SKCT. XXV. SOLAR SPECTRUM. 207 



on rays emanating from the center and borders of the 

 sun's disc ; that of the earth's, though it cannot be elim- 

 inated any more than in the case of the sun's, may yet 

 be varied to a considerable extent by experiments made 

 at great elevations and under a vertical sun, and com- 

 pared with others where the sun is more oblique, the 

 situation lower, and the atmospheric pressure of a tem- 

 porarily high amount. Should it be found that this 

 cause is in reality concerned in the production of the 

 spots, we should see reason to believe that a large por- 

 tion of solar heat never reaches the earth's surface, and 

 that what is incident on the summits of lofty mountains 

 differs not only in quantity, but also in quality, from 

 what the plains receive." 



Thus the solar spectrum is proved to consist of five 

 superposed spectra, only three of which are visible 

 the red, yellow, and blue; each of the five varies in 

 refrangibility and intensity throughout the whole ex- 

 tent, the visible part being overlapped at one extremity 

 by the chemical, and at the other by the calorific rays ; 

 but the two latter exceed the visible part so much, that 

 the linear dimensions of the three, the luminous, calo- 

 rific, and photographic, are in the proportion of the 

 numbers 25, 42, 10, and 55-10, so that the whole solar 

 spectrum is more than twice as long as its visible part. 



That the heat-producing rays exist independently of 

 light, is a matter of constant experience in the abundant 

 emission of them from boiling water. Yet there is 

 every reason to believe that both the calorific and 

 chemical rays are modifications of the same agent 

 which produces the sensation of light. Rays of heat 

 dart in diverging straight lines from flame, and from 

 each point in the surfaces of hot bodies, in the same 

 manner as diverging rays of light proceed from every 

 point of the surfaces of such as are luminous. Accord- 

 ing to the experiments of Sir John Leslie, radiation 

 proceeds not only from the surfaces of substances, but 

 also from the particles at a minute depth below it. He 

 found that the emission is most abundant in a direction 

 perpendicular to the radiating surface, and that it is 

 more rapid from a rough than from a polished surface : 

 radiation, however, can only take place in air and in 



