292 



in other words that the line in consideration is a geodesic null 

 line, which was to be proved. 



§ 4. Let Xg be the time coordinate. In three-dimensional space 

 in a point A an arbitrary direction is defined by giving detinite 



ratios to -7" (^^ U 2, 3). If inversely we assume these ratios as 

 given, we can give such values to -— that the condition 



rfr)=f^'-57rfV=" (^.(' = 0,1,2,8). 



"■ .0/ ^f^ "'•''0 ''"*'o 



is satisfied. 



The theorem which we have proved, has therefore the meaning: 

 In three-dimensional space there passes a ray of light at any 



moment through any point in any direction. 



