202 



Mr. D. Gill. On the Presence of Oxygen 



polation by our formula to employ the value of X from the above 

 table with the argument ^ ^" 8 * 



In this way the wave-lengths of the lines in the spectrum of (3 Crucis, 

 given in the following table (pp. 203 — 204), have been determined : — 



Column (1) gives the wave-lengths of all the known oxygen lines of 

 intensity 3 or brighter, between A. 4303 and 4575, according to 

 Neovius as well as Trowbridge and Hutchins.* 



Column (2) gives the wave-lengths of all helium lines according to 

 Runge and Paschen,f contained within limits of the. spectrum under 

 observation. 



Column (3) gives the results of my measurements of the negative 

 of the spectrum of f3 Crucis, those under head I being my first essays 

 n the measurement of any photographed spectrum, those under 

 head II being the results of my second measurement, including all the 

 lines which could be detected under most careful and repeated scrutiny. 

 Each result in series I depends on two pointings, each in series II on 

 four pointings. 



The lines whose wave-lengths are given to two decimal places of the 

 tenth-metre, were measured with a magnifying power of fifteen diameters, 

 those give to one decimal place with a power of only three diameters 

 — the lines of the latter class being very faint and only certainly 

 visible under a very low power. When possible, different iron lines 

 were used in series I and II for determination of the wave-lengths of 

 the stellar lines. 



The observations were not arranged for determination of motion in 

 the line of sight, but the exact coincidence of the star line 4417*06 

 with the air (oxygen) line, and the general agreement of the stellar 

 hydrogen and helium lines with their known wave-lengths, tend to 

 show that the relative motion of ft Crucis to the Earth on February 21 

 did not exceed ± 3 kilometres per second. On that date the Earth in 

 its motion round the Sun was moving towards /3 Crucis with a velocity 

 of 18 kilometres per second, and consequently /3 Crucis is probably 

 receding from the Sun with a velocity of 18 kilometres ± 3 kilometres 

 per second. 



The whole of the known helium lines within the measured range of 

 spectrum are unquestionably present, as also are all known oxygen 

 lines stronger than intensity 4. 



The exceedingly faint lines 



P Crucis 4253*9 may be coincident with Neovius 4254*1 = 1< 

 and H. 4253*42, and 

 Crucis 4303*0 or 4304*0 may be coincident with Neovius 

 4304*4 = T. and H. 4303*8, 



* Watts, ' Index of Spectra,' Appendix E. 

 t 1 A. V. Journal,' toI. 3, p. 10. 



