( 447 ) 



of ligh( in inoving- mndia. Now, tliis coefficient has been found to 

 be true by direct ineasuremeuts and may be calculated by means 

 of well founded theoretical considerations. It might be deemed strange, 

 if in these ways we arrived precisely at the value that is required 

 by a wrong theory. 



2. If we hope some time to account for the force of gravitation 

 by means of actions going on in the aether, it is natural to suppose 

 that the aether itself is not subject to this force. 



On these and other grounds, I consider Feesnel's theory as the 

 more satisfactory of the two. Prof. Planck is of the same opinion. 

 Nevertheless it will be of importance to consider the question from 

 all sides, and it is for this reason that the following remarks may 

 here be allowed. 



1. If the large condensation that has been spoken of and the 

 constancy of the velocity of propagation, whatever be the density, 

 be taken for granted, one can indeed explain all observed pheno- 

 mena. At least, I for one have been unable to find a contradiction. 

 It is true, as has already been stated, that, far away from the 

 earth, the equation (4) will no longer hold. In considering the motion 

 in those distant regions, the square of the velocity in the equation 

 (2) has to be taken into account, and the sun's attraction will have 

 to be considered. But, after all, I find that there may always exist 

 an irrotational motion, and this, in addition to a sufficient conden- 

 sation near the earth, is all that is required. 



2. If we apply to the moving aether the equations which 

 Hertz has proposed for moving dielectrics i) the propagation of light 

 will obey very simple laws. Suppose the earth to be at rest, and 

 the aether to flow, and let the axes of coordinates be fixed in space. 

 Then, if ö be the dielectric displacement, -0 the magnetic force, » 

 the velocity of the aether and V that of light, and if the electro- 

 magnetic properties of the aether be supposed to be wholly inde- 

 pendent of its density, the equation may be put in the form 



Div b — , 



di?, dS?, r3^.r 9 d 1 



4 .T I - . - + — (i\, t».r — IV bv) — TT- (IV b- — IV b-r) , etc. 



It rill OS J 



4 .T- (^ _ |l^=, _ oil _ 9 (,^ .rp..-tv..p,) + ^(.v.rx-vvrp..), etc. 

 \ 01/ 03 y at oy as 



') Wied. Ann. IW. 41, p. 36'.). 



