218 Fizeau } s Experiment and the ^Ether. 



lation, but not in the earth's motion of rotation. This 

 assumption is not permissible to the Lorentz theory, for 

 according to the latter the aether is merely empty space. 



§ 4. Let us shortly examine the consequences involved in 

 the hypothesis, that the aether has tbe same velocity of 

 translation as the earth. 



Seventy years ago this hypothesis would have been objected 

 to on a priori grounds. The belief then was, that the aether 

 was created first and then myriads of hard unchangeable 

 material particles ; these came together under the influence of 

 certain " forces " of attraction, cohesion, heat, &c, and formed 

 the planets. The object of science was to discover and interpret 

 the Divine Plan, and, of course, it was unreasonable to 

 suppose that the aether had any closer connexion with any 

 one planet than any other. 



Now the standpoint is rapidly changing ; i( laws of nature " 

 have truth only with reference to our present state of know- 

 ledge, and to the degree of skill with which we have succeeded 

 in giving expression to it. The aether is reached only by our 

 intellectual processes; it is a conception to satisfy our 

 phenomena, Hence we have a right to attach it to our own 

 planet, if necessary, and the hypothesis must be judged solely 

 on its merits. 



If the aether participates in the earth's motion of trans- 

 lation, the only motion which the Michel son-Morley expe- 

 riment might reveal, is that due to the earth's diurnal 

 rotation. This amounts at the equator to *291 miles per 

 second, and hence gives a value of v/c=l"56 X 10 -6 instead 

 of the 10 ~ 4 given by the orbital velocity. The effect to be 

 observed would consequently diminish to 10 ^ c00 of its value, 

 and hence be 100 times too small for the sensitiveness of 

 the apparatus. So the Michelson-Morley experiment is 

 accounted for. 



In addition to the annual astronomical aberration with the 

 maximum value of 20"*47 there is a diurnal aberration 

 ranging from zero to 0"*31. The peculiarity of this hypo- 

 thesis is, that it explains them in different ways. The 

 diurnal aberration is explained in the same way as the 

 annual aberration is explained on the Lorentz theory. To 

 explain the annual aberration it is necessary to proceed as is 

 shown in the fig. is the observer on the earth supposed 

 at rest in the aether, S is the star and SO the path of a ray 

 to the observer. When the ray arrives at 0, the star has 

 moved through the aether to S', the earth and aether being- 

 regarded as fixed. The star is thus displaced behind its true 

 position S' by the angle SS'sin 0/OS = vsin 6/c, where v is 



