[Footnote 16: It does not give _both_ temperature and pressure, but it gives one if the other is known. This is valuable information which may be pieced together with other knowledge of the conditions at the surface of the stars.] [Footnote 17: Hydrogen (being element No. 1) has only one planet electron.] [Footnote 18: Fig. 9 is a photograph of the ‘flash spectrum’ of the sun’s chromosphere taken by Mr. Davidson in Sumatra at the eclipse of 14 January 1926.] [Footnote 19: The helium line in the Ring Nebula on which we have already commented is not a member of the Pickering Series, but it has had the same history. It was first supposed to be due to hydrogen, later (in 1912) reproduced by Fowler terrestrially in a mixture of helium and hydrogen, and finally discovered by Bohr to belong to helium.] [Footnote 20: This, of course, is found from the other lines of the spectrum which genuinely belong to the star and shift to and fro as it describes its orbit.] [Footnote 21: As the word temperature is sometimes used with new-fangled meanings, I may add that 15,000° is the temperature corresponding to the individual speeds of the atoms and electrons--the old-fashioned gas-temperature.] [Footnote 22: Photograph taken by E. T. Cottingham and the author in Principe at the total eclipse of 29 May 1919.] [Footnote 23: We refer to calcium as it occurs in the chromosphere, i. e. with one electron missing.] [Footnote 24: There is an awkwardness in applying the term ‘apparent’ to something too small to be seen; but, remembering that we have armed ourselves with an imaginary telescope capable of showing the disk, the meaning will be clear.] [Footnote 25: Densities below that of air have been found for some of the Algol variables by an entirely different kind of investigation, and also for some of the Cepheid variables by still another method. There are also many other examples of stars of bulk comparable with that of Betelgeuse.] [Footnote 26: From a photograph taken at the Royal Observatory, Cape of Good Hope.] [Footnote 27: For comparison, the nearest fixed star is distant 4 light years. Apart from clusters we rarely deal with distances above 2,000 light years.] [Footnote 28: One cannot always be sure that what is true of the cluster stars will be true of stars in general; and our knowledge of the nearer stars, though lagging behind that of the stars in clusters, does not entirely agree with this association of colour and brightness.] [Footnote 29: The term nebula covers a variety of objects, and it is only the nebulae classed as spirals that are likely to be outside our stellar system.] [Footnote 30: This can be checked because uranium lead has a different atomic weight from lead not so derived. Ordinary lead is a mixture of several kinds of atoms (isotopes).]