1630 
=== 
Fig. 2. 
trodes is placed behind the front pane, the other before the piceine 
wall of the cuvette-space. The first electrode is perforated so that a 
small opening for the entering illuminating beam is at disposal. 
If in this cuvette the current (either ordinary or alternating) was 
closed all elongated particles disappeared from the 
field of vision. If the current was again broken, the normal 
image reappeared. 
About a minute after the circuit was closed the field of vision was 
so obscured that a sharp contour of the light-cone could no longer 
be distinguished. A few pointed images and a single flake still 
indicated the cataphoretic motion; after breaking off the current, the 
cone becomes again recognisable after about half a minute; after 
a minute a single elongated particle is again noticed; after two 
minutes the previous image has almost entirely returned. 
This observation confirms the hypothesis of the German investi- 
gators as well as our assumption that the elongated particles would 
be invisible when situated with their longitudinal axis parallel to that 
of the illuminating beam. After these investigations had been closed 
there came to oar knowledge a paper by Sirpenropr'), in which 
he discussed the same for particles which have microscopic dimen- 
sions in one direction and ultramicroscopic ones in the two other 
directions. Although the linear dimension of the V, QO, particles 
remains perhaps below the microscope demareation and the theory — 
of Siepenrorpr may, therefore perhaps not be applied here without 
modification *), the cause of this remarkable phenomenon is pointed 
out in this paper all the same. 
Incidentally, a remarkable observation has been made with this 
cuvette. Once, by mistake the first platinum electrode was connected 
with the outer wall of the front pane. On closing the current there 
still took place an obscuration of the field of vision; on closer 
examination the contact appeared to originate as follows: from that 
outer electrode, via the water immersion drop (which connects 
objective and covering glass), through a crevice between covering 
1) Stepentopr, Zeitschr. f. wiss. Mikroskopie 29, 1 (1912). Also compare 25, 
424 (1908) and Mary, 29, 48 (1912). 
2) See p. 38 of his first-mentioned treatise. 
