805 



/on 2n'VRT '^^' [dvj 

 ƒ i)'' A' dp ^ ^ ' 



dv 

 in which represent 



D distance of observation 

 ft index of refraction, 



tf^ angle of electric force in incident light with direction of 

 observation, 

 will likewise be 'found by using in (13) the value found for A' instead 

 of Fc. The exact formula then will result by inultiplying by 



the factor ^- = —^^ . 



Developing the cosine in Fc we tind 



F — Fe = ^JJJ'^- M'^) ^^-^''^/y^- 



— y. 



Representing this integral by e^ and introducing the value of C, 

 we get 



F — F, = ijt' (1 + cos ff) 



The formula of opalescence then will be: 



V RT /dnY 

 /o„. B' N ' UvJ 



I dp RT /8 



dv v^ \HA 



In the critical point itself it therefore is 



(17) 



l^dixY 

 --^ = -^ — A--'. . . . (18)^) 



I D- m^{\ + cos(r) 



The greater exactness of form. (17) as compared with (16) is 

 contirmed by the measurements of one of us (Z.)- According to these 

 measurements, which however bear upon a mixture of liquids the 



') According to tliis formula the pi-opoilioiialily of Ihc opalescence to /— <, 

 which holds for higher temperatures, changes continuously in the immediate 

 neigbourhood of the critical point, into proportionality witii >-2. 'riiis re«? "getting 

 whiter" of the opalescence should not be confused with tiie apparent changing of 

 colour which is always observed much I'artiier from the critical point. The latter 

 indeed is only a result of the method of observation, as is clearly proved by the 

 measurements of one of us (cf. Zernike thesis). 



