APR G.] HISTORICAL REMARK 217 



the stopping distance must be comparable with the elec- 

 trons' own diameter; which accordingly accounts for the 

 extreme thinness and consequent penetrating character of 

 the emitted pulse. 



APPENDIX H. 

 Faraday's Prophetic Nomenclature. 



Students of the life of Faraday will remember that when 

 he discovered the rotation of the plane of polarisation by 

 a magnetic field applied to dense bodies in which light 

 travelled along the lines of force, wresting the secret from 

 nature by strong and pertinacious experimental research 

 that w r ould not be denied, though the time was as yet by 

 no means ripe for comprehension of the fact when it was 

 discovered he labelled his discovery in a fit of enthusiasm, 

 " The Magnetisation . of Light and the Illumination of 

 Magnetic Lines of Force " : a label which puzzled con- 

 temporaries for a long time. 



It is difficult to see what meaning he can have attached 

 to, these phrases ; and for many years afterwards they 

 appeared unsuitable misnomers, indicating a foggy concep- 

 tion of his own discovery. 



It is not likely that his state of mind was really at all 

 clear on the subject, and probably he would at a later stage 

 have been willing to plead guilty to a less than lucid mode 

 of conceiving the phenomenon ; which nevertheless always 

 specially pleased him, though when it was reduced to a 

 mere rotation of the plane of polarisation, it seemed to 

 many mathematicians and physicists to have lost its unique 

 and surprising interest. It must always be remembered, 

 however, that interest was never lost by either Lord Kelvin 

 or Clerk Maxwell, and that it was the chief fact which 



