434 Dr. W. Huggins. On the Wave-length of the [Mar. 7, 



Now, though Angstrom's original value X 5567 may not be quite 

 accurate, his observation fixed a limit towards the red beyond which 

 the Aurora line cannot lie. Angstrom says, "sa lumiere etait 

 presque monochromatique, et consistait d'une seule raie brillante 

 situee a gauche " (on the more refrangible side) " du groupe connu 

 des raies du calcium."* The position of the most refrangible line of 

 this calcium-group is accurately known ; according tof 



KirchhofF X 5580*9 



Thalen 558O9 



Huggins 5581-0 



It is certain, therefore, from Angstrom's first observation in 1867 

 alone, that the Aurora line lies well on the more refrangible side 

 of wave-length 5580. This limit towards the red was confirmed 

 afterwards by Angstrom himself ; he says later that the yellow line 

 falls almost midway between the second and third line of the shaded 

 carbon group.^ The positions of these lines of comparison are, 

 according to Angstrom and Thalen, X 5538 and X 5583. 



It follows that Krafft's values, X 5586, X 5587, and X 5595, must be 

 from some cause inaccurate. A possible explanation may be found 

 in the small number of solar lines employed by Krafft for the reduc- 

 tion of the measures into wave-lengths. The curve was drawn through 

 the six Fraunhofer lines B, C, a, D, E, and b. There was no control 

 for the curve between D and E, and a very small deviation of the 

 curve from its true position here would be sufficient to account for 

 the position of less refrangibility of from X 0016 to X 0024, which his 

 measures give for the Aurora line. 



It should be stated that Krafft expresses regret that more attention 

 could not be given to the spectroscopic observations. He says : 

 " Leider gestatteten die obligatorischen Beobachtungen nicht, den 

 spectroscopiscben Untersuchungen die gehorige Aufmerksamkeit 



angedeihen zu lassen Ich glaubte ausserdem diese 



Messungen um so mehr auslassen zu konnen, als der Platz der ge- 

 wohnlichen Nordlichtlinie oft und sehr genau bestimmt ist." 



To sum up, we have the following values for the principal line of 

 the Aurora : 



(6) 1872, Yogel X 5571-3 0'92 



(5) 1874, Huggins 5571-0 0'5 



(7) Gyllenskiold's mean of 23 observers 



from 1867 to 1884 5570'0 0'88 



* ' Spectre Solaire,' Upsal, 1868, p. 42. 

 f ' Brit. Assoc. Eep.,' 1884, p. 3/2. 

 J ' Nature,' vol. 10, p. 211. 

 ' Acta Upsal.,' vol. 9, 1875 (p. 29). 



