Extract from the Index Table of Kirchhoff's Maps, showing the coincidences of the dark Solar and bright Metallic Lines.

Whether these apparently coincident lines will prove to be absolutely identical is a matter which we cannot as yet decide. Kirchhoff thinks it is necessary, for the purpose of settling this question, to use a much more delicate apparatus than even that which he employed.

Fraunhofer's line D corresponds on Kirchhoff's map to the lines 1002.8 and 10068; Fraunhofer's E to the lines 1523.7 and 15227; and Fraunhofer's b to the lines 1633 4, 1648.3, and 1655'0. Kirchhoff observed the traces of many lines and nebulous bands, which the power of even his instrument did not prove adequate to resolve. He adds: "The resolution of these nebulous bands appears to me to possess an interest similar to that of the resolution of the celestial nebulæ, and the investigation of the solar spectrum to be of no less importance than the examination of the heavens themselves." It is important to remark that it is by no means the case that

all the lines have been identified; the cause of many of them is known, but a still greater number yet remain for identification. I may again remind you that the wellknown double line D is caused by sodium, and, from the exact observations of Mr. Huggins, not only the line D but several other less distinct lines, seen on the Maps following Lecture IV., one lying nearly neutral between the D lines, are produced by sodium in the sun. The line is an iron line, and the lines c, F, and & are hydrogen

E

lines; the line b is a line of magnesium, and the line H appears from the researches of Angström to be at any rate partly produced by calcium. Many, however, of the lines seen in the solar spectrum are not due to the presence of metals in the sun, but are caused by the absorption occurring in our own atmosphere. The existence of dark bands caused by atmospheric absorption was first pointed out by Brewster in 1833, and a map of these bands was subsequently published by Sir David

BREWSTER'S AIR BANDS.

Brewster and Dr. Gladstone. Fig. 56 shows the chief of these lines compared with the solar lines and the bright lines of nitrogen and oxygen.

Some very interesting experiments were made in 1866 by the French physicist, M. Janssen: he has observed that if light from 16 jets of coal gas be passed through a long column of steam 37 metres in length, under a pressure of 7 atmospheres, the steam exerts a strong absorptive power; groups of dark lines appeared in the spectrum between theextreme red and the line D. These lines are found to coincide with lines in the solar spectrum which become intense when the sun is near the horizon, and are therefore due to ab

sorption in the aqueous vapour of our

own

J. JANSSEN.

SOLAR SPECTRUM

FROM

C TO D

IS (1) IN THE MERIDIAN, (2) NEAR THE HORIZON.

290

295

atmosphere. An accurate map of the telluric lines quite recently published by Janssen between D and c is given in Fig. 57. The important results of his researches on this subject are (1) that Brewster's dark bands are resolved into fine lines comparable with Fraunhofer's lines, and (2) that the terrestrial atmosphere produces in the spectrum a system of fine lines, so that the absorptive action exerted by our atmosphere is analogous to that of the sun in spite of the enormous difference of temperature. All the dark lines seen in the lower but not found in the upper spectrum (Fig. 57) have a telluric origin, and they have been designated by the Greek letters, and are classed in groups according to their position with regard to well-known solar lines.

I do not know that I can do better in conclusion than give you Professor Kirchhoff's exact opinions on this subject, by reading a short extract from his chapter on the "Physical Constitution of the Sun." "In order to explain," he says, "the occurrence of the dark lines in the solar spectrum, we must assume that the solar atmosphere encloses a luminous nucleus, producing a continuous spectrum,1 the brightness of which exceeds a certain limit. The most probable supposition which can be made respecting the sun's constitution is, that it consists of a solid or liquid nucleus heated to a temperature of the brightest whiteness, surrounded by an atmosphere of somewhat lower temperature. This supposition is in accordance with Laplace's celebrated nebular theory respecting the formation of our planetary system. If the matter now concentrated in the several heavenly

1 This continuous spectrum is most probably derived from incandescent solids or liquids, but may, under certain conditions, be given off by luminous gases.

bodies existed in former times as an extended and continuous mass of vapour, by the contraction of which sun, planets, and moons have been formed, all these bodies must necessarily possess mainly the same constitution. Geology teaches us that the earth once existed in a state of fusion; and we are compelled to admit that the same state of things has occurred in the other members of our solar system. The amount of cooling which the various heavenly bodies have undergone, in accordance with the laws of radiation of heat, differs greatly, owing mainly to the difference in their masses. Thus, whilst the moon has become cooler than the earth, the temperature of the surface of the sun has not yet sunk below a white heat. Our terrestrial atmosphere, in which now so few elements are found, must have possessed, when the earth was in a state of fusion, a much more complicated composition, as it then contained all those substances which are volatile at a white heat. The solar atmosphere at this time possesses a similar constitution."

I am almost afraid to allude to the physical peculiarities of the solar surface, but I must mention some remarkable results of the examination of the solar surface by Mr. James Nasmyth. He finds that the well-known mottled appearance of the sun's surface is due to the presence of peculiar willow-leaf-shaped masses, which are constantly moving with great velocity over the surface of the sun. The same phenomenon has also been observed by Mr. Stone of the Greenwich Observatory, and many other astronomers. What these are no one I believe can tell.

One most important series of observations we may all look forward to with the greatest interest; namely, the observations about to be made in India during the total solar eclipse in August next. By the kindness of the