300 Maxwell. 



the rich physical suggestiveness of Maxwell's ; the value of 

 his memoir lies chiefly in the introduction of the retarded 

 potentials. It may be remarked in passing that Lorenz's 

 retarded potentials are not identical with Maxwell's scalar 

 and vector potentials ; for Lorenz's a is not a circuital vector, 

 and Lorenz's < is not, like Maxwell's, the electrostatic potential, 

 but depends on the positions occupied by the charges at certain 

 previous instants. 



For some years no progress was made either with Maxwell's 

 theory or with Lorenz's. Meanwhile, Maxwell had in 1865 

 resigned his chair at King's College, and had retired to his 

 estate in Dumfriesshire, where he occupied himself in writing 

 a connected account of electrical theory. In 1871 he returned 

 to Cambridge as Professor of Experimental Physics; and two 

 years later published his Treatise on Electricity and Magnetism. 



In this celebrated work is comprehended almost every 

 branch of electric and magnetic theory; but the intention of 

 the writer was to discuss the whole as far as possible from a 

 single point of view, namely, that of Faraday; so that little 

 or no account was given of the hypotheses which had been pro- 

 pounded in the two preceding decades by the great German 

 electricians. So far as Maxwell's purpose was to disseminate 

 the ideas of Faraday, it was undoubtedly fulfilled ; but the 

 Treatise was less successful when considered as the exposition 

 of its author's own views. The doctrines peculiar to Maxwell 

 the existence of displacement-currents, and of electromagnetic 

 vibrations identical with light were not introduced in the first 

 'volume, or in the first half of the second volume ; and the 

 account which was given of them was scarcely more complete, 

 and was perhaps less attractive, than that which had been 

 furnished in the original memoirs. 



Some matters were, however, discussed more fully in the 

 Treatise than in Maxwell's previous writings ; and among these 

 was the question of stress in the electromagnetic field. 



It will be remembered* that Faraday, when studying the 



* Cf. p. 209. 



