ANALOGY. 287 



Herschel noticed that in three physical phenomena, a 

 screw-like form, technically called helicoidal dissymmetry, 

 was presented, namely in electrical helices, plagihedral 

 quartz crystals, and the rotation of the plane of polariza- 

 tion of light. As he himself has said b , ' I reasoned thus : 

 Here are three phenomena agreeing in a very strange pecu- 

 liarity. Probably, this peculiarity is a connecting link, 

 physically speaking, among them. Now, in the case of 

 the crystals and the light, this probability has been turned 

 into certainty by my own experiments. Therefore, induc- 

 tion led me to conclude that a similar connexion exists, 

 and must turn up, somehow or other, between the electric 

 current and polarized light, and that the plane of polariza- 

 tion would be deflected by magneto-electricity.' By this 

 course of analogical thought Sir John Herschel had actu- 

 ally been led to anticipate Faraday's great discovery of the 

 influence of magnetic strain upon polarized light. He 

 had tried as long ago as 1822-25 to discover the influence 

 of electricity on light, by sending a ray of polarized light 

 through a helix, or near a long wire conveying an electric 

 current. Such a course of inquiry, followed up with the 

 persistency of Faraday, and with his experimental re- 

 sources, would doubtless have e'fiected the strange dis- 

 covery. Herschel also suggests that the plagihedral form 

 of quartz crystals must be due to a screw-like strain 

 during the progress of crystallization ; but the notion, 

 although probable, remains unverified by experiment to 

 the present day. 



If ever men approach the investigation of the me- 

 chanism of thought, they must be guided by analogy. 

 Already many philosophers have drawn analogies between 

 nerve influence and the transmission of vibrations. Dr. 

 Briggs, Newton in his 24th Query, and Hartley, have 

 vaguely speculated concerning such vibrations, and some 

 b ' Life of Faraday,' by Bence Jones, vol. ii. p. 206. 



