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En promenant, au coucher du soleil, le spectroscope de l'horizon vers le zénith, le spectre du ciel se montre parfaitement analogue à celui de la lumière directe de cet astre; seulement, l'intensité de la partie jaune verdâtre augmente en s'approchant du zénith, en même temps que s'affaiblit celle des parties rougeâtres. Au contraire, si l'on dirige le spectroscope sur un nuage situé près de l'horizon et qu'éclairent directement les rayons solaires, cet astre étant déjà couché, on obtient le même spectre que celui du soleil même à l'horizon ou du ciel pur dans son voisinage.

L'absorption de l'atmosphère est à beaucoup d'égards semblable à celle d'une solution de chlorophylle. La lumière transmise par une couche mince de cette solution, revêt une apparence verdâtre, mais elle devient rouge, quand on augmente l'épaisseur des couches. De même, la couleur du ciel change d'apparence avec l'épaisseur de couches atmosphériques qu'ont traversées les rayons solaires, et l'on peut regarder comme de simples phénomènes d'absorption presque toutes les variations de couleur qui caractérisent l'aurore et le rouge du crépuscule, à l'explication desquelles on a proposé déjà tant de théories différentes.

A l'égard de ces phénomènes d'absorption, il est bien probable que le rôle joué par la vapeur d'eau est très-important; mais, d'après les expériences de M. TYNDALL, on peut supposer qu'il existe dans l'atmosphère d'autres substances gazeuses pouvant produire des raies, quoique la quantité de ces substances soit si minime qu'on ne les a pu découvrir jusqu'ici par une voie directe.

Pour expliquer l'origine des bandes A, B, a et d, qui sont très-constantes et ne dépendent pas sensiblement des variations de la température de l'air, il faut recourir à d'autres corps gazcux moins variables en tension que la vapeur d'eau. Parmi ces corps, j'ai indiqué déjà l'acide carbonique, et l'on pourrait même supposer que l'ozone peut produire une action analogue, pourvu qu'il existe à l'état libre dans l'atmosphère. Comme la troisième cause, on pourrait nommer enfin la fluorescence de l'oxygène, gaz qui, renfermé dans un tube de Geissler, devient faiblement phosphorescent au passage du courant électrique.

Cependant, toute l'explication des phénomènes de coloration qui se manifestent dans l'atmosphère, se montrera sans doute très-incomplète, si l'on n'apprécie pas suffisamment l'influence d'une autre cause pouvant modifier d'une manière très-sensible l'intensité des parties cohérentes du spectre, savoir la diffusion atmosphérique et la polarisation de la lumiére qui y est étroitement liée.

7) Spectre de l'aurore boréale.

Depuis l'époque où FRANKLIN faisait ses mémorables expériences sur la foudre jusqu'au temps actuel, un parallélisme complet a eu lieu entre les actions de cette force naturelle et celles de l'électricité de frottement; ainsi, l'on a pu prévoir que le spectre de l'éclair devait être le même que celui de l'air, produit par la décharge électrique ordinaire. C'est ce qu'ont parfaitement prouvé les observations faites par M. KUNDT. De plus, les deux phénomènes de l'aurore boréale et du magnétisme terrestre, étant si intimement liés l'un avec l'autre, que l'apparition de la première est toujours accompagnée de perturbations exercées sur l'aiguille aimantée, on a donc pu supposer que l'aurore boréale n'était qu'une lueur électrique, analogue à celle que produit l'air raréfié dans l'oeuf électrique, ce qui n'est pourtant

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A. J. ANGSTRÖM, SPECTRE NORMAL DU SOLEIL.

pas le cas. En effet, pendant l'hiver de 1867-68, j'ai pu observer *) plusieurs fois le spectre de l'arc lumineux qui borde le segment obscur et se présente toujours pendant de faibles aurores boréales. Sa lumière était presque monochromatique et consistait d'une seule raie brillante, située à gauche du groupe connu des raies du calcium. En en mesurant la distance de ce groupe, j'ai déterminé la longueur d'onde de la raie, qui s'est trouvée égale à

λ = 5567.

Outre cette raie, dont l'intensité est relativement grande, j'ai observé aussi, en augmentant la largeur de la fente, des traces de trois bandes très-faibles qui s'étendraient à peu près jusqu'à F. A une seule occasion, quand l'are lumineux était agité par des ondulations qui en changeaient la forme, j'ai vu les régions signalées s'éclairer momentanément de quelques faibles raies spectrales; mais, eu égard au manque d'intensité de ces rayons, on peut dire néanmoins que la lumière de l'arc lumineux est sensiblement monochromatique.

Voici une circonstance qui donne à cette observation sur le spectre de l'aurore boréale une importance beaucoup plus grande et pour ainsi dire cosmique. Durant une semaine du mois de Mars 1867, j'ai réussi à observer la même raie spectrale dans la lumière zodiacale, qui se présentait alors avec une intensité vraiment extraordinaire pour la latitude d'Upsal. Enfin, pendant une nuit étoilée, tout le ciel étant en quelque sorte phosphorescent, j'en ai trouvé des traces même dans la faible lumière émise de toutes les régions du firmament.

Un fait fort remarquable, c'est que la raie signalée ne coïncide avec aucune des raies connues dans les spectres des gaz simples ou composés, pour autant du moins que je les ai étudiées jusqu'à présent.

Il suit de ce que je viens de dire, qu'une aurore boréale intense, telle qu'on peut l'observer au-dessus du cercle polaire, donnera probablement un spectre plus compliqué que celui trouvé par moi. Supposé que cela soit vrai, il y aura donc à espérer qu'à l'avenir on pourra expliquer plus facilement l'origine des raies trouvées et la nature du phénomène lui-même. Ne pouvant pas donner cette explication pour le présent, je me réserve d'y revenir une autre fois.

* Cette observation fut communiquée à la Société des Sciences d'Upsal le 16 Février 1868. J'eus, pendant l'automne de 1867, l'occasion de communiquer verbalement cette même observation sur le spectre de l'aurore boréale à M. O. DE STRUWE, et en Mai 1868, j'ai eu la satisfaction de recevoir l'avis qu'il l'a vérifiée à son retour à l'Observatoire de Pulkowa. Dans une notice de ces observations, insérée dans les Bulletins de l'Acad. des Sciences de St. Pétersbourg, la position de la raie est indiquée, d'après l'échelle de M. KIRCHHOFF, comme égale à 1259, avec une erreur probable de 10-15 unités de l'échelle. En évaluant ce nombre en longueurs d'onde, on trouve la valeur 5552.

MESURES MICROMÈTRIQUES

pour la détermination des Longueurs d'onde des raies de Fraunhofer.

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