SOLAR ECLIPSE, MAY 6, 1883. 117 



his knowing the portion of the, corona under observation, found the polarization increasing- uj) 

 to the limb. That of the light from an area .3' in diameter and close to the limb of the moon was 

 estimated as not less than .'>() per cent. 



^ Two observers have made quantitative measurement, namely, Winter in 1871, an<l Wright 

 in 1878. 



Mr. G. K. WiN'J'ER (p. 324) found the percentage of polarization close to the limb 10; at a 

 distance estimated as 10' from the limb it was 25. Tlie tirst of these numbers is a single 

 ol)servation, the second the mean of four. 



Professor Wrioht (U. S. N. Obs. Eeport, 1878, p. 270) found at points radially disposed on 

 the corona, at the distances C, 12', and 22' from the limb of the moon, the corresponding percent- 

 ages of polarization to be 12, 7, and a trace only. If this can be regarded as yielding the law of 

 variation of polarization, it imjdies a high degree of polarization at the inner boundary of the 

 corona. 



The definite conclusion from these and other observations is that the coronal light is strongly 

 polarized radially, and that the percentage of polarization increases continuously up to flic limb 

 of the moon, where it cannot be less than from one-fifth to one-third of the whole. 



III. Photography. 



The photographic evidence as to the nature of the corona is extremely diflicult to deal with, 

 not merely because of the enormous changes in the aspect of the picture with change of duration 

 of exposure, of sensitiveness of plates, and of angular aperture of camera, but because no two 

 eclipses have been photographed in a like way as regards these variable quantities. We may, 

 however, regard the following points as demonstrated by the method : 



First. That there is no outer limit to the corona; in other words, that its extent in a photo- 

 graph is only determined by the three elements above defined. 



Second. That the structure of the corona is, generally speaking, radial. 



Thin]. That "rifts" and "streamers," that is, dark regions and bright regions radially 

 ilisjiosed, are not confined to the outer corona, but extend even to the limb of the moon. 



Fourth. That photographs taken during the same eclipse resemble each other, but those 

 belonging to different eclipses are very diverse. 



ACCEPTED EXPLANATION OF THE CORONA. 



The accepted explanation of the corona has been snjiposed to account more or less satis- 

 factorily for all the observed phenomena. It may be summarized briefly tlius : 



a. The sun is surrounded by an atmosphere of incandescent gas, chiefly of the material 

 which yields the 1474K line, aiul of hydrogen extending to a height of not less than (500,000 miles 

 above the photosphere and probably to more than 1,200,000 niiles (Stone). 



h. jNIixed with this atmosphere, i. e., suspended in it, falling into or projected from the sun, is 

 a large quantity of solid or liquid material, which is at such a temperature as to be self-luminous. 

 It is this which yields the continuous .spectrum free from dark lines. 



e. P>esides these components of the envelope, there is present .matter which reflects or diffuses 

 light much as our own atmosphere does. To this is attributed the partial radial i>olarization of 

 the corona. 



d. The streamers and rifts in the corona indicate matter repelled, in various quantities, from 

 the sun by forces which may be electrical. 



