156 0. C. Lester — Oxygen Absorption Bands. 



EXPLANATION OF PLATES II TO IV. 



Plate II. 



1. — A group ; Kirchhoff. Prismatic spectrum. 



2.— A " ; Thollon. " 



3. — A " ; Langley. Grating spectrum. 



4.— A " ; Piazzi Smyth. " " 



5. — A " ; Cornu, " " 



6. — A " ; Drawn from photographs with large Eowland 

 grating in Sloane Laboratory. The last three pairs added from 

 measurements of wave lengths. Comparison was made with 

 Higgs' photographs of A which are probably the best yet taken. 

 The drawing shows all details except the " secondary series/' 

 The eighteenth pair is cut off. 



Plate III. 



The groups as represented in Plate III are drawn from the measurements 

 of the wave-lengths. Beginning with A, they are arranged in order, one 

 above the other, with the first lines of the second bands forming a straight 

 line. This pyramid-like grouping of the bands is especially effective in 

 showing the symmetrical arrangement of the pairs from group to group and 

 their decreasing width from A to a". 



Plate IV. 



The distance between the lines marked A and B is 4 cm , representing the 

 distance in wave lengths between these two bands. The other horizontal 

 lines are drawn at distances proportional to their distances in wave lengths 

 from A. Plotted in this way the corresponding lines of the several bands 

 fall almost exactly in a straight line, this being especially true for those 

 lines whose wave lengths are most accurately measured. 



