24 ON THE APPLICATION OF INTERFERENCE METHODS 



this is very far from being true with present gratings, such a limit undoubt- 

 edly exists. The accordance between the measm-ed ^N-idths of 18 lines 

 shows fmiher that this broadening of lines in a rare gas can be fully 

 accounted for by the application of Dopi)l('r's principle to the motion of the 

 vibrating atoms in the hue of sight, and indeed furnishes what may be 

 considered one of the most (Urect proofs of the kinetic theory of gases. 



The fonn of the ultimate components of all the groups of lines thus far 

 examined is found to agree fairly well with an exponential cui-ve, 



'f (J-) = e 



-«« 



a'x- 



» 



which shows that the distribution of velocities cannot vaiy widely fi*om that 

 demanded by Maxwell's theory. 



If the limit above mentioned were due solely to the motion of the mole- 

 cule, and the radiating substance could be rendered luminous while its 

 temperature was very low, it might be possible to observe interference 

 plienomena with dilference of path of many meters. But it must be consid- 

 ered that, since every vibrating molecule is communicating its energy to the 

 sether in the form of hght-waves, its vibrations must diminish in amplitude; 

 consequently the train of waves is no longer homogeneous even though the 

 vibrations remain absolutely isochronous, and the result is a broadening of 

 the line and hmitation of the difference of path at which interference is 

 visible. 



