140 Transactions of the A Mexican Institute. 



ter of the spot. The umbra appears black, because of the intense 

 light with which it is surrounded ; but it is actually 4,000 times as 

 bright as an equal area of the full moon, according to Zollner. Not 

 unfrequently the spots are crossed by bright streaks or bridges, and they 

 are generally surrounded by feculse, which are seen to become pro- 

 tuberances as they reach the edge of the sun. Upon applying the 

 spectroscope to a spot, the same continuous spectrum is observed in 

 all parts of it, dimmed, of course, in the umbra and penumbra, 

 because there the light is diminished. None of the dark lines are 

 wanting in the umbra spectrum, but many new groups of lines or 

 absorption bands make their appearance. Moreover, certain lines, 

 as those of calcium, barium, magnesium, iron, sodium and titanium, 

 are markedly thickened in the umbra, showing an increase in the 

 absorptive power of these vapors there present; and certain other 

 lines, especially those of hydrogen, are completely reversed therein, 

 becoming bright. The bridge gives the ordinary solar spectrum, 

 except that the hydrogen lines are bright. The penumbral spectrum 

 is similar, but in it the lines of hydrogen are absolutely wanting. 

 From these results it appears clear that the umbra of a spot is a mass 

 of matter similar in all respects to that of which the photosphere is 

 made up, but which, being cooler, both radiates less light and exerts 

 a greater elective absorptive action than the surrounding parts. In 

 other words, it is a cloudy mass in the atmosphere of the sun, made 

 up of the partially condensed vapors of calcium, iron, magnesium, 

 barium, sodium and titanium, sinking gradually through the photo- 

 sphere, and broken up occasionally by an up-rush through it of a mass 

 of hydrogen, forming a bridge. 



[The previous statements were illustrated by photographs thrown 

 on the screen, showing the mottled or granular appearance of the 

 sun's surface, the appearance of a facula, various sunspots as observed 

 by Secchi, Chacornac, and others, the thickening of the D lines in 

 the umbra, and finally the spectrum of a complete spot, bridge, umbra, 

 penumbra and photosphere, as given by Secchi.] 



The third and last solar phenomenon which we have to consider is 

 the corona ; that " ring of pearly, greenish light, dazzling within and 

 fading away outwardly," contrasting so strongly with the solar promi- 

 nences. It is a sort of radiant halo or aureole, surrounding the 

 eclipsed sun and stretching away from it for a distance equal to from 

 a quarter to a half of the moon's apparent diameter. It is irregu- 

 larly quadrangular in shape, and is made up of rays, the raylets them- 

 selves being frequently curved. Many hypotheses have been put 



