Researches on Solar Physics. 181 
facts, he has furnished us with very accurate data regarding the 
sun’s rotation. 
8. Gradations in their luminosity.—We ought also to mention 
the discovery by Dawes, that what is regarded as the umbra of 
a spot consists in many cases of two well-defined and separate 
parts,* the exterior part being less luminous than the interior; 
so that we have often connected with the same phenomenon not 
less than five degrees of luminosity: these are—(1.) The facule. 
(2.) The ordinary photosphere. (8.) The penumbra. (4.) The 
borders of the umbra. (5.) The very dark central nucleus. 
Mr. Dawes’s discoveries are mainly due to his employing, 
with an eye-piece of his own invention, the full aperture of the 
telescope; but it is necessary to recall the fact that Sir William 
Herschel, in earlier times, was fully aware of the importance of 
not contracting the aperture of the objective. Moreover, we 
must not forget that Sir W. Herschel contributed to solar phys- 
ics a theory which still holds its ground. 
ames.—But there is another phenomenon connected 
with our luminary, not less curious than solar spots. e allude 
Sponds to the theory of their fixation in the sun. 
hese results were verified by Secchi, who also obtained pho- 
tions in a solar eclipse. gee 
. * In some cases, however, it is fair to assume that the appearance of lighter ai ft 
tions of the umbra may be caused by the floating across of portions of the | 
Part of the sun’s surfaze. ge : 
Am. Jour, Sci.—Srconp Series, Vou. XLII, No. 128.—Marcn, 1967. 
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