465 
one in going, and one in returning along the guides. The maximum 
velocity is practically constant over a distance of about 20 cm. 
When the fly-wheel performs 184 revolutions a minute, the maximum 
velocity rises to somewhat more than 10 meters per second. This 
is the highest value that can be reached. 
The driving apparatus was constructed by the works Werkspoor 
at Amsterdam. The execution of the mechanical parts had to be 
adapted to the optical requirements in the Laboratory. 
With regard to the optical arrangement of the experiment we 
may refer to my former communications on the Fizwav-effect. The 
quickly moving transparent, solid substance takes the place of the 
running water of the earlier experiments. The length of the moving 
column of quartz or glass ranged between 100 and 140 cm. in 
different experiments. After the successive application of numerous 
improvements it was possible to cause the beams of light to interfere 
through the moving quartz or glass, and to obtain pure interference 
lines at the greatest velocity which the apparatus admits. 
The experiment comes to this, that the interference bands are photo- 
graphed twice, first with a movement of the column to the right, 
and then with a movement to the left. These photos should be taken 
by admitting light during a time of the order of one hundredth of 
a second and at the moment of the maximum velocity. 
The optical effect to be expected, when 7 represents the length of 
the moving transparent substance is: 
4l (| 1 A du 
ie 2 de u' 2th VERDEELT Le Te (4) 
GNU. ie Se ak 
It appears from this formula, which will be proved later on (see II), 
that the optical effect is approximately proportional to u—1. In 
Fizkau’s experiment the optical effect is proportional to u? —1 according 
to (3). This difference is connected with the fact that in Frzrau’s 
experiment in its usual form, the velocity in a definite point of space 
is always the same, whereas in the experiment now considered the 
light must overtake the moving bar. 
As regards the optical effect observed, the method considered now 
will accordingly be two and a half times less favourable for a 
w—1 
pay —u+i. 
This is more or less compensated by an advantage with regard to the 
dispersion term. As follows from formulae (3) and (4), the ratio of 
the dispersion term to the principal term is in the second case 1,6 
times that in the former experiment. 
value of u = 1,5 than Fizrau’s usual method, because 
l 
