446 THE MODEKN THEORY OF LIGHT. 



other end to end by an iuductiou coil, waves are emitted on which all 

 manner of optical experiments can be performed. 



They can be reflected by plain sheets of metal, concentrated by para- 

 bolic reflectors, refracted by pi isms, concentrated by lenses. I have at 

 the college a large lens of pitch, weighing over three hundred- weight, 

 for concentrating them to a focus. They can be made to show the phe- 

 nomenon of interference, and thus have their wave-length accurately 

 measured. They are stopped by all conductors and transmitted by all 

 insulators. Metals are opaque, but even imperfect insulators such as 

 wood or stone are strikingly transparent, and waves may be received 

 in one room from a source in another, the door between the two being 

 shut. 



The real nature of metallic .opacity and of transparency has long 

 been clear in Maxwell's theory of light, and these electrically produced 

 waves only illustrate and bring home the well known facts. The ex- 

 periments of Hertz are in fact the a})Otheosis of that theory. 



Thus then in every way, Maxwell's 1865 brilliant perception of the real 

 nature of light is abundantly justified ; and for the first time we have 

 a true theory of light, no longer based upon analogy with sound, nor 

 upon a hypothetical jelly or elastic solid. 



Light is an electro-magnetic disturbance of the aether. Optics is a 

 branch of electricity. Outstanding problems in optics are being rap- 

 idly solved, now that we have the means of definitely exciting light 

 with a full perception of what we are doing and of the precise mode 

 of its vibration. 



It remains to find out how to shorten down the waves — to hurry up 

 the vibration until the light becomes visible. Nothing is wanted but 

 quicker modes of vibration. Smaller oscillators must be used — very 

 much smaller — oscillators not much bigger than molecules. In all prob- 

 ability (one may almost say certainly) ordinary light is the result of 

 electric oscillation in the molecules of hot bodies, or sometimes of bodies 

 not hot, as in the phenomenon of phosphorescence. 



The direct generation of visible light by electric means, so soon as we 

 have learnt how to attain the necessary frequency of vibration, will have 

 most important practical consequences. 



Speaking in this university it is happily quite unnecessary for me to 

 bespeak interest in a subject by any reference to possible practical ap- 

 plications. But any practical application of what I have dealt with this 

 evening is apparently so far distant as to be tree from any sordid gloss 

 of competition and comjjany promotion, and is interesting in itself as a 

 matter of pure science. 



For consider our })resent methods of making artificial light; they are 

 both wasteful and ineffective. 



We want a certain range of oscillation, between 800 and 400 billion 

 vibrations per second ; no other is useful to us, because no other has any 

 effect on our retina; but we do not know how to produce vibrations of 



