SUNLIGHT AND ITS MEASUREMENT 



159 



Turning from the extra-terrestrial influences upon the sun's 

 radiation we may consider the much more profound alterations 

 induced within the earth's atmosphere. The absorption of 

 certain wave lengths to a greater degree than others renders 

 some non-luminous objects colored, while others that absorb 

 only those waves lying outside the visible spectrum remain 

 colorless to our eyes although they absorb energy. In this 

 way the constitutents of the atmosphere produce lines and 



Fig. 2. Smoothed graph of the energy distribution in a normal spectrum of 

 the sun's radiation outside the atmosphere. 



bands in the sun's spectrum for the same reason and in the 

 same way that the Frauenhofer lines are produced. Rowland 13 

 gives about 440 atmospheric lines, exclusive of those caused by 



13 Rowland, H. A., A preliminary table of solar spectrum wave lengths. 

 Astrophys. J. 1 : 29-46; 131-145; 222-231; 295-304; 377-392. 1895; 2: 45-54; 109- 

 118; 188-197; 306-315; 360-369. 1895; 3: 141-146; 201-206; 356-373. 1896; 4: 106- 

 115; 278-287. 1896; 5 : 11-25; 109-118; 181-193. 1897. Corrections and additions 

 to Professor H. A. Rowland's table of solar spectrum wave lengths. Astrophys. 

 J. 6: 384-392. 1897. 



