its structure. He also made other bright lines appear, all of them corresponding exactly to the dark lines of the absorption spectrum. “Thus was demonstrated the possibility of observing the lines of the prominences outside of eclipses, and of finding therein a method for studying these bodies” (Annuaire du Bureau des longitudes for 1869, p. 596).

From 18 August to 4 September Janssen worked on establishing maps of prominences. He continued his observations at Simla, in the Himalayas. On 25 December he wrote that the solar photosphere is surrounded.

. . . by an incandescent atmosphere, the general, if not exclusive, base of which is formed by hydrogen. . . . The atmosphere in question is low, [its] level very uneven and broken; often it does not rise above the projections of the photosphere, but the remarkable phenomenon is that it forms a continuous whole with the prominences, the composition of which is identical and which appear to be simply raised portions of it, projected and often detached in isolated clouds [Competes rendus hebdomaddaires des seauteances de l’ Académic des sciences, sciences, 68 (1869), 181].

Several other astronomers worked in these areas during this period, notably Lockyer, who arrived at the same method of analyzing the prominences as Janssen did; but it was the latter who, in the course of his stay at Simla, created the first spectrohelioscope. He described the essential device of this apparatus as follows:

. . . a metallic diaphragam, placed at the focus of the spectroscope, and pierced by a slit at the precise point where one of the bright lines of the light from the prominences appears, permits complete separation of this light from that of the photosphere, which lacks the bundle of rays of precisely this degree of refrangibility. . . . This focal slit. . ., when combined with a rotary movement imparted to the spectroscope, makes it possible to obtain the series of monochromatic images that a luminous body is capable of furnishing [ibid., pp. 713-714].

This discovery facilitated daily examination of the sun.

For carrying out his investigations Janssen received official subsidies at the times of his missions. In addition, in 1865 he was appointed professor of physics at the École Spéciale d’Architecture. Yet, in France at least, he had little more than his home to use for his technical and experimental work. In 1869 the minister of education, Victor Duruy, tried to find him an observatory. Janssen had had the opportunity to make several measurements at the Paris observatory but was not able to intall himself there permanently, since the director, Le Verrier, considered the establishment to be his personal property. Janssen was offered the pavilion of Breteuil in Sévres, but the Franco-Prussian War prevented his using it.

In 1874 the French government decided to establish an observatory for physical astronomy. Janssen had the choice of two sites: Malmaison (the former residence of Empress Josephine) or Meudon, better located in terms of climate. Janssen chose Meudon and it was granted to him in 1876.

The estate at Meudon was in ruins when Janssen moved there in October 1876. He commenced repairs on the buildings and began to prepare the astronomical equipment. The offices and laboratories were lodged in the principal part of the estate, which formerly consisted of a modest chateau, stables, and other outbuildings. A separate building, the Chateau Neuf, built by Mansart in 1706, was restored and topped by an astronomical dome 18.5 meters in diameter.

Janssen had hoped he would rapidly acquire the means necessary to extend his investigations, which until then he had conducted with small instruments and improvised devices. Soon, however, there were financial difficulties. It became necessary to use funds budgeted for research for the completion of the buildings, a task that required twenty years. The staff was insufficient moreover—until 1906 there were only two astronomers. Nevertheless Janssen was able to endow the observatory with two large instruments: a double refractor of sixteen meters with a visual objective of eighty-three centimeters (the largest in Europe) and a photographic objective of sixty-two centimeters; and a telescope with an aperture of one meter and a focal length of three meters. For the spectral investigation of gases, he also set up a large laboratory with a steel tube sixty meters long, closed by thin transparent plates and capable of supporting a pressure of 200 atmospheres.

The most famous of Janssen’s projects at Meudon during this period was the atlas of solar photographs. Composed of a selection of exposures made between 1876 and 1903, it summarized the history of the surface of the sun during these years. Janssen employed a photoheliograph of his own design. Its telescope was achromatized for violet radiations; and its shutter, which had a movable and variable slit, permitted exposures on the order of 1/3,000 of a second.

It was not possible to make all the solar observations at Meudon. Janssen was well aware of the advantages of observing at high altitudes. Wishing to know whether the dark rays of oxygen are entirely telluric or whether certain of them are present before the radiation reaches the earth’s atmosphere, he went in October 1888 to the Mont Blanc massif, to the refuge of the Grands-Mulets at an altitude of 3,000 meters. His age and his lameness did not allow him to make the climb on foot, especially at that season. Thus he invented a conveyance to be borne by porters. It consisted of a seat fixed under a horizontal ladder: the upper part of his body emerged between two rungs in such a way that his arms were supported by the uprights. The ascent, which lasted thirteen hours, was as exhausting for Janssen as for his porters. But the instruments were installed immediately; and the observations, which he was able to make during the whole of the third day, were sufficient to provide a solution to the problem under study. The dark rays were either nonexistent or so weak that it could be deduced that they would not exist for an observer at the limit of the terrestrial atmosphere.

