28 THE POPULAR SCIENCE MONTHLY. 



average distance between the lines is about half that between the two 

 components of b 3 , so that w T ithin 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 diffi- 

 cult to resolve tbem, 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, pro- 

 ducing 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 Angstrom will always remain standards from 

 the historical point of view, they are by no means adequate to repre- 

 sent 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 spec- 

 troscope 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 wave- 

 length 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 lanre as that of Angstrom. 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 con- 

 cave 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 spec- 

 troscope, 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 vraxe-le?iffths, 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. 



