ABSORPTION LINES OF THE INFRA-RED SOLAR 

 SPECTRUM 



By C. G. abbot and H. B. FREEMAN 

 (With 5 Plates) 



In the decade 1890 to 1900, the bolometer was used under Langley's 

 direction at the Astrophysical Observatory of the Smithsonian Institu- 

 tion to feel out the positions of Hnes and bands in the infra-red solar 

 spectrum. The results were published in Volume I of the Annals of 

 the Observatory. In the spectral region A to O, about 550 lines were 

 recorded as observed in the spectrum of a 60° prism of ordinary 

 telescope flint. 



At Mount Wilson, in the summer of 1928, Dr. H. D. Babcock 

 urged that further holographic studies of the infra-red solar spectrum 

 should be undertaken with apparatus of higher resolving power. Our 



Fig. I. 



vacuum bolometer equi])ment, then on Mount Wilson, presents a sensi- 

 tive strip of approximately O.i mm. width, and the combined outfit 

 of bolometer and galvanometer was certainly not less than five times 

 as sensitive as the most sensitive outfit employed at Washington 30 

 years before. 



It appeared practicable to undertake a brief holographic study of 

 the upper infra-red solar spectrum from A to CI in the time available. 

 Accordingly we set up a spectroscope (fig. i) comprising a slit 6 cm. 

 high and (usually) 0.4 mm. wide; a collimating cylindric mirror of 

 543 cm. focal length ; a set of three telescope flint-glass prisms, two 

 of 60°, the third of 64° angle, and all presenting faces approximately 

 6 cm. square. From thence a plane silver-on-glass mirror reflected 

 the spectrum to an image-forming spherical mirror of 230 cm. focal 

 length. The vacuum bolometer, above mentioned, whose sensitive 

 strip was 16x0.1 mm. received the rays at focus. The spectroscope 



Smithsonian Miscellaneous Collections, Vol. 82, No. 1 



