laboratory the ironpiece A BCH is fixed which passes into the 
glass tube £ D. By means of the mirror S a strong light-beam can 
be thrown along the axis of the tube (diameter 2 em). The meaning 
of the window JV will be given below (§ 10). Before the water 
reaches A it is mixed with small gas-bubbles. The paths of these 
bubbles beautifully contrast with the dark back-ground, which 
enables us to pursue particulars of the motion of the fluid’). 
3. The illumination is arranged in such a way that a vertical 
plane through the axis of the tube /’D is strongly illuminated. 
On the horizontal axis of a small continuous-current motor, the 
speed of which can be regulated, a vertical disk of eard-board with 
a number of holes in it is fixed; through these we look at the tube. 
At a proper velocity of the disk the confuse image of the many 
entangled stream-lines is decomposed into simple elements. At / we 
then discern straight line-elements, of which many are horizontal, 
while others show an inclined direction. It is by the latter that the 
radial motion becomes visible, which has been discovered by OsBorne 
REYNOLDS. The further we go from / to Die. in the direction of the 
streaming water, the simpler the image becomes, until at a distance 
of 20 em. from // it does not change any longer. There are however 
1) The gas-bubbles can be introduced into the fluid by pressing compressed air 
through a fine opening. Better results are reached however with an electrolytic 
developer as ZENNECK uses in his method for demonstrating the stream-lines in 
the inner part of the fluid. (Berichte deutsch. phys. Ges. p. 695. 1914). In my 
experiments the electric current entered through two coat plates. The behaviour 
of different coal plates is not always the same. The gas-bubbles had a diameter of 
0,1 to 0,3 mm. When of one of the plates a piece had been broken off we took into 
use a new pair. Then the gas-bubbles proved to have become far too small, so 
that we took again the old plates. If the bubbles are too small the stream lines 
cannot be observed very well, at least not in the mirror of § 4. In the experiments 
of ZENNECK the bubbles might be very small, 
