398 ScientifiG Intelligence. 



OBITUAEY. 



M. HippoLYTE FizEAU, the French physicist, died recently at 

 the age of seventy-seven. His name will be long remembered 

 in connection with his determination of the velocity of light. 



"Born in 1819, Fizeau was only thirty years of age when his 

 paper, * Sur une experience relative a la vitesse de propagation 

 de la lumi^re,' appeared in the Comptes Rendus. In this he put 

 forward his plan of rotating a wheel having round its rim alter- 

 nate teeth and spaces of equal width, so that these teeth and 

 spaces should alternately intercept and allow to pass a beam of 

 light from a source, and so adjusting the speed of rotation that 

 the time occupied by the light in traveling from the wheel to a 

 mirror and back again, should be equal to the time taken by the 

 rim of the wheel to advance through a space equal to an integral 

 number of times the width of a tooth or space. Curiously 

 enough, the other experimental method of finding the velocity of 

 light was described by Foucault in the very next volume of the 

 Comptes Rendus. In some respects the latter method- — that of 

 the revolving n^irror — was even more striking than that of Fizeau. 

 It allowed the velocity of light to be determined within an ordi- 

 nary room, and, besides, enabled the question as to whether light 

 traveled more or less quickly through a more refractive medium 

 to be decided by direct experiment. 



Another ex:periment of capital importance with which the name 

 of Fizeau will ever be honorably associated is that by which he 

 determined the amount of drift of light-waves in a transparent 

 medium in motion. According to a theory given by Fresnel, the 

 velocity of drift of ether-waves in a medium moving with velocity 

 u is {\ — \Jik)u^ where /x is the index of refraction of the medium. 

 This conclusion of Fresnel was verified more lately by the experi- 

 ments of Airy and Hoek, which proved, in opposition to the 

 statement of Klinkerfues, that no change in the constant of aber- 

 ration is observed when the tube of the observing telescope is 

 filled with water. But it was tested directly by Fizeau in the 

 most simple and beautiful manner. Two tubes were arranged 

 side by side, and water was forced at a considerable speed (as 

 much as seven meters per second) along one tube and back l3y 

 the other, while a beam of light was split into two parts, which 

 were sent round the tubes, one with the stream, the other against 

 the stream, and then brought together again and tested for inter-, 

 ference produced by the virtual difference of path traversed, 

 arising from the motion of the water. The result gave exactly 

 the formula quoted above, and has been confirmed by very care- 

 ful experiments made comparatively recently by Michelson and 

 Morley."— iV^^i^re, Oct. 1. 



Sr. Luigi Palmieri died at Naples in the early half of Sep- 

 tember at the age of eighty-nine. He was director of the 

 Observatory at Vesuvius and made important contributions to 

 meteorology and seismology. 



