469 
is introduced into the beam of light, precautions being taken which 
will be mentioned in communication Il. It must then first of all 
be ascertained whether the apparatus and particularly the guides 
satisfy the high requirements on which the efficacy of the whole 
arrangement depends. 
With a slow movement of the shoes with the quartz column 
along the guides the interference fringes did not remain stationary, 
but changed with regard to distance and slope. When the apparatus 
had first been put together, this movement of the bands was very 
great. It is clear that it must also occur fora perfectly homogeneous 
column bounded by parallel planes, when the movement does not 
take place along a perfectly straight line. For then the whole column 
of a length of more than 100 em. acts as a compensator of exceed- 
ingly great thickness. In order to make the circumstances as favourable 
as possible the steel plates of the guides were laid on the supporting 
plates about in the correct position. The bolts which were used to keep 
the apparatus in place, are then screwed down till a sensitive level, 
which could be placed in longitudinal and in transverse direction, 
indicated a plane as much as possible horizontal, determined by the 
upper planes of the steel guides. It was then examined if the inner 
edges of the guides were as perfectly straight and parallel as possible, 
and improvements were made in this respect by filing and grinding. 
At last the free play of the bronze shoes in their movement along 
the guides was removed as much as possible. Great improvements 
were successively made to the apparatus in this direction, so that 
rotation and change of distance of the fringes became comparatively 
slight. It was, however, impossible to have the interference fringes 
quite steady when the apparatus was slowly moved. This is, 
however, not necessary; what is required after all is that the same 
positions of the interference bands are found again when the shoes 
have returned to the same point of the guides. The results prove 
that this is actually the case, and that the occasional deviations fall 
now in one sense, now in another. 
The excellent definition of the interference fringes, recorded with 
the quickly moving apparatus in itself proves already, that every 
time about the same position of the bands is obtained, as 20 or 30 
images are superposed (see above $ 4), which could never produce 
a definite image, when the single light impressions were not almost 
identical. Sometimes the system of fringes proved to be rotated, and 
then the photo had to be rejected. Of course care had also to be 
taken that the guides were well oiled, and there is one more 
dynamic particularity that had to be seen to. When the motor has 
