354 RADIATION OF ENERGY. 



mass 



The presence of the radiant energy, therefore, cannot be detected by an 

 side observer, at all, if the system is at rest. If it is set into motion it will h 

 evident only as apparent inertia, and cannot be distinguished from the true 

 of the system, unless the true mass is already known. 



This discussion of possible forms of electrical systems, suggests that particl 

 of matter, be they atoms or electrons, may in some cases be closed electrical 

 systems having many degrees of freedom, none of which would become evident 

 until the closed nature of the system had in some way been broken down. 



If it could be shown that all matter consisted of various arrangements and 

 combinations of an ultimate unit, such as the electron, it might be possible to 

 reduce all forms of radiation to that of the electron, and to regard the electron 

 as a very minute Hertz oscillator. It appears to me, however, that the conception 

 of an ultimate unit is impossible. 



It is in the first place impossible to conceive of a body so small that when 

 placed under a microscope it cannot be magnified to any desired extent, and after 

 it is magnified it will be impossible to conceive of it having no structure. If it 

 has structure, it must have still smaller parts of which it is built up. If it is 

 impossible for the mind to grasp the idea of an ultimate unit of matter, it seems 

 to me to be doubtful if a satisfactory theory of radiation can be built upon it. 



It is possible that all forms of matter may be found to be related, and that the 

 phenomena of discharges in rarified gases, and of radioactivity, may be due to 

 the behavior of minute particles of matter carrying electric charges, but it is 

 improbable that these particles are all of the same size and that they represent 

 an ultimate element of mass. It must be remembered that when dealing with 

 enormously large numbers of minute particles the methods are statistical ones, 

 and only mean values can be obtained. 



§4. The Principle of Relativity. 



If a medium for the transportation of radiant energy actually exists it must 

 either remain at rest while the earth moves through it, or else it must be, to a 

 greater or less degree, carried along with the earth. 



The phenomenon of aberration discovered by Bradley 1 in 1728 would seem 

 to show that the ether, if it exists, remains at rest in space, and that the earth 

 moves through it. For Bradley found that the apparent position of a star in 

 the sky depended upon the ratio of the speed of the earth in its orbit around the 



to the velocity of light. In fact he found that if v is the component of the 



earth's velocity at right angles to the line joining the earth to the star, the 



gular aberration, a, is given by 



tan 



v 



« 



c 



From his observations he found that the maximum displacement of a star from 

 its mean position, due to aberration, was 20.44 seconds. 



1 Phil. Trans., 35, p. 637J1728. 



