Electricity and Light. 343 



14. Fifth experiment. — A compensating slip is introduced 

 between the dielectric and the analyzer ; all the other arrange- 

 ments and the procedure are as in the first experiment (6). The 

 plane of polarization being at 45° to the horizon, and the initial 

 extinction in the polariscope perfect, the electric force is applied, 

 and the light is well restored. When the compensating slip is 

 now raised into the course of the beam, and stretched horizon- 

 tally, the light is weakened, and, with a proper degree of tension, 

 extinguished perfectly. As the electric action is continued an 

 increasing force has to be applied to the compensator to produce 

 extinction ; but the force is always of the same kind — tension 

 exactly or nearly parallel to the lines of electric force-, or com- 

 pression in a direction exactly or nearly perpendicular. 



After a few trials I got this phenomenon to present itself with 

 perfect regularity. The compensator is chosen carefully (13), 

 and inequalities of temperature avoided. A strong electric 

 action is kept up for any convenient time, 5 or 6 seconds to 30 

 or 40, till the light is well restored. The slip is introduced as 

 in the illustrative experiment (12), and carefully bent, so that 

 the part of it crossed by the light is stretched horizontally by a 

 continuously and slowly increasing force. Tn these circum- 

 stances, and constantly, the light fades away from a very consi- 

 derable brightness to an extinction as pure as I have ever got in 

 a polariscope. After this it is almost superfluous to say that, 

 when the compensating slip is compressed in a direction parallel 

 to the lines of force, the light restored by electric action is always 

 distinctly strengthened. 



15. Sixth experiment. — Distance of the spark- terminals small 

 (say, 3 inches), the other arrangements as formerly. The elec- 

 tric action is kept up for a minute or more, till the light in the 

 polariscope is certainly constant. The neutralizing plate is then 

 moved about, and the analyzer turned (through a small angle in 

 any. case), till the light is again well extinguished. All the op- 

 tical pieces being left untouched, the primary circuit is broken ; 

 the light reappears in a few seconds, and increases continuously 

 up to a certain permanent intensity; but the compensator, to 

 produce extinction at any stage of this increase, has to be com- 

 pressed in a direction parallel to the lines of force. And this is 

 evidently as it ought to be ; for what the compensator has now 

 to do is to reinforce the failing action of the dielectric, an action 

 which has been proved equivalent optically to compression along 

 the lines of force (14). 



16. Seventh experiment. — Distance of the spark-terminals 

 small, 1^ or 2 inches. As in the preceding experiment (15), a 

 constant effect of electric action is obtained in the polariscope, 

 and the light again extinguished by the neutralizing plate, which 



