

CH. xxv.j HOW THE LIGHT WANES. ^355 



all their glory; till again as gradually as they were withdrawn, 

 the veils fall. 



I have said gradually, but that is really not the right word 

 to use, because at certain instants the light changes suddenly. 

 In order to study this more closely let us take the first half of 

 the eclipse from the beginning to the middle of totality, in the 

 other half of course any change indicated will occur in inverse 

 order. From the time that the edge of the moon is first seen 

 biting into the disc of the sun, until the sun is very nearly 

 covered, the change is very gradual. When only a thin 

 crescent of the sun is left the darkness increases rapidly ; and 

 there is a very sudden decrease in the illumination at the 

 instant when the photosphere is finally covered. At this 

 moment, in fact, the shadow of the moon is seen sweeping 

 through the air almost like a solid thing and as if it could 

 carry devastation in its course. This is one of the most terrible 

 moments of the eclipse to the ignorant multitude. For a few 

 seconds after this sudden darkening the light still decreases 

 until the lower corona is hidden ; and then, after another 

 definite but less sudden reduction of illumination than the 

 former one, we get the period of greatest obscuration. 



Now the withdrawing of the veils only begins to be really 

 effective some ten minutes before totality, and the effectiveness 

 of this withdrawal is first indicated by the spectroscope. Lines 

 too faint to be visible in the chromosphere in full sunlight are 

 now seen for the first time, and the number increases as the 

 withdrawing of the veils goes on. Passing from the spectroscope 

 and dealing with ordinary observation, we may say that, five 

 minutes before totality, if the eye has been shielded, the corona^ 

 becomes visible, and its details can be faintly made out in a 

 telescope. 



At the instant of the disappearance of the sun the sudden 

 decrease of light naturally affects all the observations with the 

 naked eye, and in the telescope all the more delicate and 



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