1403 
the horizontal or vertical cross-lines. Else no photos are obtained 
on which measurements can be made. 
Already in the experiments with quartz-a compensator was inserted 
in one of the interfering beams of light, consisting of a plane-parallel 
circular glass plate of a thickness of 5 mm. and a diameter of 25 
mm., to which every desired position could be given. The inclina- 
tion of the interference fringes can be modified by rotation round 
a horizontal axis; a simple arrangement was, therefore, applied 
through which the observer, sitting at the eye-piece of the telescope, 
could bring about the desired rotation. Besides a plane-parallel plate 
was placed before the object glass of the telescope im such a way 
that an image of the interference fringes could be observed in a 
small telescope placed on one side, while at the same time after 
removal of the eye-glass a photo of the fringes was made with the 
large telescope. Thus the observer at the small telescope could at 
once observe an error in the position of the fringes, and if neces- 
sary, redress it during the photographing. This proved to be but 
rarely necessary when an experiment had been properly prepared. 
3. As was set forth before (I, 4), it was necessary to superpose 
20 to 30 photographs of the interference fringes, each with an 
exposure of a hundredth second, because otherwise the photographic 
image was too faint. This number could be greatly reduced by 
working without filters, hence directly with the white arc-light. 
Diminution of the number of exposures increases the sharpness 
of the photos, and renders it possible to take more in succession, 
before the disturbances through fluctuations of the temperature in 
the glass-rods, which inevitably occur in consequence of the move- 
ment of the apparatus, become troublesome. 
For the interpretation of the photo obtained it is then necessary 
to know what is the effective wave-length À of the white arc-light, 
with which the fringes have been photographed. 
The accuracy in the determination of 4 need not be very great, 
as will appear presently (see 5). 
4. Determination of the effective wave-length of the light used. 
The effective wave-length of the operative light, had to be 
measured after it had left the last mirror of the interferometer, and 
of course for that kind of plates that was used in the experiments. 
The beam from the interferometer was focussed with a cylinder 
lens on the slit of the collimator of a Hircer spectroscope with 
