200 Mr. G. Higgs. On the Geometrical Construction 



" On the Geometrical Construction of the Oxygen Absorption 

 Lines Great A, Great B, and a of the Solar Spectrum." 

 By GEORGE HIGGS. Communicated by R. T. GLAZEBROOK, 

 F.R.S. Received February 20, Read March 9, 1893. 



In the early part of August, 1890, the photographic work of the 

 normal solar spectrum which I had undertaken had been carried as 

 far as great A, or the limit of visibility in the red, and to X 8350, or 

 beyond z, in the invisible regions. 



During the two previous months of continuously dull weather, 

 while classifying and comparing results, I was interested, on making 

 a close examination of the head portion of the A line, to find the 

 symmetrical construction, the rhythmical grouping, the harmonic 

 order of sequence, and other characteristics of the B line repeated 

 here in every detail. 



These two bands, together with alpha, are composed of a number 

 of doublets or pairs, which approach each other on the more refrangi- 

 ble side with uninterrupted regularity, finally crossing, and at the 

 limiting edges of all three bands the three last pairs overlap each 

 other. 



The differences of wave-length between the components of pairs 

 increase in the same order. 



These and other properties, which will be referred to, are still more 

 obvious in the trains or flutings. 



From its holding an intermediate rank in each of its distinguishing 

 characters I was induced to adopt B as a typical group in a geometri- 

 cal representation, and to investigate the subject by means of rect- 

 angular co-ordinates. 



Before a complete analysis could be made out, a micrometer had to 

 be completed. This consisted of a platform, serving as a plate 

 holder, which was made to travel on runners between parallel ways 

 by means of a screw of such a pitch as to move the negative from 

 one division of the scale to the next, for one revolution of the 

 divided plate on the screw head, this latter being divided into 

 100 parts. 



On and over the platform, a microscope is mounted with slide 

 motions at right angles to each other ; an index of glass fibre and 

 reflector complete the apparatus. 



Over 1000 measurements of nearly 200 lines have been made, 100 

 of which belong to great A. 



In the analysis the axis of x is assumed to occupy a position coin- 

 cident with, or parallel to, the scale of 1/10 10 m. units, and the 

 positions of the various lines are set off on this scale (see fig. 1) for 





