﻿Double Refraction in Colloids. 



519 



tube was so placed that a yellow colour corresponded to an 

 expansion or positive dilatation, a blue colour to a contraction 

 or negative dilatation, parallel to the axis of the tube. An 

 eyepiece fitted with a Babinet's compensator could also be 

 used, the upper nicol being slid to the side and a second, 

 analysing prism put on top ; the indications o£ the com- 

 pensator were tested as to sign by means of a bent glass 

 strip*. Current was obtained from the town mains or from 

 a battery of cells. 



When the circuit was closed there appeared, one or two 

 millimetres from the positive end of the tube, a fine line, 

 yellow with selenite plate, indicating a positive dilatation 

 parallel to the axis of the tube. This line was to be seen 

 the sooner the higher the voltage ; its appearance varied — it 

 was sharp or blurred, quiet or in rapid movement, according 

 to the nature of the cell-walls that had been formed in the 

 jelly. In general the line had a screw or wave-motion, in 

 the direction of its length, and therefore perpendicular to its 

 motion into the tube (fig. 1 c). This wave-motion was the 

 more pronounced the higher the voltage and the more dried- 

 u p the gelatine. 



Fisr. 1. 



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Before the line, at the sides, were two blue spots 

 indicating a negative dilatation parallel to the tube axis ; 

 these blue places came only after the yellow line had gone 

 some distance into the tube, and not while its course was 

 through soaked jelly ; there the line was quiet without wave 

 motion (fig. la). This motion reappeared when the line 

 had penetrated further into the tube. The line seemed some- 

 times the boundary of a parabolic surface (fig. l^t)- 

 Before the line a yellow colour spread slowly out ; behind 



* G. Quincke, he, cit. 



t G. Quincke, Ann. d. PJujs. ix. p. 794 (1902). 



Phil. Mag. S. 6. Vol. 12. No. 72. Dec. 1906. 2 



