average distance between the lines is about half that between the two components of b3, so that within the b group the total number of dark lines is some 300, and there are seven or eight of the bright lines. This structure is most easily seen in the part of the spectrum between E and F; above F the lines are crowded so closely that it is difficult to resolve them, and below E they appear to grow wider, more diffuse, and fainter. It seems to indicate that the principal absorption which darkens the center of a sun-spot is not such as would be caused by minute solid or liquid particles—by smoke or cloud—which would give a continuous spectrum; but it is a true gaseous absorption, producing a veritable dark-line spectrum, in which the lines are countless and contiguous.
Since the notes to the second edition were written, great advances have been made in the study and mapping of the spectrum. While the maps of Kirchhoff and Ångström will always remain standards from the historical point of view, they are by no means adequate to represent what is seen by our present instruments, and a number of new ones have been recently constructed which must entirely supersede them for all detailed work. The most important of these are the maps of Thollon and Rowland. The former, for which its author received the Lalande prize of the French Academy of Sciences last January (1886), was constructed from visual observations with a great spectroscope having a train of his powerful compound bisulphide of carbon prisms. This map covers the whole length of the visible spectrum, and embodies the results of some two years' continuous labor; it was presented (as a drawing) to the Academy last year, but its engraving and publication are not yet completed, so that it will not be accessible for some time to come.
Professor Rowland's map is photographic, and extends from wavelength 5790, half-way between D and E, through the whole upper portion of the spectrum, and far beyond the visual limits. Its scale is from three to four times as large as that of Ångström. Five of the seven sheets are already published and in the hands of subscribers. The original negatives were made by means of a four-by-six-inch concave diffraction-grating, having about 90,000 lines, and a focal length of about thirty feet.
Professor C.P.Smyth, of Edinburgh, has also published a map of the whole visible spectrum, made with a very large diffraction spectroscope, having four-inch collimator and telescope, and a three-and-a-half by five-inch flat grating by Rowland. This map is constructed on a scale, not of wave-lengths, as usual, but of wave-numbers—i.e., the scale expresses for any given ray the number of its waves in the length of one "British inch." The dispersion is about the same as in Rowland's map. Important and very useful maps, on a slightly smaller scale, were published a year or two earlier by Fievez, of Brussels, and Vogel, of Potsdam.
