MICHELSON MOULTON 581 



structed apparatus he secured results of a higher order of accuracy 

 than any that had theretofore been attained. Much of his scientific 

 work throughout his long life related to light, and his last experi- 

 ment, completed just before his death, was an extraordinarily ac- 

 curate measurement of its velocity. In all of his experiments he 

 exhibited an uncanny ability to make apparatus work. For example, 

 after French physicists thought they had proved both theoretically 

 and experimentally that interference phenomena could not be se- 

 cured in white light, he set up in Paris his recently invented inter- 

 ferometer with the materials that happened to be available and aston- 

 ished the French scientists with its performance. For the purpose 

 of measuring short distances or small angles, the interferometer is 

 incomparably superior to the microscope. An outgrowth of this 

 early instrument is his later stellar interferometer with which in 

 recent years the diameters of several stars have been measured. 



Early in his scientific career Michelson, in association with E. W. 

 Morley, performed an experiment which marks a turning point in the 

 philosophy of physical science. To the mass of mankind the sur- 

 face of the earth appears fixed, but to the astronomer the earth is a 

 tiny globe which spins on its axis and revolves about the sun. If 

 one should be tempted to conclude that motion w^ith respect to the 

 sun is absolute, astronomers would inform him that the sun moves 

 with respect to the stars; and recently it has been found that our 

 galaxy is moving with respect to exterior galaxies. 



In 1887 Michelson and Morley undertook to measure the motion of 

 the earth, not with respect to the sun, or our galaxy, or exterior 

 galaxies, but with respect to the ether, an assumed all-pervading 

 medium through which light is transmitted and which, if anything, 

 would give absolute motion. The quantity to be determined was so 

 minute that it could be measured only with the aid of Michelson's 

 interferometer. To the astonishment of scientists no certain motion 

 of the earth with respect to the assumed ether was found. Though 

 light has the properties of wave motion, it appears to be propagated 

 with a speed which is independent of the motion of its source. 



Einstein's theory of relativity has its roots in the Michelson-Mor- 

 ley experiment. All the changes in point of view it has introduced 

 into physics and astronomy rest upon the experiment carried out in 

 Cleveland in 1887, and upon the subsequent verification of its sur- 

 prising results. Whatever modifications the theory of relativity 

 may undergo and whatever may be its ultimate fate, the results of 

 the Michelson-Morley experiment stand. 



In 1913, Michelson and Gale carried out their first earth-tide ex- 

 periment, the purpose of which was to determine the degree of 

 rigidity and of elasticity of the earth. Sir George Darwin had built 



