1874.] 



White Lines in the Solar Spectrum. 



221 



III. "On White Lines in the Solar Spectrum/' By J. H. N. 

 HENNESSEY, F.R.A.S. Communicated by Professor STOKES, 

 Sec. R.S. Received December 8, 1873. 



Extract from a Letter from Mr. Hennessey to Professor Stokes. 



" Mussoorie, Nov. 12, 1873. 



" MY DEAR SIR, As I cannot account for what is described and drawn 

 iu enclosed, I hasten to place the same before you, intending to look for 

 the white Unas in question so soon as I move down to a lower altitude. 

 Amongst others, no doubt Kirchhoif closely examined the region in ques- 

 tion, without notice of the lines ; and this only adds to my perplexity, un- 

 less what I see here is due (1) to altitude, or (2) is instrumental. In 

 the latter case I cannot account for the absence of the white lines at 

 Dehra, where I examined the spectrum generally several times ; I must, 

 however, add that without close examination and some experience, the 

 lines might easily be passed over. But if instrumental, to what are they 

 due ? I very much regret that the old spectroscope is not available at 

 present [it had been temporarily sent elsewhere for a special object] to 

 enable me to verify the phenomena " 



[In the drawing sent by Mr. Hennessey, the intervals between the 

 dark lines are coloured green, except in the place of the two white 

 lines. To transfer this distinction to a woodcut, an additional horizontal 

 band has been added below, in which only those parts of the drawing 

 which are left white appear as white, while in the upper part the white 

 of the woodcut represents the white or green, as the case may be, of the 

 original. G-. G-. S.] 



Part of Solar Spectrum, drawn to Kirchhoff's scale, observed at Mussoorie, 

 N. W. Provinces, India, Lat. N. 30 28', Long. E. 78 4' ; Height 

 6700 feet above sea (about), with the Spectroscope belonging to the 

 Royal Society. 



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