468 
6. Observation and Photography of the Interference-bands. The 
shutter is placed before the objective of the telescope, which was 
also used before to record the Fizrav-effect *). In the focal plane of 
the telescope, which is provided with a negative achromatic system 
of lenses to increase the effective focal distance, a system of wires 
has been placed, which is photographed at the same time with the 
interference bands. 
The position of the interference fringes with respect to the wires 
is determined. Immediately behind the wires the photographic plate 
is in a plateholder mounted quite independently of the telescope 
with the cross wires. The photographic plate can be put in the 
required position without the telescope being touched, and be shifted 
to take the successive photos. 
The telescope, the plate-holder, the interferometer, and the glass 
rectangular prism, in which the interfering beams are reversed, are 
mounted on separate freestone piers, which are cemented to the 
large pier. This last mentioned prism, which also served in the 
earlier experiments, is visible on the righthand side of the Plate. 
7. Measurement of the tune that the shutter or interrupter is opened. 
This time, which is of the order of 1 or 2 hundredths of a second, is 
dependent on the current in the coils of the cireuit of the interrupter 
and can be regulated by this and by the change of the width of 
the opening in the moving screen of the interrupter. For the deter- 
mination of the time the interrupter is placed before the lens of a 
camera, with which a small lamp is photographed, which revolves 
on a disk with known velocity. During the time that the interrup- 
ter is opened, the lamp describes part of a circle, the length of 
which is measured. 
8. Checking and regulating of the apparatus. After the inter- 
ference fringes have been made as distinct as possible, the beams 
passing only through the air, a compensator is placed in one 
beam, consisting of a plane parallel glass plate 7 m.m. thick and 
with a diameter of 25 m.m., made by Hilger. This plate can be 
rotated round a vertical and horizontal axis, and enables us to 
change the slope and the distance of the interference fringes in a 
simple way. In many cases it was unnecessary to insert this com- 
pensator, as the desired interference fringes were already obtained 
with the interferometer alone. Then the column of quartz and glass 
1!) Cf. Zeeman. These Proc. 18, 400, 1915. 
