342 Dr. Lodge on the Rotation of the Plane of Polarization 



reason to suppose it in any way inferior. In fact, experiment 

 distinctly suggests that the effect is practically instantaneous ; 

 but as to the degree of instantaneity I shall be able to make 

 a more exact numerical statement later on. 



It may be of interest to the Physical Society to have the 

 oscillatory character of the restored light demonstrated, and 

 there is no difficulty in the experiment. One sets the 

 analyser to as near darkness as possible, one receives the 

 trace of residual light upon a rotating mirror, by which it is 

 spread out into a faint band ; and on then sending sparks 

 through the coil of wire round the tube of CSg, the band 

 brightens and presents a distinctly beaded appearance at every 

 spark. 



Rotating the analyser a little, every alternate bead grows 

 fainter, while the other alternate ones brighten, thus proving 

 most directly the oscillatory character of the light and of the 

 Leyden-jar discharge. 



Still more feasible is it to spread out the light of the spark 

 itself into a serrated band, but in making this visible to an 

 audience it is well to save time by exciting the jars with a 

 large induction-coil instead of a "Wimshurst machine, because 

 not only can sparks be thus got much more frequently, but 

 each spark is multiple — the jars filling and overflowing several 

 times during the one coil-discharge. The multiple or inter- 

 mittent spark is analysed by the revolving mirror into a 

 number of serrated bands one after the other, and Mobile one 

 band is in the field of view of one part of the audience 

 another band may be visible to another. 



Although when a suitable circuit is employed the analysis 

 of the spark in a mirror such as is used for manometric flames 

 rotating not more than three or four times a second is easy, 

 yet with ordinary discharging circuits I have used small 

 mirrors spinning 200 times a second and failed to see any 

 certain trace of oscillation ; while, as is well known, Wheat- 

 stone used mirrors rotating 800 times a second, and got the 

 image-spark only barely elongated : not in the least serrated. 

 It may be worth while, therefore, to state the kind of circuit 

 which I have recently employed. The capacity consists of a 

 couple of condensers built up in the laboratory with double 

 thicknesses of window-glass alternating with tinfoil, and the 

 whole flooded in paraffin till it is a solid mass in a box of 

 teak weighing a couple of hundredweight. These condensers 

 stand a considerable length of spark, 3 or 4 inches for in- 

 stance, but their strength is not in the least called out in the 

 experiments now related. Their capacity when joined up 

 parallel is "048 microfarad. These are often supplemented by 



