Physics. — “The Propagation of Light in Moving, Transparent, 
Solid Substances”. II. Measurements on the Fizeau- Effect 
in Flint Glass. By Prof. P. Zerman, W. pr Groot, Miss 
A. Sneranace and G. C. Dipperz. 
(Communicated at the meeting of April 23, 1920.) 
More accurate than the results obtained with quartz, an account 
of which was given in communication II’); are those of the Fizeau- 
effect in moving flint glass. 
Six cylindric rods of a length of 20 em. and a circular cross- 
section of a diameter of 25 mm. were made for us by the firm 
Zeiss at Jena. The kind of glass is the ordinary silicate flint glass 
of the type 0.103 of the firm Scrorr und GeNosseN. The endplanes 
are plane-parallel in close approximation. The clearness of the inter- 
ference fringes appeared to be excellent with stationary glass column, 
while, when the necessary precautions were taken, also when the 
column was in rapid motion, the fringes remained still very good. 
The photos taken were much better than those that had been obtained 
before (Il) with quartz. This is partly owing to the excellent 
material °), to the greater cross-sections of the rods (now 25 mm. 
as against 15 mm. before for quartz), and to the smaller number 
of internal reflections *). It appeared finally possible to observe also 
the Fizeau-effect for moving flint glass directly in a telescope, as 
clearly as it is possible for moving water, and we had the privilege 
to demonstrate the effect before several physicists. 
The perfect sureness with which the rather complicated apparatus 
worked at last, was not obtained until some improvements had been 
made in the arrangement as it had been used for quartz. We will 
discuss the principal of them. 
2. Through different causes the interference fringes can take an 
oblique position during the movement of the column of glass cylin- 
ders. It is, however, necessary that the fringes remain parallel to 
1) These Proc. Vol. XXII, N°. 6, p. 512. 
*) Compare II, 2. 
