514 
Experiments were carried out with three different colours, the effective 
wave-lengths of which amounted to 6510, 5380, 4750 ALU. 
4. When white light traverses the apparatus, we easily distinguish 
the central band. Its centre is the point whose displacement we 
should wish to measure in an experiment witb white light. Also for 
incident monochromatic light we can speak of the centre of the central 
band. It is the point that remains fixed when the interference lines 
rotate, or become narrower or wider through any cause that does 
not depend on the Fizeau effect. The position of the centre can be 
determined by means of the horizontal and movable vertical cross- 
wires in the telescope, by subjecting the interference bands to some 
modification with the compensator, thus causing the centre to be 
observed clearly. 
When the centre has been determined, the movable vertical wire 
is displaced over a few bands, so that this wire can have no 
disturbing influence on the measurement on the photo. 
A series of photos is then taken on one photographic plate, in 
which the directions of the movement alternated. 
The observed effect is derived from the displacement of the centre. 
Of course plates on which a notable rotation of the interference 
bands has occurred, are rejected. 
5. The following table may serve as an example of the results 
obtained by measurement of a plate taken with: 
Green light 4 = 5380 A.W. 
Maximum | Length of | Effect reduced 
bomen velocity | column | Observed [to 1 m. of quartz aa vale 
ee in cm. | of quartz | effect jand max. velo- fate 
P per sec. in cm. city 10 meters P 
48 750 100 92 123 
137 183 
118 157 152 
99 132 
125 167 
The effects are given in thousandths of the distance of the fringes. 
Altogether photos have been taken on eleven plates with green light 
4 = 5380 A.U. In all fifty-one values have been obtained in this way 
