Nov. 6, 1879] 



NATURE 



and storm-lines ; a complex spectrum is turned into a 

 simple one, the feeble lines are exalted, the stronger 

 ones suppressed almost altogether. 



Since then the spectra of spots and prominences are 

 confessedly the spectra of the hottest region of the sun 

 available to our inquiries, we can test the nature of the 

 basic lines by seeing how they behave when we pass from 

 the general solar to these special solar spectra. 



With special reference to this point I have brought 

 together the various observations which have been re- 

 corded of the lines visible in solar disturbances at the 

 sun's limb, and those observed to be widened, brightened, 

 or otherwise modified in the spectra of solar spots. 



The finest series of observations of this kind that we 

 possess is that collected by Prof. Young near the time of 

 the last maximum of sun-spots, during his stay at Sherman, 

 at a height of 8,000 feet. The result which stares us in 

 the face when we examine the work done by Young is 

 most striking ; but although his observations of the chro- 

 mospheric lines extend over the whole visible spectrum, 

 the list of lines in the solar spots is limited to the less 

 refrangible region ; we must therefore limit the discussion 

 to this region. 



As a basis for the discussion, I have used the lines 

 given in Thale'n's admirable tables, comparing them with 

 those shown in Angstrom's map, and indicating the in- 

 tensities of the lines which are given in the tables, and 

 which particular line occurs in the map only. A glance 

 then shows which line is seen in spots and prominences, 

 and how it is affected. In short we have in one view, for 

 each metallic substance, exactly what happens to the 

 lines of that substance — which lines are not touched ; 

 those which are visibly affected both in spots and storms, 

 or those recorded in one table and not in the other. 



Taking all the lines included in the discussion, the 

 following statistics will show how they are distributed : — 



Total number of lines in Thale'n's list and map 

 included in the discussion 



Number of lines affected in spots 



Number of lines bright in storms 



Number of lines common to spots and storms 



Number of lines seen in neither spots nor storms 



So much for the list of lines as a whole. It is also 

 necessary to show the number of lines assigned to each 

 metal, and those among them which occur in both spots 

 and storms, or only in one or the other. 



345 

 10S 



183 



It will be seen that the ratio between the affected and 

 unaffected lines is very variable. What strikes one, 

 indeed, is the wonderful irregularity in the behaviour of 

 the various lines ; there is no relation, for instance, between 

 the widening of the lines in the spots and their appear- 

 ances in the prominences. 



It may here be asked, "But what has this to do with 

 basic lines ? " I answer, it would have nothing to do with 

 basic lines if Thalen had not observed them ; but in his 

 observations, which are the tie plus ultra of spectroscopic 

 accuracy, he came across them abundantly. The basic 

 lines therefore have the great advantage of not being new. 



Among ike 345 lines given by Thalen are 18 with 

 identical readings in two spectra. They are, therefore, 

 the exact equivalents of those lines which I have found to 

 be basic in work on another part of the spectrum. 



Now, for the reasons above given, if my explanation of 

 their basic character be the correct one, then we should 

 expect a considerable development of these lines in the 

 spectrum of the hottest regions of the sun, which spots 

 and storms enable us to study apart from the absorption 

 going on at higher levels. 



It is not too much to say that the result of this inquiry 

 settles this question in the most conclusive way. What 

 does come out in the strongest manner is the following 

 very remarkable fact. 



The only constant thing in the tables employed in 

 the inquiry is, that these basic lines are always widened hi 

 the spots. However badly the brighter lines of a chemical 

 substance, taken as a whole, may be represented amongst 

 the spot lines, as the basic lines, among these which are 

 often of the second or third order of intensity and some- 

 times even of the fourth, are never absent. The same 

 fact holds almost equally true with regard to the storms. 



The following comparison of Thale'n's basic lines with 

 those seen by Young in solar spots and storms shows this 

 result : — 



So far as my own knowledge of these matters goes, I 

 can imagine no severer test to apply to the hypothesis that 

 the basic lines in the above tabic are produced by the 



