PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 331 



known experiment was as follows : A beam of light was divided 

 by a semi-transparent reflector into two, at right angles to each 

 other. Each of these beams fell, at any desired distance, upon 

 a plane mirror, which reflected it back directly to the glass which 

 divided the original beam. These two beams are now in a con- 

 dition to interfere, forming the familiar rings or fringes, by 

 which the difference in path traveled by the two, beams can be 

 exactly measured. One of the two beams was sent out in the 

 direction in which the earth was traveling at the time of the 

 experiment, the other at right angles to this direction. It is evi- 

 dent that they would be differently afifected, if the ether was at 

 rest with reference to the earth. The amount of the difference 

 was shown by calculation to be within the measuring capacity 

 of the instrument. 



But Michelson found no evidence of measurable difference 

 in the length of path of the two beams, that is, no evidence in 

 favor of Fresnel's first hypothesis. The difficulties, however, 

 were great, and the apparatus was not entirely satisfactory. In 

 1886 Lorentz pointed out a small error in Michelson's theoretical 

 discussion, which when corrected reduced the effect to be ob- 

 served to about one-half of what was at first expected, bring- 

 ing the magnitude of the result perilously near the errors of 

 observation. A little later Michelson, then at the Case school 

 in Cleveland, became associated with E. W. Morley. The two 

 attacked the whole problem again with far more refined and deli- 

 cate apparatus. They repeated in i8S6 Fizeau's experiment, 

 allowing light to pass through pipes through which liquid was 

 flowing with high velocity. The change in speed of the light 

 due to the motion of the liquid was found to be quite in accord- 

 ance with Fizeau's measurements, made in 185 1, and confirma- 

 tory of Fresnel's second hypothesis. 



In 1887 Fresnel's first hypothesis, that the ether is at rest, 

 was elaborately investigated by Michelson and Morley. The 

 results, so precise now as to admit of no question, were confirma- 

 tory of Afichelson's earlier experiments. Fresnel's hypothesis 

 was seen to be without foundation, and the theory of aberration, 

 founded upon it, fell to the ground. The importance of this 



