I 



1894.] Fundamental Question in Electro -Optics. 255 



The result of the arrangement is that, when the pieces are properly 

 placed, the bright vertical slit L, as seen from E in the direction EG, 

 is crossed by a set of interference-fringes. These are well defined in 

 position by reference to a constant black line, the image of a fine wire 

 which is fixed across the slit L. It may be assumed, without argu- 

 ment, that any small increase or decrease of velocity of one of the 

 pencils BF, CG, will produce a positive or negative displacement of 

 the fringes, at the rate of one fringe- width of displacement for every 

 wave-length of relative retardation. As far as the assumption is 

 required, it is easily verified by the introduction of thin plates of glass 

 into the course of the light, anywhere between the two thick plates ; 

 nd I find in this way, definitely, that (as the pieces actually stand 

 u the diagram and in all the experiments) an ascent of the fringes 

 ndicates a relative retardation of the pencil BF. 



There are two essential pieces that remain to be noticed, of which 

 the first is the electro-optic cell. It is shown in the diagram how the 

 laterally separated component pencils pass through the cell, BF 

 through the electric field, and CG through the space electrically 

 screened by the second conductor, this conductor being always to 

 earth. The last piece is a Nicol's prism EC, which is placed in the 

 path of either of the single pencils GE, LB, with its principal section 

 laid (1) horizontally and (2) vertically. The design of the apparatus 

 will now be apparent, which is to give the means of detecting elec- 

 trically generated changes of velocity of the light BF in two succes- 

 sive cases, when the plane of polarisation is (1) perpendicular to the 

 lines of force, and (2) parallel to the lines of force. But in actual 

 experiment there is a difficulty encountered at once, which appears at 

 first sight to be insurmountable. 



Disturbance of the Fringes. Suppose all the pieces placed as in the 

 diagram, the cell nearly filled with carbon disulphide, the second 

 internal conductor put permanently to earth, and the fringes obtained 

 in good form and position. When connexion is made between the 

 first internal conductor and the knob of a charged Leyden jar whose 

 ater coating is to earth, there is an immediate disturbance of the 



inges, 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 



connected permanently with the prime conductor and an attached 



iyden jar, for the potential can then be raised regularly and very 

 slowly from zero, and the full coarse of the disturbance takes a 



nger time ; but in other respects the phenomena are the same as 



lore. 



When the fringes have been extinguished in this way by the 



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



mductor to earth, or by keeping the potential at a sensibly constant 



VOL. LV. T 



