THE MODERN TIIEOKY OF LIGHT. 447 



this rate. We can produce a detiiiite vibration of one or two hundred 

 or thousand per second; in other words, we can excite a pure tone of 

 definite pitch, and we can command any desired range of such tones con- 

 tinuously by means of bellows and a keyboard. We can also (though 

 the fact is less well known) excite momentarily definite a^therial vibra- 

 tions of some million per second, as 1 have explained at length; but we 

 do not at present seem to know how to maintain this rate quite continu- 

 ously. To get much faster rates of vibration than this we have to fall 

 back upon atoms. We know how to make atoms vibrate; it is done by 

 what we call " heating" the substance, and if we could deal with indi- 

 vidual atoms unhampered by others, it is possible that we might get a 

 pure and simple mode of vibration from them. It is possible, but 

 unlikely ; for atoms, even when isolated, have a- multitude of modes of 

 vibration special to themselves, of which only a few are of practical use 

 to us, and we do not know how to excite some without also the others. 

 However, we do not at present even deal with individual atoms; we 

 treat them crowded together in a compact mass, so that their modes of 

 vibration are really infinite. 



We take a lump of matter, say a carbon filament or a piece of quick- 

 lime, and by raising its temperature we impress upon its atoms higher 

 and higher modes of vibration, not transmuting the lower into the 

 higher, but superposing the higher upon the lower, until at length we 

 get such rates of vibration as our retina is constructed for, and we are 

 satisfied. But how wasteful and indirect and empirical is the process. 

 We want a small range of rapid vibrations, and we know no better than 

 to make the whole series leading up to them. It is as though, in order 

 to sound some little shrill octave of pipes in an organ, we were obliged 

 to depress every key and every pedal, and to blow a young hurricane. 



I have purposely selected as examples the most perfect methods of 

 obtaining artificial light, wherein the waste radiation is onl}- useless, 

 and not noxious. But the old-fashioned {)lan was cruder even than this ; 

 it consisted simply in setting something burning, whereby not the fuel 

 but the air was consumed, whereby also a most powerful radiation was 

 produced, in the waste waves of which we were content to sit stewing, 

 for the sake of the minute, almost infinitesimal, fraction of it which 

 enabled us to see. 



Everyone knows now however, that combustion is not a- pleasant or 

 healthy mode of obtaining light; but everybody does not realize that 

 neither is incandescence a satisfactory and unwasteful method which is 

 likely to be praticed for more than a few decades, or i)erhaps a century. 



Look at the furnaces and boilers of a great steam-engine driving a 

 group of dynanu)s and estimate the energy expended, and tiien look at 

 the incandescent filaments of the lamps excited by them, and estimate 

 how much of their radiated energy is of real service to the eye. It will 

 be as the energy of a pitch-pipe to an entire orchestra. 



It is not too much to say that a boy turning a handle (;ould, if his 



