SOLAR ECLIPSE EXPEDITIONS— MITCHELL I59 



by nearby clouds in the earth's atmosphere. This, however, is a mere 

 detail on a fine piece of work by Grotrian, and it is evident that the 

 measures must be repeated at future eclipses when, it is to be hoped, 

 the sky will be clear from passing clouds and haze. 



As a result of many different theories, it is now generally recognized 

 that the electron must play an important role in explaining the 

 radiation of the corona. If the continuous spectrum is caused by 

 free electrons which scatter the solar radiation, it is found that at a 

 temperature of 4,000° K., the average thermal velocity of the electron 

 would cause a Doppler effect of the order of 10 angstroms. As a 

 result, all of the Fraunhofer lines would be wiped out with the excep- 

 tion of the broad wings of the H and K lines. 



Electron scattering, therefore, furnishes an adequate explanation 

 of the absence of dark lines in the spectrum of the inner corona and 

 also is in conformity with the fact that there is no difference in color 

 between the corona and sunlight, as was found at the 1932 eclipse. 



But how are the dark lines in the outer corona to be explained? 

 If the electron is to be the agent, then it is necessary to find some 

 process whereby the random velocities of the electrons may be slowed 

 down to such an extent that the Doppler effects will not obliterate 

 the Fraunhofer lines. It is scarcely possible to assume a tempera- 

 ture in the middle corona less than 2,000° K. which would cause a 

 Doppler effect of about 7 angstroms. We might, if we please, 

 assume some (as yet) unknown influence from positrons, or that the 

 sun's magnetic field, or some other unknown source, might gradually 

 slow up the electron velocities at distances of 10' from the sun's 

 surface. If, indeed, there were a gradual slowing up of the electrons, 

 this would cause the Fraunhofer Unes to be broader in the inner 

 corona and sharper in the outer corona, which is not in accord with the 

 facts. 



Under the assumption that the light of the corona is caused by the 

 superposition of a continuous spectrum over a dark-line spectrum, 

 it is necessary to conclude that electrons are present everywhere 

 throughout the corona. As atoms and ions are present in the inner- 

 most corona causing the bright-line spectrum of coronium, it has 

 been thought by many that ionized atoms must be the cause of the 

 dark lines in the spectrum of the outer corona. If atoms were pres- 

 ent in appreciable numbers, their existence would cause a Rayleigh 

 scattering and a distribution of intensity in the coronal radiation 

 different from that of the sun — which again is not in accord with the 

 facts. 



To explain the dark lines of the outer corona and with no change 

 in color, Grotrian is forced to assume that the mechanism must be 

 solid particles with diameters greater than three times the wave 

 ength of light. Close to the sun all solid particles would be imme- 



