270 
PAUL BIEFELD 
the axes being thought of as rigid rods and the unit length a 
short rigid bar. 
Time is expressed by means of a body, or system, called a clock 
that counts off events, the intervals between them being consid- 
ered of equal length. 
Time and space as above defined serve only to represent to the 
physicist the complex of experiences relating to the events ob- 
served by him. 
To give a physical significance to the concept of time and to 
establish definite relations between different points in space a 
physical process of some kind must be used. As the propagation 
of electromagnetic radiations or light in vacuum is the one that 
has been most accurately investigated and determined, this is 
chosen for the purpose ; not for the reason, however, as has been 
brought forward by some of the objectors to the theory, that this 
particular process is essential to the theory; any other equally 
well determined process would serve as well. 
There are two postulates on which the theory rests : the prin- 
ciple of relativity and the constancy of the velocity of light; 
stated definitely as follows : 
1. The lav^s according to which the physical conditions of sys- 
tems change are independent of the fact to which of two frames 
of reference, having relative uniform rectilinear motions, these 
changes of conditions are referred. (Relativity of uniform mo- 
tions.) 
2. Light has a definite and constant velocity “c’’ in a frame of 
reference at rest, independent of the fact whether the source of 
light is in motion or at rest. 
Let a clock be placed at a point A where an observer may 
'‘time’' an event in its immediate vicinity and a second similar 
clock placed at a point B where a second observer may “time” 
an event in a similar manner ; this does not lead, however, with- 
out further consideration to the fact that the times of the events 
at A and B may be directly compared. For we have up to this 
time only defined a ‘time at A’ and a ‘time at B’ but not a ‘com- 
mon time.’ This common time will now be defined by means of 
the second postulate; stating, that it takes the same time for 
light to travel from A to B, as it does to go from B to A. 
We therefore send a light signal from A to B where it is re- 
flected to A. Let t ^ be the time at A or the ‘A-time’ and t „ be the 
A B 
