467 
diametrically opposite one another so that they take each other’s 
places as regards level after every whole revolution of the fly-wheel, 
and can make contact with a suitably fixed sliding contact. 
By means of the cam an electric current is closed in a circuit 
containing also windings of a coil that acts electromagnetically ona 
shutter or light interrupter. When the second cam makes con- 
tact, the current passes through a second coil, which closes the 
shutter. Every time the fly-wheel, and consequently the moving column, 
have arrived at the same position and move im the same direction, 
the shutter is opened, and light is allowed to pass for a moment. 
The intensity of the interference figure not being strong enough to 
give a satisfactory photo with light that has been admitted once, a 
photo is taken e.g. thirty times successively on the same plate. As 
the light is let through three times a second, this takes only ten 
seconds. 
To take the second photo, i.e. when the column of quartz moves 
in the opposite sense, a duplicate arrangement is used, placed sym- 
metrically with respect to the one sketched. By means of a double- 
pole throw-switch it becomes possible to make one series of photos 
succeed the other immediately without loss of time and without 
stopping the machine. Only the photographic plate must be moved 
a little. The two large toothed wheels, and between them the small 
one can be distinguished on the Plate near the bearing at the 
bottom on the righthand side. 
As some time passes between the moment that contact is effected by 
the cam and the opening of the shutter, this time must be taken into 
account. To do so it is necessary to perform a phase determination. 
5. Phase determination, i.e. to ascertain by a separate experi- 
ment, if really at the moment of the greatest velocity the light 
passes through the shutter. For this purpose the shutter is put at 
the place of the greatest velocity by the side of the bed. The 
wooden beam containing the transparent substance, is provided with 
a black sereen with an opening. A glowlamp is placed in the line: 
shutter-place of the greatest velocity, a line which is normal to the 
longitudinal direction of the apparatus. When the machine is running 
and when the shutter is in action, the lamp must be observed through 
it. When it does not work at the right moment, the field of 
view remains dark. Then the phase can be improved, and can at 
last be made accurate by gradually shifting the large toothed wheel 
with respect to the small one. This causes the contact to be formed at 
another moment. 
