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attraction brought about by several centres of attraction which are 
stationary with respect to each other, is not compatible with the hypo- 
thesis of equivalence. 
§ 2. Let, therefore a laboratory £/ be given, in which there is 
a statical field of attraction. With EisreiN we suppose that the rays 
of light propagate in it curvilinearly in some way or other, but so 
that the following conditions are satisfied : 
When once a ray of light may have passed through the points 
A, Bu FG of the laboratory : L'*) then 
[A] this way A, b,.../,G must always be possible for the light 
(“Constancy of the ways of light”), 
[B] the reversed way G, F,...B, A must also be always possible 
(“reversibility of the ways of light”). | 
The hypothesis of equivalence now compares this laboratory 4/ 
resting in the field of attraction with a laboratory Z which is free 
from gravitation, but has a corresponding acceleration instead. How 
must the points of this laboratory in which there is ro gravitation 
move, so that the observers in it shall observe constancy and reversi- 
bility of the ways of hght in the sense of the hypothesis of equivalence ? 
§ 3. For the sake of simplicity we confine ourselves to a two- 
dimensional laboratory £. As fundamental system of coordinates, 
with respect to which £ moves in an accelerated way may serve 
the system of coordinates 2, y, which has no acceleration, and the 
time ¢ measured in it. With respect to this system which is without 
gravitation, the rays of light move in straight lines and with constant 
velocity 1. In the corresponding w, y, ¢world-space of Minkowski 
every optical signal travelling in this way is represented by a straight 
line forming an angle of 45° with the taxis. Such a line in the 
w, y, t-space is called: “a line of light’. The motion of the different 
points A, b,.../.G of the moving laboratory / is represented by 
the same number of (curved) world lines a,b... /, 9. 
When the observers in the laboratory JZ state that they have 
succeeded in making an optical signal S, pass through the points 
A, B,... 4, G@ of their laboratory this means that the corresponding 
line of light s, intersects the world lines a,b,...f,g of these points 
of the laboratory. 
According to condition [A] of § 2 the observers in the laboratory 
L must in this case be able to send light signals S,,.S,... through 
1) These points may be imagined e.g. as apertures in the walls of the laboratory. 
