METHODS 13 



of electrons finds in an atom a very minute representation 

 of the planetary system, with a positive central sun and 

 negative little planets turning around it. A few examples 

 will help to understand this existence of parallel series of 

 different scales ; I choose them, of course, among those 

 which have some immediate relation with life, so that the 

 study of them may be of immediate use. 



First Example : Vibratory, Oscillatory, Periodic Move- 

 ments. 



Vibratory motion, which occupies so considerable a 

 place in science nowadays, was first verified in the phe- 

 nomena of sound. The first vibratory movement known was 

 that of a spring producing a sound. In the study of 

 acoustics, scholars found a first continuous series of phe- 

 nomena all comparable with each other and differing from 

 each other by simple numerical coefficients. Every one 

 knows this series the series of sounds classified in the order 

 of height and reaching from the highest-pitched sound to 

 the lowest perceptible to the human ear. The study of 

 such vibratory movements is no longer carried on by the 

 ear, classifying the height of sounds, but by the eye which 

 verifies the registering of sound movements on cylinders ; 

 and this allows us to complete the series at its two ends 

 by vibratory motion which is quicker than that of the high- 

 est perceptible sound, or slower than that of the deepest 

 perceptible sound. 



Here, then, we have a series which is complete and very 

 interesting in itself ; but it became still more interesting 

 when the genius of Fresnel, taking up an idea of Descartes 

 and Huyghens, found in it a model for those movements 

 of the ether of physicists which constitute what we now call 

 radiations. Clerk Maxwell has since modified this idea 

 of Fresnel and substituted for the conception of an oscil- 



