2S8 GREAT MEN OF SCIENCE 



already showed itself in the hands of its creator as immedi- 

 ately fruitful; he discovered the influence of the insulating 

 material through which the lines of force pass upon their 

 eff"ect, a fact that had hitherto been unknown. Every insula- 

 tor or 'dielectric,' is accordingly characterised by a peculiar 

 'dielectric constant,' which controls its application to the con- 

 struction of large electric capacities. Here also Coulomb's 

 law received a further extension, inasmuch as this constant 

 appears in it as a factor, when the electric force no longer acts 

 through empty space, but through material media. 



After this succession of extraordinary discoveries, a period 

 of exhaustion supervened for Faraday. These successes 

 had not been obtained without very hard work; for though 

 ideas came to him in plenty and without trouble, the 

 necessary innumerable experiments, and continual modifi- 

 cation of the ideas towards a convincing representation of 

 actual facts - the true work of the man of science - had led 

 to an expenditure of energy which reached the nearer to 

 the limits of the possible, the further he was led by his ideas 

 into the unknown. 



Frequent excursions to the seaside, where he could only sit 

 quietly and look into the distance, brought him some re- 

 cuperation; but it was only when he finally undertook a 

 journey to the Alps, and remained for a considerable time 

 in and near Interlaken, which at that time was a quiet 

 place, that he was completely restored to health. This 

 was followed by a series of new discoveries and important 

 works, in the time from his fifty-third to about his sixty- 

 fifth year. 



In this period he made his discovery of the magnetic rota- 

 tion of the plane of polarisation. It is noteworthy that this 

 discovery was preceded by hundreds of unsuccessful experi- 

 ments with all kinds of crystals and other materials, without 

 Faraday letting go of the idea of seeking some kind of eff^ect 

 of the magnetic lines of force upon light. 'It must be tried; 



