FUNDAMENTAL CONCEPTS OF PHYSICAL SCIENCE 23 



upon our conceptions of the constitution of matter; and since our 

 ideas concerning the nature of matter, like our knowledge of the 

 ether, is purely speculative, the science of optics has a doubly specu- 

 lative basis. One type of selective absorption, for example, is as- 

 cribed to resonance of the particles of the absorbing substance, and 

 our modern dispersion theories depend upon the assumption of nat- 

 ural periods of vibration of the particles of the refracting medium 

 of the same order of frequency as that of the light-waves. When the 

 frequency of the waves falling upon a substance coincides with the 

 natural period of vibration of the particles of the latter, we have 

 selective absorption, and accompanying it, anomalous dispersion. 

 For these and numerous other phenomena no adequate theory is 

 possible which does not have its foundation upon some assumed 

 conception as to the constitution of matter. 



The development of the modern idea of the ether forms one of 

 the most interesting chapters in the history of physics. We find at 

 first a tendency to assume a number of distinct media corresponding 

 to the various effects (visual, chemical, thermal, phosphorescent, etc.) 

 of light-waves, and later the growth of the conception of a single 

 medium, the luminiferous ether. 



In the development of electricity and magnetism, meantime, the 

 assumption of media was found to be an essential something with- 

 out which no definite philosophy of the phenomena was possible. At 

 first there was the same tendency to a multiplicity of media - - there 

 were the positive and negative electric fluids, the magnetic fluid, etc. 

 Then there grew up in the fertile mind of Faraday that wonderful 

 fabric of the scientific imagination, the electric field; the conception 

 upon which all later attempts to form an idea of a thinkable mechan- 

 ism of electric and magnetic action have been established. 



It is the object of science, as has been pointed out by Ostwald, 

 to reduce the number of hypotheses; the highest development 

 would be that in which a single hypothesis served to elucidate the 

 relations of the entire universe. Maxwell's discovery that the whole 

 theory of optics is capable of expression in terms identical with those 

 found most convenient and suitable in electricity, in a word, that 

 optics may be treated simply as a branch of electromagnetics, was 

 the first great step towards such a simplification of our fundamental 

 conceptions. This was followed by Hertz's experimental demonstra- 

 tion of the existence of artificially produced electromagnetic waves 

 in every respect identical with light-waves, an achievement which 

 served to establish upon a sure foundation the conception of a single 

 medium. The idea of one universal medium as the mechanical basis 

 for all physical phenomena was not altogether new to the theoretical 

 physicist, but the unification of optics and electricity did much to 

 strengthen this conception. 



