6 PLEASANT WAYS IN SCIENCE. 



correctly, the ethereal waves thus originated are feebler than 

 those of the same order which would have travelled earth- 

 wards from the sun but for the interposed screen of vapour. 

 Hence the corresponding parts of the solar spectrum are less 

 brilliant, and contrasted with the rainbow-tinted streak of 

 light, on which they lie as on a background, they appear dark. 



In order, then, that any element may be detected by its 

 dark lines, it is necessary that it should lie as a vaporous 

 screen between the more intensely heated mass of the sun 

 and the eye of the observer on earth. It must then form an 

 enclosing envelope cooler than the sun within it. Or rather, 

 some part of the vapour must be thus situated. For enor- 

 mous masses of the vapour might be within the photospheric 

 surface of the sun at a much higher temperature, which yet, 

 being enclosed in the cooler vaporous shell of the same 

 substance, would not be able to send its light rays earthwards. 

 One may compare the state of things, so far as that particu- 

 lar element is concerned, to what is presented in the case of 

 a metallic globe cooled on the outside but intensely hot 

 within. The cool outside of such a globe is what determines 

 the light and heat received from it, so long as the more 

 heated mass within has not yet (by conduction) warmed the 

 exterior shell. So in the case of a vapour permeating the 

 entire mass, perhaps, of the sun, and at as high a tempera- 

 ture as the sun everywhere except on the outside : it is the 

 temperature of the outermost part of such a vaporous mass 

 which determines the intensity of the rays received from it 

 or in other words, determines whether the corresponding 

 parts of the spectrum shall be darker or not than the rest of 

 the spectrum. If the vapour does not rise above the photo- 

 sphere of the sun in sufficient quantity to exercise a recog- 

 nizable absorptive effect, its presence in the sun will not be 

 indicated by any dark lines. 



I dwell here on the question of quantity, which is some- 

 times overlooked in considering the spectroscopic evidence 

 of the sun's condition, but is in reality a very important 

 factor in determining the nature of the evidence relating tc 



