38 THE CHEMISTRY OF THE SUN. [CHAP. 



so that when the sun is near the horizon, and shines through 

 the greatest thickness of our atmosphere, his light appears red 

 because all the other colours are to a great extent absorbed. 



The next work of Brewster's carries us much farther in the 

 same direction. He announced to the Eoyal Society of Edin- 

 burgh, 1 in 1833, that he had examined the lines of the solar 

 spectrum with various optical appliances, and had delineated 

 them on a scale four times greater than that employed in the 

 beautiful map of Fraunhofer. Some portions also, which were 

 more particularly studied, had been drawn on a scale twelve 

 times greater. Fraurihofer's spectrum was fifteen and a half 



FIG. 17. Diagram showing the increasing thickness of air through which the 

 light of the sun has to pass as it descends towards the horizon. When the sun 

 is in the zenith, A, its light will have to pass through a thickness of atmo- 

 sphere represented by a o. When near the horizon, this distance will be 

 increased to y o, and when actually on the horizon to x o. 



inches long ; a map on the largest scale employed by Brewster 

 would be seventeen feet long. 



But this was not all. In this same paper Sir Pavid Brewster 

 described 



" a remarkable series of dark lines and bands, which made their ap- 

 pearance in the spectrum when nitrous acid gas was interposed between 

 the prism and the source of light, whether that were the sun or a 

 burning lamp." 



In these experiments we get the first adequate glimpse of 

 those wonderful phenomena produced when the light on its way 



1 Edin, Phil. Trans, vol. xii. 1834, "p. 519 et scq. 



