the spectrum of the umbra to about the same degree of invisibility the wedge had to be moved until the part marked 20 came before the eye. A photometric examination of the wedge shows that the light intercepted at 10 is to that intercepted at 20 as 1 to 3, It may then be concluded that about three-fourths of the light which formed the spectrum of the umbra was really due to the umbra of the spot.
There were several bright granules on the umbra, but the spectra of these were seen distinct from that of the umbra. Each bright point as it came upon the slit gave a narrow spectrum like a bright thread extending along the dark spectrum of the umbra.
There still remained two sources of uncertainty. 1. In consequence of the mode in which the spectrum is formed, under similar conditions of the instrument, the dark lines should appear rather thicker when the light is feebler. 2. The increased thickness of the lines in the compound spectrum might be due to the light of the umbra, or to that of our atmosphere.
The uncertainty on both these points was removed by observing the feebler spectrum of the illuminated atmosphere near the sun’s limb. The lines in this spectrum, though they appeared very slightly stronger, were not so in a degree that could afford an explanation of the very marked increase of strength which most of them presented in the spectrum of the umbra. It seemed, therefore, satisfactorily determined that the light from the umbra had really suffered a more powerful absorption. The term umbra is used to include the cloudy stratum and the darker nucleus into which Mr. Dawes has shown the umbra of a spot may be usually resolved. It is probable that nearly the whole of the light under examination came from the part of the umbra known as the cloudy stratum. It was not possible to distinguish the spectra of these portions of the umbra.
The spectroscope was sufficiently powerful to show all the lines which are given in Kirchhoff’s maps.
I carefully examined the spectrum of the umbra with that of the adjoining parts of the solar surface from A to G, but I was not able to detect any line of absorption in the spectrum of the umbra which was not also present in that of the sun’s normal surface, or that any ordinary solar line was wanting in that of the umbra.
The increase of thickness, however, did not appear to take place in the same proportion for all lines. The lines C and F, due to hydrogen, appeared increased but very slightly, if indeed they were any thicker than would be due to a spectrum of feebler intensity. I incline to the opinion that these lines are not sensibly altered.
There is a small group of lines a little less refrangible than b, at 1601 to 1609, of Kirchhoff’s scale, and which in his map are marked as coincident with lines of chromium, which was especially noticeable from increased thickness. That this circumstance was not connected with any peculiarity of the spot under examination is shown by a similar observation having been made on other spots.
Fig. 3, Plate XXXIII. represents the appearance of the double line D in the spec-
4 G 2
tram of the umbra. The line nearly central between the two lines may be due, in part at least, also to sodium*. These lines appeared slightly broader, as if by the addition of a faint and narrowed nebulosity at both edges.
The group of lines at B are stronger, also b and E. Many of the lines marked in Kirchhoff’s map as coincident with iron appeared much stronger in the spectrum of the spot.
The absence of sensible increase in F was marked in comparison with a line or lines which were very strong, situated at a little distance on the less refrangible side of F— probably those of 2066-2 and 2067'1 of Kirchhoff’s map.
It may be well to consider some of the conditions of the solar surface by which the phenomena observed may have been produced. A cooler state of the heated vapours by which the lines of absorption are produced, would diminish the radiation from the gas itself, and'so leave more completely uncompensated the absorption by the gas of the light from behind it. This cause would produce increased blackness of the lines, but would not account for more than a slight apparent increase of breadth. The greater breadth of the lines seems to point rather to a condition of the gases in which their power of absorption embraces for each line an increased range of wave-length. That the power of absorption of gases varies in this respect is shown by the increase of breadth which some of the bright lines of some gases assume under altered conditions of tension and temperature. It will be sufficient to refer to the expansion of the lines of hydrogen as the tension increases. A similar increase in the range of its power of absorption on light passing through it should take place under similar conditions of density and temperature.
The phenomena may point to an increase of density in the vapours existing within the umbra. Such a state of things would necessarily exist at a point somewhat nearer the sun’s centre; but we do not know through how great a depth of gas below the photosphere we receive the light which comes from the umbra. Our views on this point will be connected with the interpretation we give to Mr. Dawes’s discovery of the existence within the umbrae of spots of a still darker part almost wholly devoid of light. Does this nucleus represent a more complete unveiling of the inner part of the sun ? or does it show a still cooler and less luminous part of the down-rushing solar atmosphere'? The latter suggestion, which is in accordance with the explanation of sun-spots proposed by Dr. Balfour Stewart, would seem to connect a lower temperature with the broader lines of absorption.
Some information might be gained if we could view the spectra of the dark pores of the solar surface, an observation which is perhaps not impossible, since these pores are of varying degrees of darkness, and are probably due to conditions of the solar substance
* The spectrum of the sodium at a high temperature is much more complex than was supposed. In addition to the three double lines besides D and a nebulous band, described in my paper “ On the Chemical Elements,” Philosophical Transactions, 1864, p. 147, there is a line nearly central between the lines D. It is possible there may be also other lines in the interval between the strong lines Dx and D2.
