1884.] Rotation of Light in Bisulphide of Carbon. 147 



its normal position. The amount of the rocking being suitably- 

 chosen, the comparison of the three appearances (two with auxiliary 

 current and one without) serves to exclude some imperfect matches 

 that might otherwise have been allowed to pass. 



On fifteen days sets of observations have been taken of the double 

 rotation produced by the reversal of the current in the helix on light 

 which traversed the tube three times. The double rotations varied 

 from about 9 to 19, and the currents from about ^ ampere to 

 1 ampere. Reduced to correspond with a certain standard current, 

 and corrected for temperature by Bichat's formula to 18 C., the 

 double rotations ranged from 1124'! minutes to 1132'2 minutes. 

 The mean for 18 C. is 1128'4 minutes ; and as this was about the 

 actual mean temperature of the observations, the result is nearly 

 independent of Bichat's formula for the dependence of the effect upon 

 temperature. 



Four sets of observations were also taken on light, which traversed the 

 tube but once. Multiplied by 3, and reduced to the same temperature 

 and current, the mean of these gives 112 7'4 minutes. In this case the 

 current actually used was about 1^ ampere, and the double rotation 

 about 9. 



Taking both series of experiments into account, we may adopt 

 1128'0 minutes as the sixfold rotation at 18 C. due to the passage of 

 the standard current through the helix. 



In C.G.S. measure the value of the standard current is '09722. 

 The difference of magnetic potentials at points at infinity on the axis 

 of the helix traversed by this current is- 



47rx 3684 x -09722. 



The correcting factor on account of the finite length of the tube is 

 99*449 . 



Hence if x be the rotation in minutes at 18 C., corresponding to a 

 difference of potential equal to unity, we have 



1128-0=6 x -09722 x 477- x 3684 x -99449 x x, 

 whence 



x =-042002 minute. 



M. Becquerel gives as his result for C. K)463 minute. To find 

 the rotation at 18, this must be multiplied, by '9767 according to 

 Bichat's formula : and as Becquerel's observations were in fact made 

 at about 18, this reduction does not introduce, but rather removes, 

 an extraneous element. Thus according to Becquerel 



a;="0452 minute, 

 differing by about 7 per cent, from the value found by us. 



L2 



