XXV.] 



HOW THE LINES APPEAR. 



357 



be cooler, its lines therefore will be dimmer, and the lines of C 

 will be dimmer than the lines of B, and so on. So if we could 

 really observe the strata, the longer a line is, i.e., the greater 

 the height at which the stratum which gives rise to it lies, the 

 dimmer the line will be. 



Our best chance of making such an observation as this 

 is during a total eclipse. We do not, as we have already 



FIG. 117. The shorter lines will be most intense 011 one hypothesis, 

 and not on the other. 



remarked, see the lines ordinarily in consequence of the illu- 

 mination of our air. As during an eclipse before totality the 

 intensity of this illumination is rapidly diminishing, the lines 

 first visible should be short and bright, and should remain 

 short, while the new lines which become visible as the darkness 

 increases should be of gradually increasing length ; so that the 

 spectrum should become richer in the way indicated in fig. 117. 

 And as the spectral lines due to the molecules existing in 



