On the Corona of the Sun. 



117 



an observed tendency of the brighter parts of the corona to assume a 

 square form in consequence of the greater extension of the coronal 

 matter at the latitudes between the poles and the equator of the sun — 

 that is, over the zones of maximum spot-action. The corona is fre- 

 quently less extended over the poles and over the equatorial regions of 

 the sun. 



A noticeable exception to this state of things occurred in 1878, 

 when the most remarkable features of the corona were two extended 

 equatorial rays which could be traced to a distance of several solar 

 diameters. We shall have to consider, further on, some circumstances 

 which may have had a large influence in bringing about this state of 

 the corona. 



In addition to these large changes in the external form of the corona, 

 there is a complex structure within it which appears to be in con- 

 tinual change. This inner structure was observed by Professor S. P. 

 Langley in 1878, under very favourable conditions, with a telescope of 

 5 inches aperture and a power of 50. 



He sums up his observations in the following words : — * 



1. Extraordinary sharpness of filamentary structure. 



2. Arrangement not radial, or only so in the rudest sense. 



3. Generally curved, not straight lines. 



4. Curved in different directions. 



5. Yery bright close to the edge, fading out rapidly, fading out 

 wholly (this part of corona) at 5' to 10' from it. 



In addition to this more minute structure, there are large bright 

 portions, apparently streaming outwards, and often leaving between 

 them less bright spaces, which have the appearance of rifts. There 

 are also curved forms which seem to turn round and to return to the 

 sun. 



We must not forget that the corona has thickness as well as exten- 

 sion, and that the forms seen by us must appear more or less modified 

 by projection. Rays inclined towards or from the observer would be 

 materially altered in respect of their apparent position on the sun, and 

 long rays in the nearer or more distant part of the corona would appear 

 to start from parts of the sun other than those to which they really 

 belong. For the same reason the increase of intrinsic brightness of the 

 corona towards the sun's limb must be much less than the increase of 

 brightness as seen by us, of which no inconsiderable part must be due 

 to the greater extent of the corona in the line of sight as the sun is 

 approached. Besides the real changes in the corona which have been 

 observed at different eclipses, there are several sources of apparent 

 change which may have modified the photographs taken of the corona. 

 Of these may be mentioned — the state of the air at the time ; the kind 

 of sensitive surface employed ; the length of exposure ; whether 

 * Report T. S. Eclipse, 1878, Washington, p. 209. 



