78 Mr. J. H. Jeans [March 30, 



Let US begin with a very brief consideration of the first cloud. 

 The astronomical phenomenon of the aberration of light is known 

 to all. To catch the light from a given star, the astronomer must 

 not point his telescope towards the position of the star, but in a 

 direction obtained by compounding the velocity of light with the 

 velocity of the earth in space. The principle is substantially the 

 same as that on which, when rowing through a strong current, 

 the nose of the boat must be pointed up-stream above the spot it is 

 desired to reach. If light is brought to us in the form of ether 

 waves, the phenomenon of aberration shows that the earth must be 

 moving through a stagnant ether, so that to us on the earth there 

 must appear to be a current of ether streaming past the earth. 



It is natural to attempt to measure the velocity of this stream, 

 and so determine the earth's absolute velocity in space. In the 

 famous Michelson-Morley experiment, which was designed to this 

 end, a beam of light was split into two parts. One part is sent up 

 the ether stream and comes down again to the starting point after 

 reflection by a mirror, while the other half is sent an equal distance 

 across stream, and again comes back after reflection. Thus the total 

 length of path is the same in each case, and if the earth w^ere at rest 

 in the ether, the time occupied in going and returning would be the 

 same for each ray. But if there is a stream of ether moving past 

 the earth, it is readily seen that the second half of the beam must 

 gain in time on the first, for the time lost in moving against stream 

 by the first ray is not fully compensated by the gain in time when 

 moving with the stream. The experiment w^as arranged so that any 

 difference in the time of the two rays would show itself in the 

 formation of interference fringes, and this difference would give a 

 measure of the earth's velocity through the ether. But the experi- 

 ment, repeated with all possible checks and refinements, refused to 

 disclose any motion of the earth through the ether at all, and other 

 quite different experiments designed to the same end gave one and 

 all precisely the same reply. 



Thus, assuming that light consisted of weaves in the ether, experi- 

 ment and observation led to the conclusion that the ether must be at 

 rest, and the earth at rest in the ether, so that the evidence seemed, 

 if strictly interpreted, to lead back to the geocentric universe of 

 pre-Copernican days. This was the cloud over the dynamical theory 

 of light. 



From the attempts of Lorentz and Einstein to unravel this con- 

 tradiction, the theory of relativity has arisen. It says in effect : 

 "The existence of an ether is conjectural, while the results of 

 the ]\Iichelson-Morley and other experiments are certain. Let us 

 abandon the dynamical interpretation of light, which is based on the 

 conjectural existence of an ether, and examine what laws are obtained 

 by starting from the hypothesis that all attempts to measure the 

 absoluie velocity of the earth, or of any other mass, must necessarily 



