Explanatory Notes 119 



themselves. An interesting account of all these matters 

 will be found in a scientific book by M. Jean Perrin, Pro- 

 fessor of Physical Chemistry in the University of Paris, 

 which has been translated into English by Mr. Soddy, 

 F.R.S., and published by Taylor & Francis under the title: 

 Brownian Movement and Molecular Reality. 



Pages 43 to 64 



It is as difficult to convey to general readers some idea of 

 the Principle of Relativity, and its virtual supersession 

 of the Ether, as it was to explain about the equi-partition 

 of energy and quanta, but a rough attempt may again be 

 made. It has long been a moot point whether the Ether 

 was or was not carried forward to any extent by moving 

 matter. The question was discussed mathematically by 

 Fresnel at the beginning of last century; and Fizeau found 

 experimentally that light travelled quicker down running 

 water than when it travelled against the stream about 

 half (more accurately y/i6ths) of the speed of the water 

 being added to the light, quite in accordance with the 

 teachings of Fresnel. But whether this sort of effect 

 could be detected in the immediate neighbourhood of 

 moving matter, without going actually inside it, was quite 

 unknown; and in and about 1892 and subsequent years at 

 Liverpool I made a serious attempt to examine the ques- 

 tion experimentally. The experiments are described in the 

 Philosophical Transactions of the Royal Society for 1893 

 and 1897, and the conclusion is that when a mass of iron 

 or steel is spinning so fast that it is liable to fly to pieces, 

 and when light is sent by mirrors round and round many 

 times in its immediate neighbourhood, so close as to be 

 actually grazing the spinning disks in some instances, 

 not the slightest effect of acceleration is manifested by the 

 beam of light, however delicately it be tested by means of 

 interference bands. Interference is arranged between 



