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and independent of this at about the same time Prof. F. ZeRNIKE 
at Groningen did the same in a letter dated November 11% 1919. 
The short, elementary derivation, which in the two communications 
is founded on the same idea, will follow here. 
In principle the experiment with the moving glass rod can be 
classed under the following scheme (fig. 3). From the source of 
Fig. 3. 
light L issue two beams of light, one through the glass ad, the other 
through the air. With the aid of the necessary devices the phases 
1 and 2 are compared in two cases, first when the glass ab is at 
rest, secondly when it moves e.g. to the left with the velocity w 
According to the principle of relativity the glass may as well beat 
rest, and the room with the other parts may be made to move to 
the right with the velocity w. Whether 1 and 2 are received with 
a moving or a stationary apparatus, makes no difference in the 
relative phase of the beams of light. Therefore only Z is made to 
move to the right with the relative velocity w, and approaches the 
glass rod. This only gives a Doppuer effect equal for the two beams, 
; pad cs w 
in which the wave-length varies from A to 4A—2—. When every- 
: 
thing is at rest, the phase-difference between 1 and 2 is CEE 
A 
when / is the length of the glass rod aé. 
Hence the change due to the movement is: 
_(—})! mS _ wl du 
- dì + —-—di= —_l—À— 
a + A rite Den dh 
To get the total effect the formula should be multiplied by 4 — 
i.e. a factor 2 for the movement to and fro of the rays, and a 
factor 2 on account of the reversal of the direction of the move- 
ment — so that the formula given before, appears. 
Also Fizrau’s experiment with the moving water and stationary 
glass end-plates may be treated by the method sketched above, but 
then the calculation is not so simple. . 
Mr. Zernikk still points out that an actual experiment might be 
taken with the two beams of light running in opposite directions 
and stationary glass rod, as is supposed in the calculation. It would 
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