168 



although Phmck himself considers this unnecessary. Concerning the hypothe- 

 sis the late Henri Poincarc has said, "The present state of the question is as 

 follows: the old theories, which hitherto seemed to account for all known 

 phenomena, have met with an unexpected obstacle. An hypothesis has pre- 

 sented itself to M. Planck's mind, but so strange a one that one is tempted 

 to seek every means of escaping it; these means, however, have been sought 

 vainly. The new theory, however, raises a host of difficulties, many of which 

 are real and not simply illu.'^ions due to the inertia of our minds unwilling to 

 change their modes of thought." 



Note — Since this paper was read Professor Millikan has given a masterly 

 presentation of the various atomic theories of radiation. See Science, Jan. 

 24, 1913. 



