Earth's Motion on the Rotation of a Refracted Ray^ 597 



for the difference 1-6224 — 1*5134 of the indices of the two 



x 1 



kinds of glass. Bnt — = from the previous equation, 



l~L xx / xu 



on substituting the values of v and V, where 

 ^=10~ 4 and ^=1-5134. 



Hence he finds — 77. N = tttt^. 



(j3 — a) 2o00 



For a complete reversal of the ray we should have twice the 

 increment of the rotation, or 3955. Correcting this for the 



obliquity of the ray to the drift, on account of the refraction 

 of the ray within the glass, we have a change in the rotation of 



Yg^T or, since /3 — a=6° 40' for each plate with an incidence 



of 70° and an azimuth of 20° in his experiment, we have, 



finally, y^ X 6° 40' = 16" as the probable effect on a single 



plate of the motion of the earth. 



In my experiment the incidence, 70°, and the index, 1'517, 

 were, approximately, the same as in Fizeair's experiment. 

 However, instead of an azimuth of 20°, I used an azimuth of 

 45°, each plate being adjusted with respect to the preceding- 

 one to attain this end. Furthermore, on account of the 

 inclination of the plates and the angle MM 7 A, the deviation 

 of the path of the ray within the plates was less than in 



Fizeau's experiment, so that his factor of j^ reduced to 



jTo^ in my arrangement. Direct measurements of the angles 



of rotation, for azimuths 45° and 20°, gave 8°'6 and 5°*1, 

 respectively, for each plate, using white light. Fizeau 

 obtained with a single one of his plates a rotation of 6° 40'. 

 This made his calculated probable effect 16" for each plate. 

 The corresponding effect in my experiment was 21 //, 7, for each 

 plate, which is 5/3 greater than that for an azimuth of 20°. 

 With the four plates the calculated probable effect should 

 then be +0°*024. 



The sensibility of my optical system was such that I could 

 certainly have detected a change of less than o, 02 under 

 favourable conditions with an unfatigued eye. On reversing 

 the system I was unable to detect any change whatever. In 

 order to give greater weight to my observations, I took a 

 series of settings, on several different days, whose means 

 showed that such an effect would have been detected several 

 times over. A set of observations consisted of five distinct 



