513 
2. In order to place the quartz rods in the groove, we proceeded 
in the following way. After the interference-bands had been produced 
with great distinctness, and the beam had been placed on the apparatus, 
ene quartz rod was put in the groove, and if necessary the inter- 
ference-lines were made distinct anew. It was then ascertained 
which of the four positions obtained by rotating the rod round its 
longitudinal axis, gives bands that change least, when the machine 
is made to assume different positions. Then the second rod is placed 
behind the first, likewise in four positions ete., till all the rods have 
been arranged. In order to prevent reflected light from entering the 
interferometer, each of the rods is placed in a somewhat sloping 
position by putting a piece of thin cardboard at one end. The rods 
are put in one by one. After each addition it is tried, whether the 
correct position has been obtained. 
We may still remark in this connection that the glass cylinders 
with which we have made experiments (see the following commu- 
nication) have been manufactured so exceedingly well by the firm of 
Zeiss, that on rotation about the longitudinal axis in a cylindrical 
groove there does not appear an appreciable change of the inter- 
ference bands. Hence the optical control becomes a great deal simpler 
than for quartz. The interference bands finally photographed through 
the quartz column are decidedly less distinct than the interference 
bands that are observed when the column has been removed. The 
lines have become slightly diffuse. This is not the case when the 
glass cylinders of Zriss have been introduced. The diameter amounted 
to 25 mm. with a length of 20 em. As there were used six cylin- 
ders, there were twelve reflecting planes for a total length of glass of 
120 cm. In the experiments with the quartz column of 140 em. 
length the number of reflecting planes amounted to twenty-eight. Though 
this great number of reflecting planes must have an unfavourable 
influence on the distinctness of the system of fringes, yet it was 
beyond all doubt that it was not owing to this cause that the quartz 
column had a more unfavourable influence than the glass column. 
We might still have eliminated the reflections on the interfaces by 
introducing a liquid of the mean index of refraction of quartz be- 
tween the successive rods. The complication of the apparatus, which 
would ensue from this, and the unfavourable experience which 
we had with moving liquids, made us resolve to put up with the 
reflections. 
4. As source of light a 12 Amperes are-lamp was used, the light 
of which was made sufficiently monochromatic by means of filters. 
