CHAPTER XXV. 



TESTS AFFORDED BY ECLIPSE OBSERVATIONS. 



DURING an eclipse of the sun, the moon by gradually coming 

 between us and that body shields our atmosphere from the 

 brilliant light of the photosphere. 



If the eclipse is to be a total one, this shielding effect will be 

 more decided, and if the totality is a long one it will be most 

 decided. The reason of course is, that, as under the last condition 

 the angular diameter of the moon is greater than that of the sun, 

 the air will be shielded, not only from the light of the photo- 

 sphere, but even from that of the brighter layers of the 

 superincumbent atmosphere. 



Naturally after the middle of totality the amount of shielding 

 will be gradually reduced, as it was gradually increased up to 

 that instant. 



Now the sun's atmosphere above the photosphere is masked 

 ordinarily, as are the stars, by the illumination of our air due to 

 the photospheric light, and more effectively because the particles 

 in our air, the reflection by which produces this illumination, are 

 in a position to reflect more light when they lie nearly between 

 us and the sun than they are elsewhere. 



When the moon, therefore, during an eclipse shields our 

 atmosphere from this illumination, the effect is as if a series of 

 veils was very gradually withdrawn ; and the sun's atmosphere 

 and the stars, first dimly and finally magnificently shine out in 



