68 LIGHT SCIENCE FOR LEISURE HOURS. 



possible, of course, that there are in reality dark lines 

 across this rainbow-tinted streak, but that these lines 

 remain undetected owing to the extreme faintness of 

 the spectrum itself across which they lie. If we 

 adopted this view we might assume that the corona 

 shone in part at least, by reflecting the sun's light. 

 As far as the evidence goes, however, we have the 

 theory presented as, on the whole, more probable, that 

 the matter of which the corona consists is, in large 

 part, incandescent through intensity of heat. It is 

 difficult to suppose that such skilful observers as have 

 studied the coronal spectrum would have failed to 

 detect dark lines, had any existed. On the other hand, 

 we have a priori reasons for believing that the matter 

 of the corona, or at least of that part which has been 

 analysed with the spectroscope, must be intensely 

 heated. A portion of the corona which appears to lie 

 but eight minutes from the sun's edge, must lie in 

 reality so close to his orb that the sun, instead of 

 appearing as a disc but about half a degree in width, 

 would seem nearly ninety degrees wide, and the amount 

 of heat received from him would be many thousand 

 times greater than that received on the hottest day of 

 a tropical summer. We can form an opinion of the 

 effect of such heat as this, in the same way that Sir 

 John Herschel estimated the heat received by the great 

 comet of 1843, when nearest to the sun. 'To form 

 some practical idea of this,' he writes, * we may compare 

 it with what is recorded of Parker's great lens, whose- 

 diameter was 32^ inches, and focal length 6 feet 8 



