445 



vertical directions. The new tigiire in this {)a|ier lias been adjusted in 

 bucli a position, that it will correspond to the text of p. 797, if only the 

 words vertical and horizontal (line 9 and 10 from beneath) are inter- 



changed on readint>- 



Gronimjen, Jane 1914. 



Laboratories for Physics and for Inorganic 

 Chemistry of the University. 



Physics. — ''P'resnel'.*, coefficient for light of different colours^ 

 (First part). By Prof. P. Zeeman. 



One of the empirical foundations of the electrodynamics of moving- 

 bodies in the domain of optics is Fizeal's celebrated experiment on 

 the carrying along of the light waves by the motion of water, l^et 

 10 be the velocity of water relative to an observer, then for him 

 the velocity of light propagated in the water would be 



c 

 ft 



if the dynamical laws for the addition of velocities were perfectly 



general. 



In this equation n designs the index of refraction of water, c the 



velocity of light in vacuo, and we must take the upper or the lower 



sign, according as the light goes with or against the stream. Fizeau 



demonstrated that not the entire velocity w but only a fraction of 



it comes into action. This particular fraction appeared to be approxi- 



1 

 mately equal to 1^ , Fresnel's coefficient. Hence we must write 



in place of the above given formula: 



Ca = -±f«' (1) 



where 



^ = 1-A (2) 



For water 8 is equal to seven-sixteenths. 



The extremely important role which the formulae (1) and (2) have 

 had in the theory of aberration, in the development of Lorentz's 

 electronic theory needs not to be exposed here, and it is hardly 

 necessary to state that equation (1) is now regarded as a simple 

 confirmation of Einstein's theorem concerning the addition of velocities. 



I may be permitted however to point out the smallness of the 



