THE ECLIPSE OF 1870. 69 



inches. The effect of this, supposing all the light 

 and heat transmitted, and the focal concentration 

 perfect (both conditions being very imperfectly satis- 

 fied), would be to enlarge the sun's effective angular 

 diameter to about 23^ degrees.' This, he shows, w^ould 

 give a heat 1,915 times greater than that received by 

 the earth, 'and when increased seven-fold, as was 

 usually the case, would give 1 3,400 times ' the heat 

 received by the earth. The heat received by the matter 

 of tlia. corona would be fully twice as great as this ; 

 ' yet,' says Sir John, * the lens, so used, melted cornelian, 

 .agate, and rock crystal.' 



And here a somewhat curious subject presents itself 

 for consideration a subject which has not hitherto, so 

 far as I know, been very carefully attended to. It 

 may seem that material so diffused and tenuous as 

 that of the corona would be altogether invisible, how- 

 ever intensely heated and illuminated. For, beyond 

 question, the actual quantity of matter in the corona 

 must be indefinitely small by comparison with the 

 space which this object fills. It may be doubted, 

 indeed, whether all the matter in a portion of the corona 

 as large as our earth might not be outweighed by half- 

 a-dozen peppercorns. 



But so far as the visibility of the corona is con- 

 cerned, the extremely fine division to which its material 

 substance is almost certainly subject, would tend to 

 compensate for the quantitative minuteness of that 

 material. A very simple illustration will explain my 

 meaning. This earth of ours reflects a certain amount 



