Electricity and Light. 347 



being left at the centre of the plate. The action of the crystal 

 in the polariscope is well neutralized by a contrary plate of 

 equal thickness ; the other arrangements are as in the fifth ex- 

 periment. With my rough apparatus the adjustments are so 

 troublesome that I have executed only one short series of obser- 

 vations, of which I append all the notes preserved. 



Distance of spark-terminals i inch : a faint but clear resto- 

 ration of the light from almost perfect extinction ; the com- 

 pensator not working distinctly ; the insulation perfect. 



Distance 1 inch : the light clearly restored by electric action, 

 and then well extinguished by tension of the compensator parallel 

 to lines of force. The insulation is now failing, sparks passing 

 occasionally over the surface of the lac near the crystal. 



Distance j inch : the insulation still defective ; the light well 

 restored by electric action and then well weakened by horizon- 

 tal tension, and strengthened by compression. From what I 

 have seen in some other experiments, I think that the effects 

 in this case and the preceding may have been produced wholly 

 or partially by slight changes of temperature, due to occasional 

 spark-discharge over the surface of the crystal. 



Distance again \ inch : the insulation again good. In this 

 case it is noticed that, before the electric force is applied, while 

 the body of the light is barely perceptible in the polariscope, 

 there is a short length of it from the apex downwards (due 

 exactly through the centre of the electric field) which is per- 

 fectly extinguished. By electric action this upper part of the 

 light is restored very clearly, and is then as clearly extinguished 

 by tension of the compensator parallel to the lines of force. 



23. Upon the whole, though better experimental results are 

 desirable, I consider it proved that dielectrified quartz (like 

 glass) acts upon transmitted light as if compressed along the 

 lines of force, while dielectrified resin (unlike glass) acts as if 

 extended along the lines of force. 



24. Theory. — Faraday's views as to the constitution and func- 

 tion of dielectrics apply here very aptly. 



When the induction-terminals are charged, the particles of 

 the dielectric throughout the field are electrically polarized, and 

 tend accordingly to arrange themselves end to end, and to co- 

 here in files along the lines of force, just as iron filings do in 

 a magnetic field. As far as this tendency of the polarized 

 particles towards a file arrangement along the lines of force 

 takes effect, there is a new molecular structure induced in the 

 dielectric. 



If we neglect the influence of ordinary strains transmitted 

 from point to point of the solid, and assume, as a good first ap- 

 proximation warranted by facts (19), that the change of mole- 



