DR L. BECKER ON THE SOLAR SPECTRUM. 101 



at the beginning of September of that year. I hope, however, to have an opportunity of 

 completing the survey next summer. 



The weather in 1887, though fine for low sun observations, was generally unfavour- 

 able when the sun was at a considerable altitude. The Barmekin, too, being forty minutes' 

 walk from the Observatory, it was not, of course, possible to utilise every opportunity. 

 In 1888, however, the remaining parts of the high sun observations were supplied. 



2. Short Summary. 



In this memoir we publish a catalogue of 3637 lines of the solar spectrum between 

 the wave-lengths 6024 and 4861 Angstrom units, including 928 telluric lines. They 

 are deduced from 26,107 observations, yielded by 47 sunsets and 32 sunrises, and from 

 8325 observations made when the sun was at medium altitudes. 28 lines excepted, the 

 whole telluric spectrum is found by these observations to consist of three bands ranging 

 from A = 6020 to 5666 A.U., A = 5530 to 5386 A.U., and A = 5111 to 4981 i.U. They 

 contain respectively 678, 106, and 116 lines. 



All the darker lines of these bands are due to water-vapour. For the fainter lines, 

 however, the small variations in the amount of water- vapour in our atmosphere did not 

 suffice to produce different intensities of blackness at the same altitude of the sun. 

 Nevertheless, investigations on the behaviour of the lines, combined with the results 

 obtained by former observers, led us to assume that the water-vapour lines of the first 

 band are split into two distinct groups by a band of faint lines, which are probably due 

 to oxygen. These two groups have been called the Eain-band and the S-band. They 

 were known to Sir David Brewster more than fifty years ago, and the description 

 he gives of them virtually contains the assumption, that they are caused by the 

 absorption of water-vapour. His drawing of telluric absorption bands gives also our 

 other two bands under the designation £ and t, besides some other bands, which our 

 observations do not attribute to atmospheric absorption. The £-band has never been 

 mentioned by later observers to my knowledge. 



Of all the telluric bands within our zone, the Eain-band is the only one which has 

 hitherto been resolved into lines. The names of Angstrom, Kirchhoff, Hofmann, 

 Janssen, Piazzi Smyth, and Cornu, mark a continuous progress in our knowledge of its 

 structure. Whilst Angstrom's drawing (1869) of this band contains only 19 lines, M. 

 Cornu resolves it into 170 lines, by observations made with a Eutherfurd grating in 

 the years 1879 to 1882. Of the S-band there is an interesting account in Angstrom's 

 Recherches sur le Spectre Solaire* We have also to mention that in Professor Piazzi 

 Smyth's maps of The Visual Solar Spectrum in 1884,\ the region where the S-band 

 begins is marked as Region of Low Sun-Band of Thin and closely -set Telluric Dry-Gas 

 Lines. But as this note stands within the spectrum copied from Angstrom's map for 



* Upsal, 1868. t Trans. Roy. Soc. Edin., vol. xxxii. part 3. 



