Question in Electro- Optics. 



385 



charged with water, and placed in the course of the pencils 

 BF,0G, immediately 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 disturbance. And when the 

 fringes, extinguished in this way mechanically, 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 

 experiment, 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 birefringent action of the medium. 



Regular Dislocation of the Fringes. — The electric arrange- 

 ments are the same as formerly, the two internal conductors 

 being connected permanently, the first with the prime con- 

 ductor and the second with earth. There is only one change 

 made in the apparatus : the nicol N is withdrawn, and a 

 small rhomb of Iceland spar (about 3 centim. 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 (£,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 



<x 



a 



T 



S € 



Fig 3. 



visible, the sets (j3) and (7) are seen to be relatively displaced, 

 the system (/3,y) being changed into the system (8, e). The 

 extent of the dislocation increases as the potential rises : that 



