Researches in Solar Chemistry. 175 



cost him very considerable thought, would probably have been 

 less obvious. 



Dr. Draper was so kind as to send me some little time ago 

 a photograph of the solar spectrum confronted with the lines 

 of oxygen ; and the result which this photograph is claimed 

 to show is, that a considerable number of the oxygen-lines are 

 coincident with bright lines in the solar spectrum. I will 

 throw this photograph of Dr. Draper's on the screen, in order 

 that we may have common ground of thought. The lower 

 part of the photograph gives the lines of oxygen ; the middle 

 part gives Dr. Draper's photograph of the sun, and the upper 

 part a photograph of the sun taken in England, which I have 

 put side by side with Dr. Draper's in order that the definition 

 of the two photographs may be contrasted. 



On examining the upper photograph with a very consider- 

 able magnifying-power, the detail comes out marvellously, and 

 the spectrum between the more marked lines is found to be 

 occupied with extremely fine lines in those regions where Dr. 

 Draper's photograph gives ribbed structure, which, I fear, 

 may not be due to the solar spectrum at all. In the silver-on- 

 glass gratings, one of which Mr. Rutherfurd was so kind as 

 to give me, I find that, in consequence of the grating being- 

 ruled on the back surface of the glass and the double trans- 

 mission of the light through the plate, there is a considerable 

 formation of Talbot bands, and the solar spectrum is in some 

 regions entirely hidden and absolutely transformed. Lines 

 are made to disappear ; lines are apparently produced ; so that 

 if one compares a part of the spectrum taken with one of 

 these silver-on-glass gratings with an ordinary refraction- 

 spectrum, the greatest precaution is requisite. Indeed I 

 think I am not going beyond the mark when I say that the 

 positions of all lines below the third or fourth order of inten- 

 sity must be received with very great caution indeed when 

 these gratings are employed. So well is this known to Mr. 

 Rutherfurd himself, who prepared these gratings for another 

 purpose, that he is now, with equal generosity, distributing 

 gratings containing the same number of lines to the inch 

 (17,300, or something like that) engraved on speculum-metal 

 in order that these defects may be obviated. 



With regard to this work of Dr. Draper's, then, I wish to 

 point out that the photograph in which these comparisons with 

 the oxygen-lines have been made is not one which is compe- 

 tent to settle such an extremely important question. Secondly, 

 upon examining these oxygen-lines, I do not find the coinci- 

 dences to which he refers with bright solar lines and oxygen - 

 lines in that part of the spectrum with which I am most fami- 

 liar, for the reason that there are no bright lines whatever in 



