1880.] On the Spectra of the Compounds of Carbon, fyc. 509 



as approximately that of the least refrangible edge of the ultra-violet 

 set of bands next above K, whereas in our former paper we had given 

 it as 3883' 5. Now, as we have before explained, we have never set 

 ourselves to the exact determination of wave-lengths, we have rather 

 used them as convenient handles by which to designate particular 

 lines ; but a repetition of the measures of our photographs does not 

 confirm Mr. Lockyer's number. The photographs were those of the 

 arc from a De Meritens machine in one of our lime crucibles, and 

 gave the iron lines well developed ; and we used the iron lines, wave- 

 lengths 3906, 3902, 3898-5, 3894"8, and the pair between 3877 and 3878 

 as lines of reference. These measures give, as a minimum, the wave- 

 length 3882' 7. In Cornu's map a manganese line is given with the wave- 

 length 3881. None of our photographs show this line, though man- 

 ganese was purposely introduced into the crucible, and the other man- 

 ganese lines came out well and strong. We should not put a deter- 

 mination made in this way against a direct determination of the wave- 

 length by a diffraction grating ; but unless there be an error in the 

 identification of the iron lines, or Cornu's wave-lengths for those lines 

 be erroneous, neither of which suppositions is at all probable, we con- 

 clude that the wave-length is more nearly 3883 than 3881. If this be 

 correct, the band covers a markedly luminous portion of the solar 

 spectrum as shown in Cornu's map. 



Mr. Lockyer has forced us to go again over some of the ground 

 which we had previously traversed, by making such a statement as 

 the following : — 



" I have therefore thought it desirable to go over the ground again, 

 modifying the experimental method so as to demonstrate the absence 

 of impurities. Indeed, I have started upon a somewhat lengthy re- 

 search, and have taken a large number of photographs under various 

 conditions, so that to reduce them all will be a work of time. I besr 

 permission, therefore, in the meantime, to submit to the notice of the 

 Society an experiment with a tube containing CC1 4 , which, I think, 

 establishes the conclusions arrived at by prior investigators. And I 

 may add that it is the more important to settle the question, as 

 Messrs. Liveing and Dewar have already based upon their conclusions 

 theoretical views of a kind which appear to me calculated to mislead, 

 and which I consider to have long been shown to be erroneous." 



As we have now demonstrated the utterly unsatisfactory character 

 of the crucial experiment on the strength of which Mr. Lockyer con- 

 demns in so sweeping a manner oar conclusions, it is unnecessary to' 

 add any further comment.* 



* Mr. Lockyer lias coinnumicated to the Society a " Further Note on the Spec- 

 trum of Carbon," since the above was written. We see no reason to modify our 

 conclusions regarding the carbon spectrum on account of any new facts brought 

 forward. The discussion of the paper we leave until a future occasion. 