Encouraged by this experiment, Janssen repeated it in 1890, this time at the summit of Mont Blanc (4,800 meters). The measurements confirmed the earlier results. Despite the difficulties encountered (the caravan left Chamonix on 17 August but did not arrive at the summit until 22 August), Janssen decided to erect an observatory there for conducting studies in physical astronomy, terrestrial physics, and meteorology. By July 1891 he had gathered the necessary funds and equipment, and two years later the observatory was completed. The initial stages were completed at Meudon, where a fifteen-ton building had been set up, which was then transported to Mont Blanc in pieces. Each piece had to weigh less than thirty kilograms so that it could be carried by porter to the summit.

Although his observatory did not withstand the rigors of the weather, Janssen had set a splendid example by his energy and unfailing courage. In 1897 the annual Mont Blanc expedition set out to determine the solar constant. Janssen had broken a leg on the staircase of the large dome at Meudon and was unable to manage the climb. He nevertheless arranged to be carried on a stretcher to Chamonix in order to organize the work of his collaborators. He was then seventy-three.

The most famous instance of Janssen’s adventurous spirit occurred during the Franco-Prussian War. He had planned ro observe the eclipse of 22 December 1870 in Algeria. On 24 October, while Paris was under siege, he wrote to the Academy of Sciences:

Despite the very critical circumstances . . . that prevail in our country at this moment, it does not seem that France should abdicate and renounce taking part in the observation of this important phenomenon. Despite the siege . . . an observer would be able, at an opportune moment, to head toward Algeria by the aerial route; he would carry with him only the most indispensable parts of his instruments.

A balloon, the Volta, was placed at his disposal, and Janssen left Paris with an assistant on 2 December. He headed west at an altitude of 2,000 meters and descended in sight of the Atlantic coast. In spite of a violent wind he succeeded in making a good landing, and his instrument cases remained intact. The weather proved to be unfavorable for the observation of the eclipse.

Janssen later profited from his experience as a balloonist to think out an aeronautic compass designed to furnish instantly direction and speed of flight by observation of the apparent movement of the ground. He never ceased to be interested in aeronautical problems, the importance of which he foresaw. Opening the International Aeronautical Congress held at Paris in 1889, he declared,

. . . the twentieth century . . . will see the realization of great applications of aerial navigation and the terrestrial atmosphere navigated by apparatuses that will take possession of it to make a daily and systematic study of it or to establish among nations communications and relations that will take continents, seas, and oceans in their stride,

A prophet of aviation, Janssen was perhaps also the precursor of observations from outside the atmosphere. He was interested, for example, in the meteor shower of the Leonids, which appears about mid-November, a period generally unfavorable for observations in Europe. The determination of its intensity, which attains a maximum every thirty-three years, was of great interest to celestial physics. Janssen had the idea of undertaking the observations in a balloon above the cloud layer. He obtained a balloon for this exploit in 1898 and for several others in the following years (in the course of which balloon observations were also carried out abroad at his request). In 1900 he wrote,“The application of balloons to astronomical observations is destined to render to this science services whose extent is difficult to measure today” (Comptes rendus hebdomadaires de séances de l’ Académir de sciences, 131 [1900], p.128).

Janssen, who thought that “the photographic plate is the retina of the scientist” (L’astronomie, 2 [1883], 128), was one of the first to understand that a photograph can do more than record what the eye perceives.“I realized that photography ought to have distinct advantages over optical observation in bringing out effects and relationships of light that are imperceptible to sight” (Association française pour I’avancement des science [Le Havre-Pairs, 1877], p. 328). He subsequently made photographs and, through the brevity of the exposure and a special method of developing, he obtained images which disclosed a new and scientifically true aspect of the solar surface. The technique of short exposures led Janssen in 1873 to conceive of a device of historical interest, the photographic revolver.

