On the Chief Line in the Spectrum of the Nebula. 



I am extremely gratified to find that in an inquiry dealing with, 

 roughly, some 10,000 observations, which has taken me two years, 

 and the details of which occupy some 235 pages in the ' Proceedings/ 

 the point in my argument to which Dr. and Mrs. Huggins take 

 exception is a subsidiary one. 



II. THE ACCURACY ATTAINED IN EARLIER INVESTIGATIONS. 



My astronomical work has so far been almost exclusively devoted 

 to the Sun, in which case considerable dispersion is easily utilised. 

 When my Sun work drove me to try to obtain some information from 

 other celestial bodies, I had to enter a comparatively unfamiliar field 

 of observation. It may well be, therefore, that I ignorantly over-esti- 

 mated the difficulties of such observations, and a passage in Dr. and 

 Mrs. Huggins's paper seems to suggest that such is the case. They 

 refer to observations of nebulae with a dispersion approaching that 

 given by 8 prisms of 60 U , with the Royal Society telescope. It must 

 be pointed out, however, that although this instrument has been in 

 Dr. Huggins's possession for nearly twenty years, so far as I know 

 no such observations have been continuously made with it previously, 

 or indeed by any other inwtruments in the hands of any other 

 observers. I was justified in this view by noting that in Dr. 

 Huggins's important research published in 1879 only one prism was 

 employed in obtaining the photographic spectra of some of the 

 brightest stars in the heavens, made with the same telescope, and that 

 some two years afterwards, in 1881, he wrote, with reference to some 

 observations made by him of the comet of that year : " I am also 

 able to see upon the continuous solar spectrum a distinct impression 

 of the group of lines between G and h which is usually associated 

 with the group described above. My measures for the less refran- 

 gible group give a wave-length of 4230, which agrees, as well as can 

 be expected, with Professor Liveing and Dewar's measures, 42*20."* 

 A diagram of the spectrum of the comet was published in the paper 

 in which this passage occurs, to which some importance seems to have 

 been attached. 



As, judging from this, the position of the lines in question could 

 have been read to about two in the fourth place, I am justified in 

 regarding this statement as a practical use of a three-figure reference. 

 I gather from Dr. and Mrs. Huggins's criticism that Dr. Huggins 

 now expresses wave-lengths by five figures, that is, he states wave- 

 lengths to the hundred-millionth of a millimetre. It is convenient, 

 therefore, to express the discrepancies between different measure- 

 ments in terms of this quantity as a unit. It will be seen that in 

 1881 he accepted with complacency a variation of 100 units of such 



* ' Roy. Soc. Proc.,' vol. 33, p. 2. 



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