THE PROGRESS OF PHYSICS. 157 



We owe to Clerk Maxwell, and to Hertz, for 

 experimental corroboration, the image of a plane 

 wave of light as a propagation of an ethereal dis- 

 turbance, in which there is electric and, at the same 

 time, magnetic intensity, varying as a simple har- 

 monic function of the time. In what may seem to 

 be plainer words, we regard light as an electric phe- 

 nomenon, and the term electric light as a tautology. 



Invisible Light. From what has been said it may 

 be inferred that light has many forms, and that it 

 is not necessarily visible. Even in sunlight there 

 are components which are not visible to our eyes. 



One of the most recent additions (1896) is that 

 of an invisible radiation which Becquerel discovered 

 to be emitted by many fluorescent substances and 

 especially by Uranium salts. The radiation can be 

 polarised and by means of it (as by the Rontgen 

 rays) photographs can be obtained through opaque 

 bodies. Moreover, like the Rontgen rays, the Ura- 

 nium-radiation causes an electrified body to lose its 

 charge, whether positive or negative.* 



SUMMARY. By Young and Fresnel, Fizeau and 

 Foucault and l>y others the emission theory of light 

 was replaced by the undulatory theory. Light was 

 interpreted in terms of ethereal waves, and Clerk 

 Maxwell and Hertz subsequently showed that it was 

 essentially similar to electro-magnetic radiations. 



THEORY OF ELECTRICITY. 



Beginnings. In the last quarter of the eighteenth 

 century, the Italian Galvani whose name has given 

 our language several new words had discovered 



* See J. J. Thomson. Address Section A, Rep. Brit. Ass. 

 for 1896, p. 703. 



