2." T. Dr. J. Kerr. Experiments on a [Mar. 15, 



valne, 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 (twenty to eighty 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 formerly, 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 polarised in the 

 two principal planes. 



This optical disturbance is evidently a remote effect of the electric 

 action, prodnced 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 pervade 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 upward 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 observa- 

 tion. 



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

 with water, and placed in the course of the pencils BF, CG, imme- 

 diately behind the electro-optic cell ; and the fringes were obtained in 

 good form and position. The stirring of this water gave a set of 

 optical effects that could not be distinguished from the former dis- 

 turbance. And when the fringes, extinguished in this way mechanic- 

 ally, were well restored and made moderately steady by regular 

 stirring kept up for a time, I found that a disturbance of the same 

 kind could be obtained at pleasure, either by an interval of rest (the 

 longer the better), or by the addition of a little warm water. But 

 leaving this and returning to the electro-optic experiments, I proceed 

 to show how, in spite of these irregular movements of the fringes, 

 and in the midst of them all, it is possible to obtain a steady effect, 

 which corresponds perfectly to the known bi-refringent action of the 

 medium. 



Regular Dislocation of the Fringes. The electric arrangements are 

 the same as formerly, the two internal conductors being connected 

 permanently, the first with the prime conductor, and the second with 

 earth. There is only one change made in the apparatus ; the nicol 



