384 Dr. John Kerr on a Fundamental 



the fringes obtained in good form and position. When con- 

 nexion is made between the first internal conductor and the 

 knob of a charged Ley den jar whose outer coating is to earth, 

 there is an immediate disturbance of the fringes, a set of large 

 and irregular movements, with deformations, ending in the 

 disappearance of the whole system in one or two seconds. 

 The effects are seen better when the first internal conductor 

 is connected permanently with the prime conductor and an 

 attached Leyden jar ; for the potential can then be raised 

 regularly and very slowly from zero, and the full course of 

 the disturbance takes a longer time ; but in other respects 

 the phenomena are the same as before. 



When the fringes have been extinguished in this way by 

 the electric action, it is easy to recover them, either by putting 

 the prime conductor to earth, or by keeping the potential at a 

 sensibly constant value high or low for a little time. If with 

 this view the machine be kept working at a constant rate 

 throughout the experiment, the extinguished fringes return 

 gradually into the optical field, and in a little time (20 to 80 

 turns of the plate) they are as clearly visible as they were 

 before disturbance ; their forms also are good, and their 

 positions approximately constant, though they do not often 

 continue quite motionless in such circumstances even for a 

 fraction of a second. If the prime conductor be now put to 

 earth for a little, and the experiment be then repeated, the 

 disturbance passes through all the same phases as before, 

 though it is more violent at starting as the preceding interval 

 of rest is longer. All these effects come out equally well with 

 common light and with light polarized in the two principal 

 planes. 



This optical disturbance is evidently a remote effect of the 

 electric action, produced immediately — not by electric strain 

 — but by irregular changes of density in the medium. We 

 know that in the present cell, as in every like arrangement, 

 the electric action throws the liquid into currents, which per- 

 vade all parts of the cell and are very intense at high potential. 

 These material currents explain the changes of density ; for, 

 at starting, they give rise to a rapid process of mixture, 

 forcing denser masses upwards into the course of the light, 

 &c; and, afterwards, when the mixture is completed, they are 

 still accompanied by irregular variations of pressure in the 

 liquid. It should be easy therefore to imitate the effects by 

 means purely mechanical ; and of this I can give an example 

 from actual observation. 



A plate cell, about an inch thick and open at the top, was 



