MICHELSON'a RECENT KESEAKCHE8 ON LIGHT. 463 



combined. The differeuee in tlie values of the rotation according as 

 the light came iroiu the west or east was consistent with a change iu 

 the index of refraction corresponding to Fresnel's hypothesis. Fizeaii 

 indicated his intention of renewing the research with improved appa- 

 ratus, but no further i)ublication on the subject by him can be found. 



Faye has criticised this investigation of Fizeau, on the ground that 

 he has taken no account of the motion of the solar system towards the 

 constellation Hercules. This motion, recognized by astronomers on 

 substantial evidence, amounts to 25,880 feet per secoiid (7,89-4 meters) 

 at its maximum. Its intiucncc is almost zero at noon of the solstices. 

 But it increases after noonday. Faye examines Fizeau's observations 

 at t r. M., and finds discrepancies of 12' or 15' between the results of 

 theory and observation. By neglecting the term which corresponds to 

 the motion of the solar system, Fizeau's observations accord better at 

 all hours of the day. Must the inference be, Faye asks, that the solar 

 system does not move? Tessan, in reply to Faye, saj^s that the sun, 

 from which Fizeau derived the light used in his experiments, moves 

 with the rest of the solar system; and that therefore Fizeau was justi- 

 fied in neglecting tlie term which expresses this motion, as of no effect on 

 his calculations. Fizeau's theory depends only on the relative velocity 

 between the source of light and the body which receives it; that is, the 

 velocity of revolution and rotation of the earth. 



In 1881, Professor Michelsou published the results of his investigation 

 on this delicate problem. He iirst calculates the probable difference of 

 time taken by the light iu going and returning over a given distance, 

 according as that distance lies in the direction of the earth's motion or 

 at right angles to it. If the distance were 1,200 millimeters, the differ- 

 ence of time translated into space would be equal to one twenty-fifth of 

 a wavelength of yellow light. The apparatus w^as ingeniously devised 

 so as to bring about fringes of interference between the two rays which 

 have travelled on rectangular paths. The whole apparatus was then 

 turned round bodily through 90°, so as to exchange the conditions of 

 the two interfering rays. Special apparatus was made for this experi- 

 ment by Schmidt and Haeusch of Berlin, and Avas mounted on a stone 

 pier at the Physical Institute of Berlin. It was so sensitive to accident- 

 al vibrations that it could not be used in the day-time, nor indeed earlier 

 than midnight. To secure greater stability the apparatus was moved 

 to the Astrophysikalisches Observatorium in Potsdam, iu charge of Pro- 

 fessor Vogel. But even here the stone piers did not give sufficient pro- 

 tection against vibration. The apparatus was then placed in the cellar, 

 the walls of which formed the foundation for an equatorial. But stamp- 

 ing with the feet, though at a distance of 100 meters, made the fringes 

 disappear. 



The experiments were made in April, 1881. At this time of the year, the 

 earth's motion in its orbit coincides roughly with the motion of the solar 



system, yigj towarclg the coustell^tiou Herculeg. This diiectiuu k iU' 



