Physics. — “The Propagation of Light in Moving, Transparent, 
Solid Substances. IL. Measurements on the Fizwav-fject in 
Quartz’. By Prof. P. Zeeman and Miss A. SNETHLAGE. 
(Communicated at the meeting of May 3, 1919) 
1. In communication | the apparatus has been described that has 
proved suitable for the investigation of the Fizeau effect in solid 
substances. 
We have now carried out experiments with quartz, which was 
traversed by beams of light in the direction of the optical axis. We 
were led to the choice of this substance by the consideration that 
in general, crystals are the most homogeneous bodies that we know, 
and the scattering of light in a crystal must be exceedingly slight 
on account of its regular structure *). 
It appeared to us later that the best optical glass, for our purpose, 
can be compared in some respects with quartz, in others it is 
even preferable. 
In some series of experiments 10 quartz rods were used, supplied 
by the firm of Srere and Reuter, with endplanes normal to the optical 
axis, and of the dimensions 10 Xx 1.5 1.5 em. Later on four similar 
rods supplied by the firm of A. Hrreer, Ltd. were added to them. 
For a series of experiments the rods were joined together to form 
a column of a length of 100 em.; in a second series of 140 em. 
They were placed one behind another in a groove which was milled 
in a wooden beam, fastened to the driving apparatus by means of 
four solid screws. The different rods are separated from each other by 
rubber dises with round apertures of a diameter of about 13 mm. 
Each quartz rod rests in a groove 13 mm. deep, and is pressed 
down by two brass plates, fastened with screws in the upper 
surface of the wooden beam, a thin piece of cork being placed under 
the plates. The space remaining at the ends of the groove is filled 
up by a piece of brass tubing. Solid brass plates, which clasp the 
beam, shut off the ends of the groove. 
‘) Lorentz. Théories statistiques en thermodynamique, p. 42. 
