396 PROCEEDINGS OP THE AMERICAN ACADEMY 



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 25,889 feet per second (7,894 meters) 

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

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

 at 4 p. M., and finds discrepances 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, says 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 the 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 Michelson published the results of his investiga- 

 tion on this delicate problem. He first calculates the probable differ- 

 ence of time taken by the light in going and returning o^jr 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 1200 millimeters, 

 the difference of time translated into space would be equal to 5*5 of a 

 wave-length of yellow light. The apparatus was 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 experiment by Schmidt and Haensch of Berlin, and was 

 mounted on a stone pier at the Physical Institute of Berlin. It was 

 so sensitive to accidental vibrations that it could not be used in the 

 daytime, nor indeed earlier than midnight. To secure greater stability 

 the apparatus was moved to the Astrophysikalisches Observatorium 

 in Potsdam, in charge of Professor Vogel. But even here the stone 

 piers did not give sufficient protection against vibration. The appara- 

 tus was then placed in the cellar, the walls of which formed the foun- 

 dation for an equatorial. But stamping 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, viz. towards the constellation Hercules. This 