In planning for the observation of the transit of Venus, which he was to observe in Japan on 9 Deembe 1874, Janssen decided to substitute for visual observation at the time of transit a series of photographs taken in rapid succession, which would permit him to measure the successive positions of the planet in relation to the solar limb. He ordered the construcion of an apparatus consisting of three circular disks with the same axis: the first, pierced by twelve slits, served as the shutter;the second contained a window; the photographic plate, which was circular, was fixed to the third. The first two disks turned with a synchronized movement, the shutter disk continuously and the other irregularly in the intervals of time in which the window was not swept by a slit. A series of separate images laid out on a circle was thus obtained on the plate. In a general manner the apparatus provided an analysis of a motion on the basis of the sequence of its elemental aspects. Here Janssen realized one of the operations necessary for cinematography, which was invented twenty years later, and which required, besides analysis, the synthesis of images.

Even in fields in which he was not a specialist, Janssen displayed astonishing insight. In 1865, in a course designed for architects, he set forth the Pinciples of effective illumination:“In retail stores light is squandered in the least intelligent manner;. . .” instead of seeking power, . . . would it not be preferable to adapt the luminous intensity to the objects, . . . and to avoid above all those radiant points that are so fatiguing for our sight?” (Oeuvres scientifiques, I (1929), 111-112). At the same time he recommended the soundproofing of apartment buildings:

Since the increasing value of space in our big cities has imposed the necessity of joining, under the same roof, a large number of families. . . since the rooms have diminished in area and in height in order to increase in number, and all the common walls . . . have become thinner, our dwellings present an intolerable resonance to which the promptest remedy urgently needs to be applied. On this point, almost everything remains to be done [ibid., p.110].

Janssen had acquired an international reputation from the start of his scientific work. He was elected to the Academy of Sciences in 1873 and to the Bureau of Longitudes in 1875 and was also a member of the academies of Rome, Brussels, St. Petersburg, Edinburg, and the United States. He carried out his duties as director of the Meudon observatory until his death, which was the result of a pulmonary congestion.

It is not surprising that Janssen was able to accomplish so much that he undertook. As he himself wrote,“There are very few difficulties that cannot be surmounted by a firm will and a sufficiently thorough preparation”.


I. Original Works. Between 1859 and 1907 Janssen published approximately 350 items, almost all of which were reprinted in Oeuvres scientifiques de Jules Janssen, H. Dehéerain, ed., 2 vols. (Paris, 1929-1930). Janssen “scientific works originally appeared mainly in the Comptes rendus hebdomadaires des séeances de L” Acadéemie des science and in the Annuaire du Bureau des longitudes. Janssen collected about thrity of his speeches in Lectures acadéemiques. Discours (Paris, 1903). In addition, in 1896 he founded the Annales de l’Observation d’astronomie physique de Paris, sis au Parc de Meudon. The first volume, which he himself wrote, is devoted to the establishment of the Meudon observatory.

II. Secondary Literature. A list of biographies and obituaries is included in the Oeuvres scientifiques, II, 632-634. The most important accounts are G. Bigourdan, “J. Janssen,” in Bulletin astronomique, 25 (1908), 49-58; A. de la Baume Pluvinel, “Jules Céesar Janssen,“in Astrophysical Journal, 28 (1908), 88-99; and R. Radau and H. Deslandres, “Discours prononcées aux funéerailles de Janssen,” in Annuaire du Bureau des longitudes (1909), pp. CI-C11.

See also G. Bigourdan, H. Deslandres, Prince R. Bona-parte, C. Flammarion, P. Renard, and M. Dubuisson, Inauguration de la statue de Jules Janssen (Paris, 1920).

Jacques R. LÉvy

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"Janssen, Pierre Jules César." Complete Dictionary of Scientific Biography. . 29 Mar. 2017 <>.

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Janssen, Pierre Jules César

Pierre Jules César Janssen (pyĕr zhül sāzär´ zhäNsĕn´), 1824–1907, French astronomer. In 1857–58, in Peru, he worked on the determination of the magnetic equator; in the Azores (1867) he examined magnetic and topographical conditions; and in Japan (1874) and in Algeria (1882) he observed the transit of Venus. Janssen accompanied various solar eclipse expeditions, notably that to Guntur, India, in 1868, where he devised a new method of studying the solar prominences spectroscopically and discovered, almost simultaneously with J. N. Lockyer, the chemical constitution of the prominences. He was active in the establishment of the astrophysical observatory of Meudon in Paris and in 1876 became its director. There he gathered an important series of solar photographs included in his Atlas de photographies solaires (1904). He later became director of the observatory on Mont Blanc.

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"Janssen, Pierre Jules César." The Columbia Encyclopedia, 6th ed.. . 29 Mar. 2017 <>.

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