On the Chief Line in the Spectrum of the Nebuloe. 171 



a systematic error in Dr. Huggins's results of about thirty of the 

 units suggested above, aud anyone acquainted with spectroscopic 

 work will see how very easily this might arise from, the absence 

 of perfect adjustment. 



This brings me to another point which also influenced me in 

 arriving at the view I held, however erroneously. 



For the last fifteen years I have been employed, among other 

 matters, in taking photographs of the solar spectrum compared with 

 arc spectra. Of the thousands of photographs taken (with a disper- 

 sion such that the distance between H and K covers about half an 

 inch on the plate) many hundreds have been rejected on account of 

 the want of exact coincidence between the solar and terrestrial lines 

 of the same element, the slightest variation in the rate of the clock of 

 the heliostat or siderostat employed, or the occasional changing of the 

 arc from the centre to either side of the pole, being enough to pro- 

 duce this result. 



Hence, when I wrote my paper of November, 1887, I held (and I 

 stili hold, although I may have erred in overrating the difficulty of 

 observing stellar and nebular spectra) that short titles of the lines 

 compared, extending to three figures, sufficiently refer to positions, 

 and do not really underestimate the accuracy generally attainable. 

 Indeed, if this be not so, the-n from the single instance I have quoted 

 Dr. Huggins's classic paper on the spectra of the white stars is mis- 

 leading, and his series of lines in the ultra-violet cannot be due to 

 hydrogen. 



These general remarks being premised l I next give in historical 

 sequence the observations of the nebula line now in question. 



III. WAVE-LENGTH OF THE CHIEF NEBULA LINE. 

 A. Historical Notice. 



Dr. Huggins's considers the nebulae to be masses of gas, and he has 

 suggested that the chief nebula lino may owe its origin to- some 

 unknown form of nitrogen. 



When I commenced my experiments on meteoritic glows, I saw a 

 line in the position of the chief nebula line, with the dispersion 

 employed. Thinking it might arise from the magnesium in the 

 silicate, I tried terrestrial olivine, and I again saw the line in its 

 spectrum. Subsequent work with a large model Steinheil spectro- 

 scope (four prisms and a high power eye-piece) showed that the line 

 was coincident with the least refrangible member of one of the 

 flntinos seen in the flame-spectrum of magnesium, the wave-length of 

 which had been given as follows : 



