Researches in Solar Chemistry. 173 



In a paper communicated to the Royal Society in 1868 I 

 find these words : — "Attention has recently been drawn to cer- 

 tain bright regions in the ordinary spectrum." The position 

 of these bright lines in the ordinary spectrum was then stated, 

 and attention was called, among others, to one between b and 

 F. I call especial attention to that line now because the re- 

 quisite amount of dispersion is now so common that any one, 

 whenever the sun shines, may turn to b and see that bright 

 line for himself. It will be found just as much outside the 

 fourth line of b as the third line is on the other side of it. 

 This bright line, lying in the most visible part of the spectrum, 

 is exactly similar to many others, some of them in the yellow 

 and some of them in the red, A careful list of these lines 

 was made some years ago ; and, I am sorry to say, the list 

 was unfortunately lost by one of my assistants in a Metropo- 

 litan Railway-carriage ; at all events, enough was said in this 

 and other countries about these bright lines in the years 1869 

 and 1870 to have given rise, at all events, to the hope that 

 Jiny. one interested in solar physics would be perfectly familiar 

 with them. Among other matters which called attention to 

 their existence was a correspondence which took place in the 

 Comptes Rendus of the Academy of Sciences in Paris between 

 Father Secchi and another observer in connexion with solar 

 spots. I have remarked that a large dispersion is requisite to 

 see these bright lines, because with a small dispersion bright 

 regions of another kind in the solar spectrum are very obvious. 

 When this small dispersion, however, is changed for a large 

 one, one sees that these bright regions in the solar spectrum 

 are due to the absence of fine lines ; and, indeed, if one ob- 

 serves the solar spectrum with considerable dispersion through 

 a cloud which prevents the fine lines from being seen, then 

 there is a very considerable relative diminution in the intensity 

 of some parts of the spectrum, and a very considerable rela- 

 tive increase in others, where these very fine lines are present 

 and absent relatively, so as to give rise to the appearance of a 

 very considerable change indeed in the background of the 

 spectrum. 



When, however, a very considerable dispersion is employed 

 and photography is brought into play, if precautions be 

 taken to give sufficient exposure, these bright regions, as op- 

 posed to the bright lines, entirely disappear. I have here, bv 

 the kindness of two friends, Mr. Rutherfurd and Captain 

 Abney, the means of showing you exactly what I mean. A 

 Rutherfurd grating containing 17,000 lines to the inch has 

 been used as a means of obtaining the spectrum ; and the film 

 employed was kindly put on the plate for me by Captain 



