172 Mr. J. N. Lockyer. 



*Lecoq de Boisbaudran 5006 



t Watts 5006-5 



JLiveing and Dewar (1878) 5000 



: , (1888) .... 5006-4 



Now for the determination of the wave-length of the chief nebula 

 line. I give a condensed statement of the observations available when 

 my paper was written. 



(1864) : " The strongest line coincides in position with the 

 brightest of the air lines."|| The diagram which accompanies 

 this description represents the line about midway between the 

 two components of the bright, coarse, nitrogen double. The 

 wave-lengths given by Dr. Watts for the nitrogen lines from a 

 reduction of Dr. Huggins's measures are 4999 and 5003 ;^[ so that, 

 according to these measures, the wave-length of the nebula line would 

 be somewhere between those positions.** Taking Thalen's measures 

 of the nitrogen lines (5002 and 5005), the position of the nebula line 

 assuming that it fell according to the drawing would be 5003'5. 



Taking Thalen's measures and Dr. Huggins's reference to the 

 coarse double line, which with the dispersion then used appeared as 

 a single one, the wave-length might be 5002'! or 5005'! or any 

 value between these. We have thus a limit of error of thirty units 

 by Thalen's values. Kirchhoff 's values for the two lines, as given by 

 Watts, are 5004'6, 5000'6. 



1865. Secchi observed the spectrum of the Orion nebula in 1865. ft 



* ' Spectres Lumineux,' p. 86. 



t ' Phil. Mag.,' 1875, p. 85. 



I ' Roy. Soc. Proc.,' vol. 27, p. 353. 



' Roy. Soc. Proc.,' vol. 44, p. 242. 



|| Dr. Huggins, ' Phil. Trans.,' 1864, p. 438. 



^[ ' Index of Spectra,' p. 3. 



** Dr. and Mrs. Huggins, in their paper, call these figures of Watts into question. 

 They say, " Watts's reduction of my (sic) measures to wave-lengths is clearly not 

 accordant with the measures of air-lines immediately preceding and following this 

 line. I have therefore reduced my original measures to wave-lengths, and find for 

 N L the value 5004'5 



" Kirchhoff. 5004'6 



" Thalen 5005'1 



" Thalen's value is clearly too high, as Thalen gives for the lead line coincident 

 with N! A.5004'6 and Nj is seen on the more refrangible side of the solar iron line 

 given by Angstrom as A.5004'9. In Angstrom's map N! is laid down on the more 

 refrangible side of the iron line 5004'9 at about 5004'5. The same position is given 

 to Nj in KirchhofFs map. I have made a new determination of the position of N,, 

 using the second spectrum of a grating 17,300 to the inch, relatively to the solar 

 iron line at 5004 - 9 according to Angstrom. The value came out 5004'6." (' Roy. 

 Soc. Proc.' vol. 46, p. 45.) 



ft ' Comptes Rendus,' vol. 60, p. 543. 



