1410 
then only be required in the experiment described before to make 
the glass prism slide to and fro, and keep the glass rod at rest. 
For different optical and mechanical reasons the execution seems to 
me attended with greater difficulties than the experiment made. 
2. In a letter of October 22nd 1919 Prof. M. von Lave had the 
kindness to draw my attention to a thesis for the doctorate of 
P. Harress of 1912), which he sent me, and in which a subject 
is treated closely related to our investigations. In Harrrss’s experiment 
the light runs to the right and to the left in a cycle of glass prisms, 
which as a whole is in rotatory motion. 
The comparison of the observed displacement of the interference- 
fringes and the theory elaborated by Harress gave a very unsatis- 
factory agreement. This is chiefly owing to the theory, in which 
the absolute and the relative velocity of the light are mixed up. 
Von Lave has redressed this error, which greatly improves the 
agreement between theory and observation. Harrgss’s experiment 
closely resembles SaGnac’s experiment of 1913, of which, very 
remarkably, von Laur gave the relativistic theory already in 1911 *). 
In Sagnac’s and Harruss’s experiments the displacement A of the 
interference fringes expressed as fraction of their distances is: 
in which / is the length of the path passed over in the rotating 
apparatus, 7 the distance of this to the axis of rotation, w the angular 
velocity, the sum extended to all the different paths. 
Index of refraction and dispersion do not occur in this formula, 
which in itself is already a difference with our experiments. 
It seems unnecessary to enter into a fuller discussion of HarrEss’s 
work, as in an interesting paper by von Laur®) the experiments by 
Fizeau, Sacnac, Harress, and those made by us are discussed and 
compared, and as with exclusion of the influence of the dispersion 
the two last-mentioned experiments have also already been treated 
in the fourth edition of von Lauw’s Relativitatstheorie *). 
3. I pointed out on an earlier occasion that it might be interesting 
d 
to examine substances in which “ is great. 
1) Re-edited in O. Knorr, Ann. d. Phys. 62, 389, 1920. 
2) Miinchener Sitz. Ber. 1911, 404. 
5) Von Lave, Ann. d. Phys. 62, 448, 1920. 
4) Cf, p. 23, 25, 185—189. 
