462 michelson's recemt kesearches on light. 



polarization was 18° 40' aud 24° 58' for the two bimdles. By Fresuel's 

 hypothesis the change iu the velocity of light from the motion of the 



medium is :L( ^^ ■ V- The greatest available velocity for the medium 



is that of the earth in its orbit, viz, 101,708 feet per second (31,000 

 meters). At the time of the solstices this motion is horizontal, and from 

 east to west at noon. If the incident light comes from the west, the 

 velocity of light is diminished by FresnePs fraction of the velocity of 

 the earth. If the light comes from the east, its velocity is increased by 



the same amount. The change in the index of refraction ( or— ^ \ is 



v' 

 equal to -{/i~ — 3 ) ; th is for an index of 1.5 13 amounts to x jy4 (,. Measure- 

 ments show that in glass, the index increasing by a certain fraction, 

 the rotation increases by a fraction four and one-half times greater, and 

 the consequent change in the plane of polarization would be a^Vo- ^^^ 

 total change on reversing the direction from which the light came would 

 be YaVo' I^ the incidence is 70°, and allowance is made for the change 

 of direction inside of the glass, the fraction becomes yjoo* When a ray 

 of light falls on a single plate of glass at an angle of 70°, if its plane of 

 primitive polarization makes an angle of 20° with the plane of refrac- 

 tion, this plane is changed by 0° 40'. This multiplied by yJg_ gives 

 sixteen seconds for the probable effect of the earth's motion. With 

 forty such i^lates the effect would be increased to ten and two-third 

 minutes. Two mirrors were used, one to tlie east and the other to the 

 west, and light could be sent by a heliostat upon either one. The ap-. 

 paratus was easily turned through 180° so as to receive successively 

 the light which travelled with or against the earth's motion. 



With a single pile of plates highly inclined and a second pile less 

 inclined, of more highly tempered glass and in the opposite azimuth, a 

 rotation of 50° could be i^roduced, while the tendencies to elliptical 

 polarization were exactly balanced. The motion of the earth could 

 modify this result to the extent of only two minutes; which is too small 

 for accurate observation. Fizeau then resorted to a device already in- 

 dicated by Botzenhart for amplifying this effect. A small variation in 

 the primitive plane of polarization produces a greater effect the smaller 

 the azimuth of this ])lane. If the original azimuth is only 5°, a small 

 change in the azimuth trebles the value of the rotation. A large rota- 

 tion is first produced on a ray whose azimuth is large, and then this 

 rotation is largely changed by another pile so placed that the ray enters 

 it under a small azimuth. More than two thousand measurements were 

 made under various conditions. For noon observations at the time of 

 solstice the rotation was always greater when the light came from the 

 west, and was less at other times of day. The excess iu the value ot 

 the rotation when the light came from the west varied between 30' and 

 1'55'5 according to the different ways in ^vhicL] the piles of plates were 



