150 Prof. R. Birkeland : An Attempt to explain 



is not influenced by reflexion on mirrors, or reflecting 

 surfaces; from those now described by me, it results that the 

 said velocity does not change by the movement of the source. 

 These facts are surely in harmony with the theory of 

 relativity; but really, in spite of their evident interest, they 

 cannot logically be cited as sure experimental proof of this 

 theory. In fact, two experimental circumstances must not 

 be forgotten: first, the presence of materials which are 

 traversed by the interfering rays (air, glass, metals) ; and 

 second, the gravitation field of our earth. While it is 

 possible to imagine experiments entirely apart from the 

 former, it cannot be foreseen if later experimental results 

 will bring into evidence the eventual influence of the 

 second. 



XII. An Attempt to explain the Michelson. Interference- 

 Experiment. By Richard Birkeland, Br. phil., Pro- 

 fessor of Mathematics at the Technical High School, 

 Trondhjem *. 



1. rtlHE problem to determine the influence, if any, 

 JL exercised by the earth's motion on optic phenomena 

 on the earth's surface, is one of great theoretical interest 

 and importance, and a vast amount of speculation and 

 research has been devoted to the subject. 



The earth's mean velocity in its orbit is v = 30km./sec. 

 Even compared to the velocity of light c = 300,000 km./sec, 

 v is not a negligible quantity in all circumstances. All 



v 

 attempts to register effects of the— = 10~ 4 order of magni- 



c 



tude have been in vain. In 1881 Professor A. A. Michelson 

 devised his now famous experiment f , by which it would be 



v 2 

 possible to discover effects of the ~ z =10~ 8 order of magni- 

 tude. The expected effect was not registered. Th:- experi- 

 ment has afterwards been repeated with still greater accuracy, 

 and at present most physicists feel sure that the effect, 

 which was to be expected, does really not occur. 



It had been possible to explain all previous experiments 



* Communicated by the Author. 



f American Journal of Science, (3) xxii. p. 128 (1881). 



