288 
PAUL BIEFELD 
We may hope that also here experimental tests may present 
themselves that may bring us nearer to the goal of comprehend- 
ing the true nature of matter and energy, or more likely the two 
as one, and that one would have to be energy. 
The author acknowledges as his principal source for the above the 
“Princeton Lectures^’ of Prof. Einstein, whose definitions will be recognized. 
The section on a rotating system of coordinates is taken from Prof Edding- 
ton’s “Report.” 
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