96 LIGHT SCIENCE FOR LEISURE HOURS. 



able, that it is with a sense of wonder one hears that 

 Arago called it in question. To use the words of Sir 

 John Herschel, 'the fact is so palpable that it is a 

 matter of some astonishment that it could ever fail to 

 strike the most superficial observer.' And, again, not 

 only the light but the heat of the outer portions of the 

 sun's image has been estimated. In this case we do 

 not depend upon the perhaps fallible evidence of the 

 eye, but on that of heat-measuring instruments. Fr. 

 Secchi, measuring the heat of different parts of the solar 

 image, has found that of the part near the centre nearly 

 double that from the borders. Lastly, photography 

 gives unmistakable evidence on the subject. 



Now, when Kirchhoff discovered the meaning of the 

 solar spectrum, it seemed clear to him that he had 

 determined the nature and constitution of the solar 

 atmosphere. Let us consider the nature of Kirchhoff's 

 discovery. 



He found that the dark lines across the rainbow- 

 tinted streak forming the background (as it were) of 

 the solar spectrum, are due to the action of absorbing 

 vapours. The vapours necessarily lie outside the source 

 of that part of the sun's light which produces the rain- 

 bow-tinted streak. If those vapours could be removed 

 for a while, we should see a simple rainbow-riband of 

 light. Or if the vapours could be so heated as to be 

 no less hot than the matter beneath them which pro- 

 duces the rainbow spectrum, they would no longer 

 cause any dark lines to appear ; but being cooler, and 

 so giving out less light than they intercept, they cut 



