4 PRESIDENTIAL ADDRESS. 



to move about in it perfectly freely and without hindrance, and 

 yet behaves like a mass of jelly in the matter of transmitting 

 these waves of light. Such a jelly seems at first sight rather 

 hard to believe, but according to modern views electrical and 

 magnetic phenomena also require the existence of an ether, and, 

 what is most remarkable, the same ether that will account for 

 electricity and magnetism will also account for light. Indeed, the 

 recent researches of Prof. Hertz and numerous others following in 

 his footsteps have shown that electrical waves can be propagated, 

 possessing properties exactly analogous to those of light-waves, and 

 from these and other experiments we have abundant evidence that 

 light-waves are in reality electrical oscillations. 



But whether we regard the ether as a material jelly-like solid, 

 or adopt the theory that light is an electric phenomenon, the fact 

 always remains that the light-vibratio7is take place in a direction 

 transverse to the line along which the light is travellings and this is 

 the only point upon which I wish now to lay stress as affecting 

 the phenomena of polarisation. 



Polarised Light. — If we take a string stretched horizontally 

 across the room, we can make it vibrate transversely in any 

 number of different directions. We can shake its end backwards 

 and forwards horizontally, when the whole will vibrate in a hori- 

 zontal plane, or we can shake it up and down and make the string 

 vibrate vertically, or by shaking it in any slanting direction we can 

 make it vibrate in any other plane. In just the same way, if we 

 suppose a beam of light travelling through the ether in the direc- 

 tion of the string, the particles of ether might vibrate horizontally 

 or up and down, or in any of the other directions. In this case 

 the plane of vibration of the ether, or the corresponding plane in 

 which the string vibrated, is called the plane of polarisation and 

 the light is said to be pla?ie polarised. By revolving the end of 

 the string in a circle, every other point will be made to revolve in 

 a circle, and this kind of motion may be taken as representing a 

 beam of what is called circularly polarised X\^X.. 



Finally, if we shake the string about indiscriminately in differ- 

 ent directions, we get a more general kind of vibration, and this 

 represents the kind of motion that goes on in the ether when a 



