The Evolution of the Sciences 



a practically flat surface, that is to say, equally 

 illuminated throughout. 



We are now brought to admit the existence 

 of a gaseous atmosphere surrounding the photo- 

 sphere, and are in a position to understand and 

 interpret the characteristic property of the solar 

 spectrum. This bright spectrum is not rigorously 

 continuous ; it is crossed by a multitude of black 

 lines, occupying fixed positions; physicists have 

 located thousands of these lines, which are 

 evidently due to the absorption caused by the 

 gaseous media interposed between us and the 

 photosphere. First of all the properties of one 

 of these media, the terrestrial atmosphere, are 

 well known to us; in fact, we find in the solar 

 spectrum all the absorption lines of oxygen and 

 water vapour, with the characteristics which 

 guarantee their origin. They are more marked 

 when the sun is near the horizon, because its 

 light then traverses a greater thickness of our 

 atmosphere and, on the other hand, all the lines 

 due to water vapour disappear in times of intense 

 cold, that is to say, when our atmosphere is 

 entirely free from water vapour. But in addition 



to these lines, called telluric lines, which cannot 



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