418 Mr. F. McClean. Comparison of Oxygen with the 



The special lines referred to are the extra lines which distinguish 

 these spectra from those of the remaining helium stars of Divi- 

 sion lb. 



The indications in the spectra of the northern stars that these 

 extra lines are due to oxygen are slight, as the lines at best are 

 indistinct. Among the southern stars, however, there are several in 

 the spectra of which these lines are better denned, and there is one, 

 viz., ft Crucis, in which they are very fairly denned. 



The following stellar spectra are mounted on the accompanying 

 plate, viz., k Orionis, ft Scorpii, ft Oanis Majoris, ft Centauri, and 

 ft Crucis. These photographs are intended to show the gradual im- 

 provement in the definition of the extra lines, between k Orionis and 

 ft Crucis, and to indicate their identity of origin throughout. 



The extra lines in the spectrum of ft Crucis are singled out by 

 comparison with another helium star, viz., k Argus, of Division lb, 

 in which- the extra lines do not appear. The lines are drawn out by 

 themselves below the spectrum of ft Crucis. They are then com- 

 pared directly by juxtaposition with a drawing of the spectrum of 

 oxygen as tabulated in the spectrum of air by Neovius (Stockholm, 

 1891, and Appendix E, 1894, of ' Watts 's Index'). 



This comparison shows a close correspondence in the grouping of 

 the extra lines with the spectrum of oxygen. The most remarkable 

 correspondence is in the case of the large group on either side of 

 H£. A slight shift of about a tenth metre is required to bring the 

 groups into identical positions. However, the close similarity of 

 the whole grouping of the two spectra as they appear on the plate 

 admits of little doubt that the extra lines actually constitute the 

 spectrum of oxygen. If this be established the spectrum of the 

 first division of helium stars would be due to hydrogen, helium, and 

 oxygen. 



The scale attached to the spectra is based on standard lines that 

 can be identified with certainty in the stellar spectra. It is inter- 

 polated between the standard lines. Its position in relation to the 

 spectra is determined by the hydrogen lines. The wave-lengths em- 

 ployed are in accordance with Angstrom's scale. - 



On the original negatives the distance between (H) and (F) 

 measures about 1 inch. The negatives are enlarged about eight and 

 a half times. It is difficult to fix the position of the lines — and 

 especially of the hydrogen lines — on these enlargements with suffi- 

 cient accuracy. A further correction than this would account for is 

 however required in order to reduce the two spectra to exact coinci- 

 dence. I believe it should be sought to some extent in a re- 

 examination of the adopted wave-lengths of the hydrogen and of 

 the oxygen spectra. 



The spectrum of 7 Argus is given on the plate in order to identify 



