208 O.N. Rood on action of Electric Light on Iodized Plates. 
The positive electrical brush consists, as is well known, of 4 
short stem with widely branching ramifications; these latter are 
r ; 
very faint even in the 
2, 
on the wood-cut, fig. 1, whic 
is from a photograph magnified 
ten diameters. 
t is well known that the s 
negative brush is much smaller than the posl- 
tive, and it is often spoken of as a star or mr 
nute point of light; the photograph, however, 
shows that this is not the case, but that is 
structure is analogous to that of the positive 
brush, only that the ramification begins lower 
down on the stem, as it were, nearly at its root, as is seen im the 
wood-cut, fig. 2, which is from a magnified photograph. 
Negative electric brush, 
Positive electric brush, 
Action of weak electric light on the plate in the presence of daylight. 
—The Geisler tubes in the physical cabinet of the college enabled 
me now to put the probability of Dove’s suggestion to the test 
of experiment; some of these were connected with an induction 
coil and photographed in broad daylight, when it was found tha 
the image formed by the electric discharge could be easily traced 
through the length of the tubes, and that even the stratification 
was still partially visible. In these cases, however, the electri¢ 
light was still visible to the eye during the discharges. 
Aecordingly, to make an exact experiment on this point, & 
sheet of white paper was placed behind one of these tubes and 
invisible. Nevertheless an intense photographic image 0 it 
envelope, and a very distinct image of the diffused electri¢ lig 
_ This experiment is indeed a very erat ak proof of the tee 
ical activity of the electric light, the more so, as according 3 
e of my experiments, the iodized plate is by no means 
sensitive to slight differences in illumination as the human ey® 
