SOLAR ECLIPSE OF JANUARY 3, I908 ABBOT 45 



From the figures just given it appears that the corona of 1908 

 equaled the moon in radiation transmissible by glass only at the 

 brightest observed part of the inner corona. Referring to the con- 

 clusions made by the writer from the bolometric observation of the 

 eclipse of 1900, it will be recalled that it was assumed by him that 

 the region of the corona then observed was equally as bright as the 

 moon visually. It now seems probable that this was not so, and ac- 

 cordingly the argument he made for an exceptional richness of visible 

 light in coronal radiation, which depended on the assumption just 

 referred to, is weakened. In actual fact the coronal radiation proves 

 to be almost, but not quite, as rich in visible light as the ordinary 

 solar radiation coming from points 0.7 radius from the center of 'the 

 sun's disk, as shown by the measurements of 1908 made with and 

 without the screen. 



Probable Nature of the Corona 



The nature of the radiation of the inner corona has been supposed 

 by some to be principally reflected solar radiation ; by others, prin- 

 cipally due to the incandescence of particles heated by reason of their 

 proximity to the sun ; by others, principally luminescence, perhaps 

 similar to the aurora ; and by some as a combination of all these 

 kinds of radiation. 



A satisfactory theory of the corona must take cognizance of the 

 following facts at least : 



I- The color of the corona does not appear to change at varying 

 distances from the limb of the sun, and the transmissibility of its 

 rays to the asphaltum screen is the same at 1.5' and 4' from the limb. 



2. Its brightness is very small and falls off rapidly with increas- 

 ing distance from the limb. 



3. Its spectrum is mainly continuous near the limb, but shows 

 dark Fraunhofer lines, more and more distinctly, at increasing dis- 

 tances therefrom. A few not very conspicuous bright spectral lines 

 are present near the limb and perhaps in the outer corona also. 



4. Its light is polarized in the outer regions, but polarization 

 grows less marked, and at length disappears near the limb. 



5. Its brightness is almost, but not quite, as little transmissible to 

 the asphaltum screen as that of the sun itelf, and is far less so than 

 the reflected brightness of the moon, but far more so than the re- 

 flected brightness of the sky. 



6- Any kind of matter so near the sun must be hot and must also 

 reflect solar rays. 



7. There is no evidence of high pressure in the corona. 



