284 Anniversary Meeting. [Nov. 30_, 



» 



expansion and the alteration of the refrangibiUty of .bodies when heated, 

 is not disposed to question any conchisions regarded by Fizeau himself as 

 well founded. 



In 1862 he published "Kecherches sur les modifications que subit la 

 vitesse de la lumiere dans le verre et plusieurs autre corps solides sous 

 I'influence de la chaleur," — the first of a series of memoirs on the change of 

 dimensions and refractive powers of various kinds of glass, and many crys- 

 tallized substances. 



On the 23rd of May, 1864, he read before the Institut " Recherches 

 sur la dilatation et la double refraction du cristal de roche echauife and 

 on the 21st and 28th of Pvlay, 1866, " Memoires sur la dilatation des 

 corps solides par la chaleur." 



In these observations he has availed himself of the possibility of forming 

 Newton's rings v/ith the monochromatic sodium light when one of the in- 

 terfering rays is 52,205 waves in advance of the other, a fact which, con- 

 jointly with M. Foucault, he announced in 1849. Using the length of a 

 v/ave of sodium light (0*0005888 millimetre) as the standard of measure, 

 the position of a ring being observable to within -^^ of the distance be- 

 tween two consecutive rings, the variation of the distance between two 

 surfaces producing the Newton's rings can be measured to within -ya^-g-f 

 millimetre. 



A plate of the substance to be experimented on (let us suppose it to be 

 fluor), usually from 10 to 15 millimetres thick, bounded by parallel plane 

 surfaces, rests upon the platform of a metal tripod (the metal was steel in 

 the earlier observations, platinum with j\j of iridium in the later). The 

 feet of the tripod are screws of equal lengths terminating above in obtuse 

 points. On these points, at a distance of about millimetre above the 

 upper surface of the fluor, rests a plate of glass. By counting the number 

 of rings or bands that pass over a mark on the upper surface of the fluor 

 during a given change of temperature, the corresponding variation of dis- 

 tance between the lower surface of the glass and the upper surface of the 

 fluor is given, and the expansion of the metal being known by a similar 

 process, the expansion of the fluor is found. 



The Newton's rings formed between the two surfaces of the fluor depend 

 upon its thickness and its refractive power. The number of rings that 

 pass over the mark on the fluor during a given change of temperature being 

 observed, and its expansion having been found by the preceding observa- 

 tion, the change of its refractive pov/er due to the change of temperature 

 becomes known. 



In this way M. Fizeau obtained several very unexpected results. Of 

 these a few may be noticed. 



The indices of refraction of most substances were found either to increase 

 or to remain unaltered with an increase of temperature, but in fluor the 

 index of refraction diminishes with an increase of temperature. Diamond, 

 cuprite, and beryl have a maximum density, like water — diamond at 



