68 Prof. A. S. Herschel on the Spectrum of the Aurora. 



yellow line, no interruption or appearance of lines could be 

 made out. It was probably also through not opening the slit 

 that I missed seeing a red line which another observer, using a 

 similar instrument and looking with me at the aurora, saw very 

 plainly. Although its red colour was intensely brilliant, I failed 

 to see the slightest trace of light on the red side of the yellow 

 line. Had I opened the slit, or perhaps opened and closed it 

 alternately (as the yellow line, though line, was still very 

 bright), the result would probably have been different. 



1 send with this a copy of the Plate of spectra accompanying 

 Angstrom's original paper (see fig. p. 71) ; and I can confirm the 

 appearance of the negative or "cathode" spectrum which he gives, 

 from the results of some examinations of it which I havelately made. 

 On projecting the recorded lines in wave-lengths, there is a very 

 exact agreement with the chief lines and shadings as figured in 

 the Plate. Some fainter lines, however, are visible, which 



o 



Angstrom has perhaps omitted purposely, to avoid encumbering 

 the drawing. As regards the aurora spectrum, considering 

 the numbers of lines which have been mapped and measured, 



1 am not so well prepared to say how complete Angstrom's 

 selection of its principal ones may be. It may, I think, be 

 taken for granted that his map includes every line of whose 

 existence, approximate place, and brightness there is really no 

 doubt, and that it does represent the normal spectrum of the 

 aurora when sufficiently bright. During the years 1871 and 

 1872 there were several resumes of the subject, accompanied 

 with new measurements, in PoggendorfFs Annalen and the 

 'American Journal of Science ' (by Vogel, Barker, Q and others), 

 with which, from his comments on them, Angstrom was 

 evidently familiar. Adopting his selection, therefore, and 

 the descriptions which he gives, from his own and other obser- 

 vers'' notes, of relative brightness and appearance, I have endea- 

 voured to condense the information in the annexed Table as a 

 guide for further observation. 



Professor Piazzi Smyth, who has given much attention to the 

 auroral spectrum, has published, in the introduction to vol. xiii. 

 of the ' Edinburgh Astronomical Observations/ a set of simple 

 comparison spectra, with notes of desiderata, which would be of 

 great service to observers well furnished with instrumental means 

 and applying them to measurements of the aurora ; and some- 

 thing similar would very much assist observers using direct- 

 vision spectroscopes to map their spectra, where auroras are 

 frequent and of great brightness, in high latitudes. 



Hoping that these notes may be of use for the purpose of 

 comparison with occasional observations of auroras, 



I remain, &c, 

 Newcastle-on-Tyne, July 24, 1874. (Signed) A. S. H. 



