PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 327 



with the great work apparently ended, with progress only in de- 

 tails, rounding out and polishing a fabric which in its funda- 

 mental characteristics was finished long before. This was the 

 time when men began to say with sadness that the fundamental 

 facts of physics were probably all or nearly all known, and that 

 the future of the science lay in the sixth place of decimals. And 

 indeed a corroboration of these views might be seen in the ex- 

 quisite instruments for fine measurements of all sorts which are 

 characteristic of this period. I can only mention without de- 

 scription a few, as the interferometer of Michelson, with the 

 modification, increasing considerably its sensitiveness, added by 

 Chamberlain, the entirely dififerent interferometer of Fabry and 

 Perot, the almost errorless clocks of Riefler, the exquisite bal- 

 ances of Rueprecht, the echelon and the giant diffraction grat- 

 ings, both of these last two due to Michelson, and the great as- 

 tronomical instruments developed especially at Mt. Hamilton in 

 California. But despite all appearances the road to the modern 

 physics did not lie that way. In 1890 there was no indication, 

 there was hardly a dream that any new realms could exist com- 

 parable to the great regions already explored. Even in the year 

 1900 Dr. Mendenhall gives fourteen lines, out of an article filling 

 several newspaper columns, to the phenomena grouped about the 

 discovery of Roentgen. 



And perhaps it can hardly be said that the advances of the 

 past twenty-five years hold so commanding a place as the monu- 

 mental achievement of the nineteenth century. They are charac- 

 terized rather by their unexpectedness, their contravention of 

 accepted ideas. The nineteenth century was constructive, the 

 twentieth is largely revolutionary. 



The more important .advances cannot be grouped, like those 

 of the preceding period, under the classical divisions, as lights 

 heat, or what not. They occupy new fields, related to all the 

 others, but different, and not their least interest is the manner 

 in which they unravel and disintegrate the web of theory, the 

 weaving of which was begun a hundred years ago by Young and 

 Dalton and Fresnel. They may be included broadly under the 

 head of radiation, and this word, radiation, embracing in itself 



