140 CARNEGIE INSTITUTION 



spectroscopic Observations. — The solar spectrum has been studied 

 with epoch-making results by Rowland and his associates at the 

 Johns Hopkins Universit)', and in the infra-red region by L,angley 

 and Abbot at the Smithsonian Astrophysical Observatory. The 

 marked success of Higgs in photographing the extreme red on 

 plates prepared by himself points to the possibilit}' of extending this 

 work to the infra-red, a result greatly to be desired. All of this 

 work relates to the integrated light of the Sun, and not to any par- 

 ticular point on its surface. Our knowledge of the spectra of Sun 

 spots is derived almost entirely from visual obser\^ations made with 

 the spectroscopes of twenty years ago, and is purely qualitative in 

 character. An adequate study of the characteristic widened lines, 

 which may involve the application of photographic methods, should 

 be based upon measurements of both wave-length and intensity. A 

 large solar image and a powerful spectroscope are indispensable for 

 this work. The following programme for the study of Sun spots 

 may serve to illustrate the opportunity for research in this field : 



Size of image. Purpose. 



(i) Photograph of form (direct) ... 17 cm To identify spot and give 



general form and helio- 

 graphic position. 



(2) Photograph of form (enlarged) 40 to 100 cm. To record details of structure. 



(3) Photograph of form (in calcium 17 cm To show distribution of cal- 



light). cium vapor for comparison 



with (i). 



(4) Photograph of form spectrum. . 17 cm To record widened lines. 



(5) Photograph of form (H and K 17 cm To show radial motion of cal- 



lines, wdth very high disper- cium vapor, 



sion.) 



(6) Heat measurement with bolom- 40 to 100 cm. To give heat radiation, com- 



eter or radiometer. pared with that of neigh- 



boring photosphere and 

 center of the Sun. 



The slit used in photographing the spectrum and in measuring 

 the heat radiation is to be set successively at various points in the 

 spot, which are recorded on the photograph of its form. 



It would be easy to name many similar investigations of spots, 

 faculse, and prominences, which would be certain to add greatly to 

 our knowledge of the Sun. It is perfectly feasible, for example, to 

 study some of the most important phenomena of the reversing layer 

 without an eclipse ; indeed, many lines which are apparently too 

 faint to be photographed at eclipses in the spectrum of the ' ' flash ' ' 



