SOLAR ECLIPSE EXPEDITIONS— MITCHELL X57 



lines of the spectra, and 4086 with 3601. As stated above, 5303 and 

 6374 cannot be placed in the same group. 



Without an eclipse Lyot found that the green and red lines and 

 also 6702 have widths of about an angstrom. His wave lengths com- 

 bined with the best values from eclipses are the values given above. 

 His wave length for the red line, however, differs by 0.2 angstrom 

 from the best eclipse value which gives further evidence of the fact 

 that better wave lengths of the coronal lines are urgently desired. 



With the great advance made in recent years in atomic structure, 

 there have been many attempts to find the origin of the mysterious 

 coronium. The general consensus of well-informed opinion seems to 

 be that although neutral oxygen may eventually be found to be one 

 of the chief constituents of coronium, at present that fact is far from 

 being proved. In brief, none of the scientific guesses made up to 

 date have been verified by observational evidence. 



SPECTRUM OF THE MIDDLE AND OUTER CORONA 



In addition to the bright line spectrum caused by the hypothetical 

 element "coronium," the corona shows the continuous spectrum in 

 the inner corona and dark lines in the middle and outer corona. In 

 1929, with three spectrographs, Grotrian obtained spectra of the 

 corona which were compared with the solar spectrum taken with the 

 same instruments but weakened by known amounts. Measurements 

 of intensities give the following interesting results: (1) The position 

 of energy maximum in the coronal spectrum is independent of the 

 height in the corona and agrees exactly with the energy maximum in 

 the solar spectrum; (2) within the errors of measurement, the distribu- 

 tion of energy in the coronal spectrum is independent of height in the 

 corona and is identical with the intensity of radiation in the solar 

 spectrum. After many years of uncertainty and conflicting evidence, 

 we at last seem to have the positive information that the color of the 

 corona is identical with that of the sun, or in other words, the corona 

 derives its radiation from scattered sunlight. 



In the remarkably clear sky at the Australian eclipse of 1922, Moore 

 seemed to prove quite conclusively that the Fraunhofer lines in the 

 middle and outer corona have their origin in the corona itself and are 

 not caused by sunlight scattered in the earth's atmosphere. By 

 means of a registering photometer, Grotrian measured the widths of 

 the stronger Fraunhofer lines in the coronal spectrum of 1923. These 

 widths were compared with those of the solar spectrum taken by the 

 same spectrograph but weakened by known amounts. In every case 

 the widths of coronal lines were compared with the widths of both 

 stronger and weaker solar spectra. Heretofore, nearly all observers 

 have been in agreement that the coronal lines looked both wider and 

 less distinct than similar lines in the sun. On the contrary, the photom- 



