1894.] 



Fundamental Question in Electro- Optics. 



257 



N is withdrawn, and a small rhomb of Iceland spar (about 3 cm. 

 long) is put in its place at E, with principal section horizontal. In 

 this way the two systems of fringes which were given by the nicol N 

 in succession are now given simultaneously, side by side, and each 

 the exact prolongation of the other ; the successive systems (a) of 

 the next diagram are changed into the double system (/3, 7). 



The machine is now set in motion. The system (/3, 7) is disturbed 

 as was the system () formerly ; but in the midst of the disturbance, 

 and as long as the fringes are clearly visible, the sets (/3) and (7) are 



oo 



seen to be relatively displaced, the system (y3, 7) being changed into 

 the system (, e). The extent of the dislocation increases as the 

 potential rises ; that shown in the diagram, which is about three- 

 fourths of the fringe-width, is not much below the highest that can 

 be got with the apparatus. The direction of the dislocation is con- 

 stant, and indicates a relative retardation of that vibration in the 

 electric field which is parallel to the line of force ; and this agrees 

 with the known character of the medium CS 2 as a positive di- 

 electric. 



It is very interesting to watch the two sets of fringes (S) and (<?), 

 and to see them sometimes moving rapidly and very fitfully, but 

 moving always as one system, with its two parts dislocated un- 

 changingly, except so far as the extent of the dislocation varies 

 "with varying potential. It is equally interesting to see the effect 

 of spark- discharge of the prime conductor, especially from high 

 potential. At the instant of the spark there is a sudden disap- 

 searance of the dislocation, an extremely quick jump of the fringes 

 ito line with each other, and this without perceptible check or sud- 

 len change of any kind in the disturbance-motion common to the 

 two sets at the time. The best way of observing the effect is to take 

 sparks from prime conductor to earth at stated intervals, while the 

 shine is kept working at some constant rate. The dislocation then 



T2 



