SCIENTIFIC THOUGHT. 



Young and in the nineteenth century by Eumford and Young, were 



Fresnel. 



united into a consistent physical theory by Augustm 

 Fresnel, who has been termed the Newton of optics, and 

 who consistently, and all but completely, worked out one 

 great example of this kind of reasoning. He has the 

 glory of having not only established the undulatory 

 theory of light on a firm foundation, but still more of 

 having impressed natural philosophers with the import- 

 ance of studying the laws of regular vibratory motion and 

 the phenomena of periodicity in the most general manner. 

 His work was carried through, as was that of Xewton, 

 by a combination of observation, measurement, and calcu- 

 lation; of experimental skill with mathematical ability. 



tion of the same twenty-five years 

 before in his Berlin memoir, " Sur 

 la lumiere et les couleurs" (1745). 

 Euler was as much opposed to 

 Descartes' and Leibniz's views as 

 he was to those of Newton, and 

 though he admits having forerun- 

 ners, he hardly refers to the 

 principal one, viz., Huygens, whose 

 well - known and useful prin- 

 ciple he absolutely ignores. In 

 fact, in spite of his great name and 

 reputation, his ideas on the ether as 

 continuously filling space, and his 

 attempts to explain the phenomena 

 of light, heat, magnetism, and 

 even gravitation by means of this 

 continuum remained isolated, and 

 had hardly any influence on physi- 

 cal science. His great friend and 

 correspondent, Daniel Bernoulli, 

 remained a firm believer in action 

 at a distance, and thought Euler 

 had put forward his hypotheses 

 with too much assurance. It is, 

 nevertheless, remarkable how 

 closely the terms in which Euler, in 

 his posthumous work ' Anleitung 

 zur Naturlehre' (edited by the 

 Petersburg Academy in the second 



volume of the ' ' Opera posthuma 

 . . . anno 1844 detecta," 1862), 

 describes his ether as continuously 

 filling empty space and existing in 

 a strained (gewa/tsam) condition, 

 agree with quite modern ideas on 

 the subject. Accordingly Eulers 

 ether theory has in recent times 

 been studied again by several 

 writers abroad, of whom I will only 

 mention E. Cherbuliez, ' Ueber 

 einige physikalische Arbeiten 

 Eulers ' (Bern, 1872) ; F. Rosen- 

 berger, ' Die Geschichte der 

 Physik' (vol. ii., 1884, p. 333 sqq.}; 

 C. Isenkrahe in ' Zeitschrift fiir 

 Mathematik und Physik ' (Hist. Lit. 

 Abth., vol. xxvi.) and 'Abhand- 

 lungen zur Geschichte der Mathe- 

 matik,' vi. ; and E. Miething, ' L. 

 Eulers Lehre vom Aether' (Berlin, 

 1894). The first-mentioned author 

 tries to answer the question why 

 Euler's ideas remained so isolated. 

 He says (p. 49): "If we combine 

 the results of Huygens' and Euler's 

 investigations, we see that in the 

 'fifties of the eighteenth century the 

 undulatory system formed a largely 

 developed scientific doctrine. . . . 



