Dr. H. Draper on Diffraction- Spectrum Photography. 423 



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ruinations of Mascart and Angstrom and Thalen differ nearly to 



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the extent mentioned above. The same remark is true of Ang- 

 strom compared with Ditscheiner, while the difference between 



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Angstrom and Van der Willigen is more than three times the 

 amount necessary to remove my discrepancy. In any case the 

 photograph is correct, as it is the work of the sun, and is only 

 open to errors arising from imperfect flatness in the field of 

 a fine lens, and that field only subtending an angle of about 4°. 

 The angular aperture of the lens viewed from the sensitive plate 

 is 20'. T trust, therefore, that the photograph may be of perma- 

 nent value to physicists ; for any one can affix another scale if 

 this be erroneous. 



An examination of the photographed spectrum shows many 

 points of interest, some of which are best seen in the spectrum 

 with the scale above, and some in the portion enlarged below. 

 The latter is magnified about twice, and comprises the region 

 from wave-length 3736 to 4205. I have also made photographs 



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on the same scale as Angstrom's map, but have not as yet printed 

 them. The capital letters which are attached to the region 

 above H are according to the nomenclature of Mascart, although 

 the wave-lengths assigned by him to those letters do not coin- 

 cide exactly in all cases with the lines in my photograph ; for 

 instance, the line L, which he regards as single, is in reality 

 triple, and does not correspond to 3819 but to 3821 ; M is cor- 

 rectly designated by 3728, but it is double; N is really at 3583 

 and not at 3580. It has been suggested that it would be proper 

 to return to the old nomenclature of Becquerel and J. W. Dra- 

 per, who simultaneously discovered these lines in 1842-43; but 

 the designation of position by wave-length in reality renders the 

 letters unnecessary. 



The spectrum above H, when compared with the region from 

 G to H, is marked by the presence of bolder groups of lines; and 

 most conspicuous are those between 3820-3860, 3705-3760, 

 3620-3650, 3568-3590, 3490-3530. The first of these groups 

 is strikingly shown in the enlarged photograph. I am not as 

 yet able to offer an opinion as to the chemical elements produ- 

 cing these groups; for almost all the photographs of the ultra- 

 violet spectra of metalline vapours that I have thus far made were 

 produced by a quartz train, and have not yet been reduced to 

 wave-lengths; indeed that is a separate field of inquiry, and 

 could not be comprised in a memoir of this length. I have 

 also tried to utilize the photographic spectra of the late Professor 

 W. A. Miller, published in the Transactions of the Royal Society 

 for 1862 ; but, for some reason (probably insufficient intensity 

 of the condensed induction-spark), his pictures do not bring out 

 the peculiarities of the various metals in the striking manner 



