124. =A. A. Michelson—The relative motion of the Earth 
made to serve as an “interferential refractor,” and has the two 
important advantages of small cost, and wide separation of the 
two ils. 
The apparatus as above described was constructed by 
Schmidt and Heensch of Berlin. It was placed on a stone pier 
in the Physical Institute, Berlin. The first observation 
showed, however, that owing to the extreme sensitiveness of 
the instrument to vibrations, the work could not be carried on 
during the day. The experiment was next tried at night. 
When the mirrors were placed half-way on the arms the fringes 
were visible, but their position could not be measured till after 
twelve o’clock, and then only at intervals. When the mirrors 
were moved out to the ends of the arms, the fringes were only 
occasionally visible. 
It thus appeared that the experiments could not be per- 
formed in Berlin, and the apparatus was accordingly removed 
tig. 3 
to the Astrophysicalisches Observatorium in Potsdam. Even 
here the ordinary stone piers did not suffice, and the apparatus 
was again transferred, this time to a cellar whose circular wa 
formed the foundation for the pier of the equatorial. 
_Here, the fringes under ordinary circumstances were sufli- 
ciently quiet to measure, but so extraordinarily sensitive was 
the instrument that the stamping of the pavement, about 100 
meters from the observatory, made the fringes disappear 
entirely | 
inclined at an angle of about +26° to the plane of the equator, 
* 
