168 



altlioufili Planck liinisclf coiisidcis this uiiiicccssary. Coiiccrniiifi thf hyi)()tlie- 

 sis the late Henri Poincurc has «aid, "Tho present state of tlie <|iiestii)ii is as 

 follows: the old tlieories. which hitherto seemed to account for ail known 

 phenomena, have met with an unexpected obstacle. An liy])othesis has pre- 

 sented itself to M. Planck's mind, hut so strange a one that one is 1einpte(l 

 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 illusions due to the inertia of our minds unwilling to 

 change their modes of thought." 



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

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

 24, 1913. 



