OF ARTS AND SCIENCES. 3G1 



XXIII. 



ON THE CONDITIONS THAT DETERMINE THE 

 LENGTH OF THE SPECTRUM. 



By Amos E. Dolbear. 



Communicated February 10, 1886. 



Professor Langlet's conclusion as to the absence in the sun's rays 

 of wave lengths as long as those to be found in the moon's rays has 

 been a surprise to many, who for theoretical reasons have thought it to 

 be wellnigh certain that all possible wave lengths were to be found in 

 the sun's rays. 



The common implication is that, when a body is being heated, the 

 shorter wave lengths that appear are simply added to those already 

 present, so that the spectrum's length is in a manner proportional to 

 the temperature of the radiating body. If that was so it is difficult to 

 see how there could be economy in electric glow lamps by simply using 

 higher potential. It might give more light, but if at the same time 

 the lower so-called heat-waves were just as numerous and with greater 

 amplitude, that is, if the visible waves were not in any way the repre- 

 sentatives of the energy of the lower end of the spectrum, then the 

 amount of light would be proportional to the amount of energy 

 expended, which is not the case. 



Instead of that it increases as the 3d power. This great increase 

 of the visible waves is then at the expense of the lower end of the 

 spectrum, and any measure of the length of the spectrum from such a 

 source would probably show a marked decrease in the length of it 

 between a low red and a white heat. This, too, is in accordance with 

 molecular dynamics. If there be a gaseous volume of elastic molecules 

 at any assigned temperature, the molecules collide and vibrate between 

 impacts. Their rates of vibration must depend upon their molecular 

 weight and elasticity, and similar molecules vibrate at equal rates. 

 The characteristic wave lengths, or those by which a given gas may be 

 identified, are produced by the vibrations between impacts, and the 

 number of impacts per second will depend upon the gaseous density. 



