SECTION B PHYSICS OF ETHER 



(Hall 11, September 23, 3 p. m.) 



CHAIRMAN: PROFESSOR HENRY CREW, Northwestern University. 



SPEAKER: PROFESSOR DEWITT B. BRACE, University of Nebraska. 

 SECRETARY: PROFESSOR AUGUSTUS TROWBRIDGE, University of Wisconsin. 



THE ETHER AND MOVING MATTER 



BY DEWITT BRISTOL BRACE 



[Dewitt Bristol Brace, Professor of Physics, University of Nebraska, b. Wilson , 

 New York, 1859; died, October 2, 1905. A.B. Boston, 1881 ; A.M. Boston, 1882; 

 Ph.D. Berlin, 1885; post-graduate, Massachusetts Institute of Technology, 1879- 

 81; Johns Hopkins University, 1881-83; University of Berlin, 1883-85. Acting 

 Assistant Professor of Physics, University of Michigan, 1886. Vice-President 

 of American Association for the Advancement of Science; Vice-President of 

 American Physical Society. Author of Radiation and Absorption.] 



THE question whether the luminiferous ether passes freely through 

 matter or participates in the translation of the same, considered as 

 a moving system, stands to-day without positive answer, notwith- 

 standing the numerous experimental attempts and the varied hypo- 

 theses which have been made since the discovery of aberration by 

 Bradley in 1726. The simple explanation of this phenomenon on 

 the corpuscular theory may have caused the century of delay in the 

 closer examination of the question until it became necessary to 

 consider it from the standpoint of undulations in an ether. As com- 

 pared with the many efforts to examine the question in the second 

 or ether period we have perhaps but two belonging to the first or 

 corpuscular period. Boscovich, in 1742, reasoning from this theory 

 on the ground of a difference of velocity in air and water, proposed 

 to examine the aberration of a star with a telescope whose tube was 

 filled with water. This experiment was not carried out till long after 

 by Airy in 1872, who found that the variation in the aberration was 

 absolutely insensible. Arago, in the second instance, reasoning on 

 the same theory, concluded that the deviation produced by a prism 

 would vary with the direction of the earth's motion; but he was 

 unable to detect any such change, a result verified later by more 

 delicate means in the hands of Maxwell, Mascart, and others. This 

 experiment, which demonstrated the absence of any effect of the 

 earth's movement on refraction is of great historical interest. This 

 negative result, which to Arago was inconsistent with the corpuscu- 

 lar theory, suggested to Fresnel the important hypothesis of a qui- 

 escent ether penetrating the earth freely but undergoing a change 



