in connexion with the Spectrum of the Sun. 



283 



It is important to bear in mind that the lines recorded in the fore- 

 going tables are in most cases the very longest visible in the photo- 

 graphic region of the respective spectra. In some cases they are 

 limited to the region 39-40, which I have more especially studied. So 

 that the fact of their being reversed in the solar spectrum must be 

 considered as the strongest evidence obtainable in favour of the exist- 

 ence in the sun of the metals to which they belong, pending the com- 

 plete investigation of their spectra. 



Where, however, there is only one line, as with Li, Rb, &c, the 

 presence of these metals in the sun's reversing layer can, for the 

 present, only be said to be probable. Neither must it be forgotten that 

 in addition to the long lines which a spectrum may contain in the red, 

 yellow, or orange, long lines may exist in the hitherto unexplored 

 ultra-violet region, so that the necessity for waiting for further evi- 

 dence before deciding finally upon* the presence or absence of such 

 metals in the sun will be rendered obvious. 



It will be thought remarkable that if the long lines of such metals 

 as lithium and rubidium are found in the photographic region of the 

 spectrum, the long lines (Li, W.L. 6705; Rb, W.L. 6205 and 6296), 

 should have escaped detection. 



To this it may be replied that, although these red lines may be appa- 

 rently the brightest to the eye, it by no means follows they are the 

 longest, since they are situated in a part of the spectrum which affects 

 the visual organ more strongly than the photographic region does. It 

 is possible also that the reasoning I have lately used in a paper com- 

 municated to the Society on the spectrum of calcium may be applied 

 in these cases. 



Since a sensitized film is affected by some rays more strongly than 

 by others, in determining the lengths of lines from a photograph, it is 

 not fair to compare together portions of the spectrum separated by 

 too great an interval. 



Furthermore the fact of these red lines having been overlooked in 

 the solar spectrum is not conclusive proof of their absence, inasmuch 

 as this portion of the spectrum is both brighter and less refrangible, 

 and a greater degree of dispersion would be necessary when prisms are 

 employed to render visible faint dark lines which are easily detected in 

 the photographic region.* I hope to be able to make special search for 

 these lines on some future occasion. 



For metals having long lines in the green a special search was 

 made. The long thallium line (W.L. 5349) was photographed, but 

 no distinct evidence of a corresponding solar line was obtained. 



Two long silver lines were found also, about W.L. 4018 and 4212, 



* It is significant that there is a dark line near the position of the Li line hoth in 

 Angstrom and KirchhofFs maps not assigned to any metal. 



VOL. XXVII. U . 



