OF ARTS AND SCIENCES. 395 



refraction lor —J is equal to - (fj.^ — 1); this for an index of 1.513 



amounts to xx74^. Measurements show that in glass, the index in- 

 creasing by a certain fraction, the rotation increases by a fraction 

 4^ times greater, and the consequent change in the plane of polari- 

 zation would be ^500* "^'^^ ^^^'^'^^ change on reversing the direction 

 from which the light came would be x^o* ^^ ^^^^ incidence is 70°, 

 and allowance is made for the change of direction inside of the glass, 

 the fraction becomes y3^oo* 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 refraction, this plane is 

 changed by 6° 40'. This multiplied by xsV^ gives 16 seconds for 

 the probable effect of the earth's motion. With 40 such plates the 

 effect would be increased to lOf minutes. Two mirrors were used, 

 one to the east and the other to the west, and light could be sent by a 

 heliostat upon either one. The apparatus 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 produced while the tendencies to elliptical 

 polarization were exactly balanced. The motion of the earth could 

 modify this result to the extent of only 2 minutes ; which is too small 

 for accurate observation. Fizeau then resorted to a device already 

 indicated 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 plane. If the original azimuth is only 5°, 

 a small change in the azimuth trebles the value of the rotation, A 

 large rotation 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 2,000 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 in 

 the value of the rotation when the light came from the west varied 

 between 30' and 155', according to the different ways in which the piles 

 of plates were combined. The difference in the values of the rotation 

 according as the light came from the west or east was consistent with 

 a change in the index of refraction corresponding to Fresnel's hypoth- 

 esis. Fizeau indicated his intention of renewing the research with 

 improved apparatus, but no further publication on the subject by him 

 can be found. 



