394 



Transactions of the Society. 



The same thing holds in all cases of the development of energy 

 in the form of oscillation. A mobile medium will take more 

 energy from a given source of power in a given time than a 

 sluggish medium and more energy can be imparted to a sluggish 

 medium from a high potential source of power than from one of 

 low potential. Hence we should expect that if a focus of light- 

 discharging energy is found to maintain a certain rate of output 

 in a sluggish medium, it will necessarily discharge from a higher 

 potential, than if it were discharging at the same rate in a more 

 mobile medium. 



Bearing this principle in mind, we may take up again the 

 problem of the focus formed in glass. It will be simplest to 

 imagine this as a minute sphere radiating light in every direction. 

 Suppose that it radiates for some small unit of time — a 650th part 

 of a millionth of a second for example — during which time it will 

 have filled a sphere 2 feet in diameter ; and imagine further, that 



at that radius the glass medium is 

 bounded and that the expanding 

 wave-fronts pass without refraction 

 into a vacuum. When the foremost 

 wave-front reaches this boundary 

 surface, let the glow be extin- 

 guished. Then sjk-tt-tks of a second 



Fig. 83. 



650 x 10 6 

 later all the light will have escaped 

 from the glass globe and will be 

 occupying a spherical shell around 

 it, as is indicated in fig. 83. 



Furthermore let it be assumed 

 that at the surface which the fore- 

 most wave-front has now reached, 

 there is a spherical mirror with internal reflecting surface on which 

 the centrifugal light impinges and by which it is reflected back. 

 Lastly, assume that somewhere in the shell of the mirror there is a 

 hole through which we can look in and examine its interior. 



If now there were no dissipation of light in other forms of 

 energy the wave-fronts set up in this enclosed space by that 

 momentary glow would go on rebounding from the surface and 

 crossing the centre indefinitely and finally we should have two 

 sets of standing wave-fronts established as in a Lippmann film. 

 The one set would cling to the surface of the mirror, and the other 

 set would cluster about the centre. These being stationary and 

 permanent could be seen, and it would soon occur to an observer 

 that every one of these standing wave-fronts was an image of the 

 illuminated part of the mirror. Opposite the hole in the mirror 

 would be a dark image of the hole on the opposite side of the 

 mirror, and there would be two corresponding dark spots in all 



