186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



has its puzzling constituent. Sir Robert Ball, in 1893 (when he was 

 Royal Astronomer of Ireland), said that we have good reason to 

 think that there are two other elements at least in the Sun which are 

 foreign to our terrestrial chemistry. At that time these elements 

 received the provisional names of coronium and helium. 



Helium is now a well-known gas. But coronium, the hypothetical 

 element which is a constituent of the solar corona, has not, as yet, 

 definitely yielded up its secret, although there are indications that, 

 like nebulium, it is a well-known gas, or gases, acting in a strange way. 



The solar corona is only observed during a total eclipse of the Sun. 

 It is an irregular halo of exquisite and delicate pearl-colored light 

 arrayed round the eclipsed solar disk. Its form varies. Sometimes 

 it is very prominent, whereas at other periods it is weak. It would 

 appear from observations and photographs taken during totality 

 that its general size changes in accordance with the number of sun- 

 spots or eruptions that appear on the solar disk. These, as is well 

 known, vary in average number over a cycle of approximately 11 

 years. 



Some observers have noted that the corona form is constantly 

 changing even during the short period of totality, but as a general 

 rule it may be said that when there are the least number of spots on 

 the Sun, the corona is at its largest. During this period of minimum 

 sunspot activity long streamers, or wings, of the corona stretch from 

 the solar equator with lesser streamers or tufts at the poles of the 

 Sun. 



This winged shape has given rise to an ingenious theory (suggested 

 first by E. W. Maunder) that the symbol of Ra, the Sun god, a deity 

 of the ancient Egyptians, is an attempt to delineate the shape of the 

 corona. If such is the case these ancients must have been keen 

 observers of eclipse phenomena. 



According to various modern authorities who base their theories 

 on spectroscopic observations, the corona consists mostly of gaseous 

 matter with a sprinkling of meteoric or cosmic dust, shining partly 

 by reflected light from the photosphere, and partly by its own in- 

 trinsic light. 



Spectroscopic observations on the corona are, like those of the 

 nebulae, full of interest. The main characteristic of the spectrum 

 is a very strong line in the green (wave length A 5303), while other 

 emission lines, some very faint, can also be seen. These lines are 

 superposed upon a relatively weak continuous spectrum which can 

 probably be attributed to the reflected or scattered light from the 

 minute particles near the sun. 



Now the line A 5303 does not coincide with that of any of the 

 known elements. There has been a suggestion in some quarters, 



