of Light hy the Discharge of a Leyden Jar. 341 



effect in various substances, are well known ; and what I have 

 to show to-day is practically nothing more than a repetition 

 of their experiments with the theory worked out. 



I take either a bit of heavy glass in its helix, or^ for pro- 

 jection, a tube of CS2 a yard long surrounded by four large 

 helices, each containing about 80 yards of gutta-percha-covered 

 No. 16 wire ; and on passing the discharge from a battery of 

 several jars the field flashes out bright, in what may be (if 

 one is looking direct towards the hot lime) quite a dazzling 

 manner, 



I have just received a post-card from M. Bichat, in answer 

 to an inquiry, saying that the coil they used was the secondary 

 bobbin of a KiihrnkorfF coil with a resistance of 5000 ohms. 

 Now, whether this was by accident or by design, it was difficult 

 for them to use a coil more suited to the purpose, or one that 

 would give a larger effect, as I shall show directly. The 

 Leyden jars employed by them were either one or two, about 

 18 inches high and 6 inches in diameter. We shall find that 

 the effect increases in direct proportion to the capacity of 

 the jars. 



To find out whether any time was required for the develop- 

 ment of the effect, they made the light coming through the 

 tube illuminate a slit, the spark being made to illuminate 

 another slit close above the first, and then both slits were 

 examined in a rotating mirror. Both were spread out into a 

 discontinuous band, and the serrations of the one agreed as 

 nearly as could be seen with the serrations of the other. Thus 

 proving in a beautiful manner that there was practically no 

 lag of the effect behind its cause, and thereby contradicting 

 the conclusion of Yillari*. 



Meanwhile I had been doing similar experiments, but with 

 a bobbin of much smaller inductance, and using a still smaller 

 capacity, my object being to find the greatest frequency able 

 to show the effect distinctly. If, for instance, heavy glass or 

 CS2 was able to follow oscillations of some million per second, 

 there could be no further question but that Yillari^s conclusion 

 was wrong. I find that the CSg is able to show the effect 

 when the rate of alternation is 70,000 per second ; and though 

 the short length of heavy glass available does not enable me 

 at present to make quite the same statement for it, I have no 



* They also mention an unpublished experiment by MM. Curie and 

 Ledeboer, in which a disk of glass was spun between the poles after the 

 manner of a Foucault disk of copper, so that the path of the light was 

 parallel to the axis of rotation, instead of perpendicular to it as in Vil- 

 lari's drum arrangement, and in that case no diminution was obserTed. 

 If this experiment is yet published I am ignorant of it. 



