166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



lively cool gases of the transitional layer to which we ascribe the term 

 "reversing layer." 



It is the transparent gaseous region above the opaque photospheric 

 surface that shows so spectacularly at eclipses. The radiating gases 

 of this zone, shining both by faint continuous light and by intense 

 monochromatic light, form one of the principal realms of study for 

 modern astrophysical devices, and comprise the objective of most of 

 the work of astronomers who travel to distant lands to be present at 

 the brief moments of total eclipse. 



The intercession of the moon between the earth and the sun at a 

 total eclipse gives us a unique opportunity to observe the faint atmos- 

 pheric gases above the sun without the overpoweringly distracting 

 effects of the light of the photosphere. Normally, the light from the 

 face of the sun so intensely illuminates the atmosphere of the earth 

 that the gases above the surface of the sun make no impression at all 

 upon our viewing instruments. But at eclipse we are unhindered by 

 this normal handicap, and are able to take photographs and spectro- 

 grams that reveal the very remarkable characteristics of the sun's 

 atmosphere. 



The very transparency of the atmosphere makes it accessible to 

 study. We can see through it, and thus we can study what goes on in 

 it. By virtue of the fact that its light is radiated, for the most part, in 

 discrete lines of the spectrum, we can study any one constituent of its 

 gaseous atmosphere quite independently from the others. The interior 

 workings of the sun are screened off from view by the very opacity of 

 the photosphere of the sun, so that we cannot glimpse beneath the su- 

 perficial markings of the sunspots and the other surface features, such 

 as the f aculae, and the granulations. To be sure, we study the surface 

 markings in all possible detail, and have learned many remarkable 

 things from these studies — as, for example, the completely unexplained 

 equatorial acceleration discovered many years ago by the English 

 astronomer Carrington. This baffling acceleration causes the sun's 

 equator to turn a full revolution in a shorter time than do the higher 

 solar latitudes. People in the path of a total eclipse become acutely 

 aware in the final minutes before totality of the rapid diminution of the 

 brightness of the sunlight illuminating the landscape. But the truly 

 spectacular phenomena do not appear until the final climax, known 

 technically as "second contact," when the moon finally cuts out all 

 the light from the photosphere, leaving only the atmosphere in view. 

 The first thing that one sees is the striking effect of the "Baily's Beads," 

 which result from the tiny portions of the bright photosphere that 

 shine through the depressions of the moon's surface where there are 

 craters with low areas in the line of sight. Then immediately fol- 

 lowing comes the brilliant "chromospheric flash." 



