150 The Smithsonian Institution 



well, taken in connection with the experiments of Hertz, have 

 demonstrated that optics is a department of electricity. 



"To produce radiation," comments Barker, "it is necessary- 

 only to produce electric oscillations of sufficiently short period. 

 An atom of sodium vibrates five hundred million times in one 

 millionth of a second. Could we produce electric atomic 

 oscillations at this rate and permanently maintain them we 

 could produce light. The problem of the age is, how to con- 

 vert some other form of energy into the energy of light. 

 That this is possible in theory, Rayleigh long ago showed. 

 That it is actually accomplished in nature, Langley's remark- 

 able measurements upon the glow-worm abundantly confirm."^ 



Another evidence of the penetration, as well as the inde- 

 pendence of his thought is shown by the fact that he was 

 among the earliest of American men of science to approve 

 the theory of evolution, as announced by Darwin. In 1864 

 he wrote to Asa Gray, who soon after the publication of the 

 " Origin of the Species" had become one of the warmest and 

 most influential of its authorized champions, in these words: 



"I have given the subject of evolution much thought, and 

 have come to the conclusion that it is the best working hy- 

 pothesis which you naturalists have .got. It, in fact, gives 

 you the first basis or real scientific foundation to stand upon 

 which you have ever had." 



Doctor Gray was at that time in the midst of a vigorous 

 controversy upon this subject with many of the principal 

 American naturalists, the most uncompromising of whom was 

 Agassiz. Although Henry's views were not made public, it 

 was generally understood that he sympathized with Darwin 

 and Gray. Agassiz, at that time a Regent of the Institution, 

 earnestly remonstrated with him and urged that he should 



1 Barker, George F., " Physics," New York, 1892, page 873. 



