December 27, 1895.] 



SCIENCE. 



865 



nomena, but against its assumj)tion of the ex- 

 istence of a medium in which light is propa- 

 gated." The mathematical formula which 

 describes the law of action at a distance of 

 a force like gravitation, electric aud mag- 

 netic attractions and repulsions was, in the 

 opinion of most mathematical phj'sicists of 

 the last century, the most essential element 

 of and the ultimate goal in our knowledge of 

 this physical concept. For if the direct ac- 

 tion at a distance doctrine be accepted what 

 else remains there to enquire into ? How 

 much of this scientific creed received a di- 

 rect support from Newton personally is 

 difficult to tell. One thing is certain ; he 

 gave no support to the beautiful undulatory 

 theory of light originated by his friends, 

 Hooke and Huyghens, and it is interesting 

 to observe here that it was owing to the in- 

 tellectual rebellion of those very men who 

 supported the undulatory theory of light of 

 Hooke and Huyghens agaipst Newton's 

 corpuscular theory that the belief in direct 

 action at a distance began to lose ground. 



UNDULATORY THEORY OF LIGHT AND THE 



DOCTRINE OF DIRECT ACTION AT 



A DISTANCE. 



There is one aspect of the Undulatory 

 Theory of Light which, in mj' opinion, de- 

 serves much more attention than is gener- 

 ally devoted to it. This theory gave ex- 

 pression to a current of thought which ran 

 diametrically opposite to the direct action 

 at a distance doctrine of the Newtonian 

 school. For we should observe here that 

 the development of Newtonian dynamics 

 of rigid bodies was accompanied by a 

 steady though somewhat less rapid pro- 

 gress of the dynamics of compressible and 

 incompressible fluids and of compressible 

 solids, that is the Sciences of Hydrody- 

 namics and of ElasticitJ^ Now, Hydrody- 

 damics and Elasticity consider more par- 

 ticularly the modern extension of our orig- 

 inal statical concept of force, that is, force 



considered as a pressure, a tension, or as a 

 stress of anj' kind in a continuous materia 

 system in which each part, no matter how 

 small, is capable of a relative displacement 

 with respect to the adjacent parts, each 

 such displacement being accompanied by 

 an elastic reaction having a perfectly defi- 

 nite relation to the displacement. This re- 

 lation cannot be found by abstract reason- 

 ing based on the concepts of Statics or on 

 those contained in the Newtonian axioms, 

 but must be determined by actual experi- 

 ment. Observe now that Hooke, one of 

 the earliest investigators in this experi- 

 mental field, was one of the founders of the 

 Undulatory Theory of Light. Hydrody- 

 namics and Elasticity containing as they do 

 an additional experimental element mark an 

 advance in our physical knowledge of force 

 and substance over that contained in the 

 Newtonian axioms. The most important 

 element in this advance is the recognition 

 of the very important physical fact that 

 matter is capable of propagating force 

 between the various parts of a con- 

 tinuous material system with a perfectly 

 definite velocity and in a perfectly de- 

 finite manner, both the velocity and the 

 form of propagation, that is, the form of 

 the wave, depending not only on the distri- 

 bution of the masses of the system, but also 

 on the elastic property of each elementary 

 mass. Hence, whereas the so-called New- 

 tonian school of physicists, influenced by 

 the many unsuccessful attempts to explain, 

 gravitational force by mechanical hypothe- 

 ses, considered force principally in its for- 

 mal or mathematical aspect and also in as- 

 pect of an objectively active property of 

 matter, capable of acting directly at the dis- 

 tance, there was another school of physi- 

 cists, with Hooke and Huyghens at its head, 

 who focused their attention upon just the 

 opposite aspect of force, that is, force con- 

 sidered as a state of stress, and hence in- 

 capable of being communicated from one 



