450 Dr. J. Kerr on a new Relation between 



are attracted from the most distant parts of the cell, and show 

 themselves at once about the centre of the field, either as a set 

 of sparkling and irregularly moving points, or as a chain con- 

 necting the terminals and remaining always bright in the polari- 

 scope. But whatever the form which they assume, they inter- 

 fere very materially with the principal effect, if they do not mask 

 it altogether. 



29. First experiment. — The room well darkened ; the light a 

 narrow flame immediately in front of the first Nicol ; the two 

 Nicols about 4 feet apart ; the dielectric about a foot from the 

 eye. Before extinction by rotation of the analyzer, the light is 

 seen through the plate of liquid as a narrow and very well- 

 defined vertical band, midway between the terminals. 



Plane of polarization at 45° to the horizon. When the 

 prime conductor is charged, the light comes out in the polari- 

 scope steadily and very clearly from pure extinction. When the 

 prime conductor is discharged by a spark upon the knuckle, the 

 restored light disappears instantly and totally. When the light 

 is again restored by electric action, and a compensating slip of 

 glass „is introduced and compressed horizontally, the light is 

 either extinguished or greatly weakened; when the compen- 

 sator is extended horizontally, the light is always strengthened. 



With one gentle turn of the plate, the light comes out conti- 

 nuously from initial extinction till, at the end of the turn, it is 

 very well restored : in this case the extinction by compression 

 of the compensator parallel to the lines of force is perfect. 



With a strong electric action, with or without occasional 

 sparks through the liquid, the light is much more strongly re- 

 stored : in this case, compression of the compensator parallel 

 to lines of force does not act so regularly, never extinguishes the 

 light all at a time, but produces generally a very dark broad 

 band, approximately straight and horizontal, which descends 

 from the outer parts of the electric field towards the centre as 

 the compression increases. This proves that the amount of bi- 

 refringent action varies considerably throughout the part exa- 

 mined of the electric field. In the black band itself, the actions 

 of the dielectrified bisulphide and horizontally compressed glass 

 compensate one another ; at other equatorial points of the field 

 one or the other action predominates — that of the glass outside 

 of the dark band, that of the liquid inside. 



Plane of polarization at 0° or at 90° to lines of force. One 

 gentle turn of the plate has generally no sensible effect, but 

 sometimes gives a barely perceptible restoration from perfect ex- 

 tinction. A strong electric force gives generally a clear restora- 

 tion from extinction ; the effect, however, is not nearly so intense 

 as in the case of 45°, and it tends to evanescence as the lines of 



