22 



THE POPULAR SCIENCE MONTHLY. 



tablishing this very interesting result, was led to express the opinion 

 that the sun's atmosphere has no such extent as had been imagined, 

 and that the corona is an appearance (only) in our own air, " an at- 

 mospheric effect merely," "due to the passage of the sun's rays 

 through our own atmosphere." 



This conclusion was, however, not very generally accepted. Sev 

 eral astronomers at once pointed out that the air which lies toward 

 the place on the heavens where the corona is seen, is not illuminated 

 at all by the sun's rays during total eclipse. I also pointed out that 

 whatever light that particular part of the air receives during totality 

 not direct sunlight, but light from the prominences, and so much of 

 the corona as might be admitted to be solar would extend over the 

 very place of the moon, and gradually increase thence on all sides 

 instead of gradually diminishing, as happens with the corona. This 

 would not be the place to exhibit the reasoning by which these results 

 can be demonstrated ; for mathematical considerations, not altogether 

 simple, are involved in the complete discussion of the matter. Let it 

 suffice to say, as respects the air between the observer and the moon, 

 that, since the observer can see the colored prominences and the inner 

 bright corona during totality, the air all around him (toward the 

 moon as well as elsewhere) must be lit up by their light. And as 

 respects the gradual increase of brightness on all sides of the place 

 where the eclipsed sun is, let the reader consider that, if, at any time 

 during totality, a bird were to fly (with enormous rapidity) from the 

 observer's station directly toward the moon's centre, that bird would 

 remain in the moon's shadow as he so flew ; but if he flew in any other 

 direction he would presently pass out of the shadow that is, he would 

 reach a place where the air is illuminated. And he would so much 

 the more quickly reach the illuminated air, as he flew more directly 

 from the moon's place on the sky. So that, putting the line of the ob- 

 server's sight instead of the swiftly-flying bird, we see that this line 

 will so much the sooner reach illuminated air, according as it is turned 

 farther from the place of the moon on the heavens. Thus the air 

 toward the place of the moon, though illuminated, is less brightly 

 illuminated than that lying toward any other part of the sky; and the 

 atmospheric illumination must gradually increase the farther we turn 

 our eyes from the moon's place. 



So matters stood when preparations were being made for the expe- 

 ditions to view the eclipse of 1870. Evidence had, indeed, been ob- 

 tained during the eclipse of 1869 in America, which seemed to show 

 that the substance of the corona is gaseous ; and singularly enough it 

 appeared as though this substance, whatever it might be, shone with a 

 light resembling that of the aurora borealis. But those who regarded 

 the corona as a mere glare in our own atmosphere, rejected these re- 

 sults because they seemed " bizarre and perplexing in the extreme." 

 The American astronomers, however, were not willing to have their 



