168 Mr. J. N. Lockyer. 



once provided for such observations both in laboratory and observatory, 

 and they were commenced last May. 



So far, however, only one branch of the observatory work has been 

 commenced, in consequence of delays on the part of the instrument 

 maker. I should here state that my main endeavour in one direction 

 will be to obtain photographs of the spectra of nebula? and reference 

 spectra under such conditions that any instrumental error will 

 register itself on the plate in such a way that a proper correction for 

 it can be made. For such work as this much light and great stability 

 are required ; I have therefore erected a 30-inch reflector at West- 

 gate-on-sea, having received a grant in aid from the Government 

 Grant Fund ; the mirror has been figured and presented to me by my 

 friend Mr. Common ; the flat (7 inch diameter) by other friends, the 

 Brothers Henry, and I am anxious to take this opportunity of 

 expressing my obligations to them for this magnificent help in my 

 work. 



The laboratory researches, consisting of the application of higher 

 dispersion, have made more progress ; but they are not yet finished, 

 and in this paper I shall confine myself to observations made with 

 reference to the chief nebula line. 



A paper by Dr. and Mrs. Huggins, which has appeared in the 

 ' Proceedings ' during the recess, contains criticisms of some points in 

 my recent papers which require a reply. They hold that I am wrong 

 in my identification of the origin of some lines in the spectrum of the 

 nebulae, chiefly those two which Dr. Huggins himself has formerly 

 ascribed in one case to an unknown form of nitrogen and in the other 

 to hydrogen under some conditions which we cannot match in the 

 laboratory ; I, on the other hand, suggesting that possibly they may 

 be produced by magnesium, a substance which occurs in most 

 meteorites. 



I gather from this paper of Dr. and Mrs. Huggins that my object 

 in using a three-figure reference to the lines has been misunder- 

 stood. I think it important, therefore, that I should at the present 

 time return to the subject, giving my reasons for the three-figure 

 references I used in the first instance. For this purpose it is 

 necessary to state the history of the subject less eclectically than 

 Dr. and Mrs. Huggins have done. I shall then give the work that 

 has been since accomplished, though, as I have stated above, it is 

 not yet finished ; and reply to Dr. and Mrs. Huggins's criticisms as 

 best I can. 



As in my replies to the various objections raised by Dr. and Mrs. 

 Huggins, I am most anxious not to even unconsciously misrepresent 

 their views, I shall deal with each line separately, take each objection 

 seriatim, and give their own words as far as possible. I confine myself 

 in the present paper to that at A, 500. 



