of Oxygen Absorption Lines in the Solar Spectrum. 203 



of A. They are, with two or three exceptions, given in a frag- 

 mentary state. At the same time, there is nothing to prevent their 

 hypothetical positions being carried further, except that the greater 

 density of the principal series precludes the possibility of obtaining 

 any check in regard to their conformity. 



Generally, a couple of numbers of the head bands are common to 

 two separate series ; this arises from their complexity being sug- 

 gested by the nature of the analysis, and, as a matter of fact, some 

 of these have been observed as doubles by Professor Rowland, of 

 Baltimore. 



In all cases of this kind a greater density is observable on the prints, 

 and is doubtless the cause of the extra density of 7608'83, which 

 belongs to two sub-groups; the line 76LO'10 is known to be a double, 

 but cannot with safety be measured as such. 



Owing to their incompleteness, the elements of the curves for the 

 sub-groups in head of A have not been made out, but a glance at their 

 second differences is sufficient to establish their agreement with the 

 preceding form, since an interval is equal to d' + (n l)d", where d' 

 and d" are first and second differences, and n any interval from the 

 commencement of the series. 



Note. Since writing the above I find that Mr. Johnstone Stoney 

 has written a note which was published with a paper by Dr. Huggins 

 on the spectrum of hydrogen, in which he refers to the conditions 

 under which members of a harmonic series might fall near to, but 

 not on, a curve. 



Fig. 2 is an enlargement of part of A 



