214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



Owing to its proximity, it is far easier to study the composition 

 and structure of the sun than that of any other star. The information 

 obtained from the sun can often be applied to other stars and thus 

 can suggest explanations for phenomena which might otherwise re- 

 main obscure. Owing to the fortunate presence of the moon at a 

 distance from the earth that enables it to eclipse exactly the solar 

 disk when it happens to come between the earth and the sun, much 

 knowledge of the atmosphere of the sun has come from the few 

 moments when the bright disk is obscured and the thin atmosphere 

 becomes visible. Plate 2, figure 1, shows a magnificent photograph 

 of the solar corona taken by the Naval Research Laboratory expedi- 

 tion at Khartoum at the eclipse of February 25, 1952. Plate 2, figure 

 2, shows a large-scale photograph of a large sunspot taken recently 

 at Mount Wilson. The detail in the spot and the so-called rice-grain 

 structure of the sun's surface are well shown. 



R. R. McMath and his associates, at the McMath-Hulbert Observa- 

 tory of the University of Michigan, have combined a motion-picture 

 camera with the spectrohelioscope designed by G. E. Hale. An image 

 of the sun is formed on a slit of a large-grating spectroscope, and the 

 dispersed image falls on another slit, which is adjusted to transmit 

 only the position of an absorption line. With this instrument, the 

 McMath-Hulbert Observatory staff have made beautiful photographs 

 of solar eruptions. 



Photographs of the solar prominences can also be made by the 

 instrument known as a coronagraph, designed originally by B. Lyot 

 of the Meudon Observatory in France. This coronagraph consists 

 essentially of a simple telescope designed to give a minimum of 

 scattered light and carefully trapped by baffles. The image of the 

 sun is caught in a light trap so that only the area surrounding the 

 sun is projected into the field of the instrument. By working at 

 altitudes above 10,000 feet, where the scattered sky light is at a 

 minimum, Lyot succeeded in photographing the outline of the corona. 

 The instrument operated by Harvard and Colorado Observatories 

 erected at Climax, Colo., has been used to take some very excellent 

 photographs of the prominences, using film specially sensitized to 

 the H-alpha line and a special filter by which the transmitted spec- 

 trum is confined to a very narrow band. Plate 3, figure 1, shows a 

 huge arch prominence photographed in the light of H-alpha on June 

 4, 1916. A new solar phenomenon was discovered at Climax. The 

 edge of the sun photographed by the light of the hydrogen red line 

 near the North Pole shows a great number of tiny prominences, termed 

 "spicules," which have an average life of only about 4 minutes. The 

 spicules are always present in inactive regions and seem constantly 



