SECT. IT. ABSORPTION BANDS. 1 3 1 



Brewster observed that certain dark bands and lines in 

 the red and green parts of the spectrum are only visible 

 when the sun is near the horizon, whence he concluded 

 that they are occasioned by the absorption of the solar 

 light while traversing a thicker stratum of air than 

 when the sun is in the zenith. Various groups of these 

 absorption bands are to be seen at times on the solar 

 spectrum, especially a remarkable one near Fraun- 

 hofer's line D, and Dr. Miller observed that temporary 

 dark lines appeared during a heavy shower, which 

 vanished when the rain ceased. 



When the sun was high, M. Kirchhoff mentions that 

 he had noticed traces of lines and nebulous bands in 

 different parts of the spectrum, which he thinks might 

 be resolved by a greater number of prisms than those 

 in his apparatus. 



Sir David Brewster was led to his discovery of at- 

 mospheric bands by observing that the brownish red 

 vapour of nitrous oxide has the property of absorbing 

 solar light, resolving the spectrum into a series of bright 

 and dark bands, alternating. Professors Daniel and 

 Miller found that bromine, iodine, and chlorous acid do 

 the same, and Sir John Herschel observed a multitude of 

 similar bands in the flame of cyanogen ; but Dr. W. A. 

 Miller, who has particularly studied the phenomena 

 of absorption bands, has proved that the colour of a 

 vapour does not necessarily determine the position 

 or even the existence of dark bands. He has shown 

 that some simple substances which do not occasion 

 dark bands produce them abundantly by the absorptive 

 power they acquire when in composition, while lines 

 that are produced by a simple vapour, vanish when 

 it is in combination. Dr. W. A. Miller has proved 

 also that none of the preceding vapours exist in 

 the atmosphere. He computed that if free bromine 

 constituted only one in a thousand million parts of 



K2 



