S48 Light Propagated in a Dielectric. 



magnetic field. This mean rotation of the plane of polariza- 

 tion for five turns of M was 2°*075. Assuming mean <f> = <fi D , 



2 o- 075 

 H= -tprr =2500 C.Gr.S. units, approximately. 



^fx 198x0022 

 bO 



The value obtained with the bismuth spiral was 2600 

 C.G.S. units. 



The discrepancy was probably due to the assumption of the 

 mean wave-length being that of sodium, while in fact it was 

 greater. 



A slight calculation will show the greatest variation in the 

 mean refractive index which could have occurred without 

 being detected. 



If v is the difference in phase between the two component 

 vibrations, and 6 the angle of a component with the resultant, 

 then if 



^=rcos (27n fp — j- ) — L ')' 



y = rsin27r(}-|), 



.'. tan0 = cot27rf ™ — — J— sin v, 



since v is small, and 



dv cos v 1 100 



dO cos 2 cos 2 49-9 



= 2 when 6 is 45°. 



Since d0< -01°, dv< -02°, or < -000055\ for a field of 

 2600 C.Gr.S. units and a distance of 198 cm. If X^ and X 2 

 are the wave-lengths of the components, and n x and n 2 the 

 reciprocals of the velocities, we have the following equation : — 



A/i A.Q 



w 2 — n 1 = n 1 — . 



Knowing dv for 198 cm., we can find the same for one 

 wave-length, or \ — X 2 . Substituting in the above equation, 

 we have 



n 2 - ni <2Sx 10- 7 for 2600 C.G.S. units. 



If the effect on each component was of an opposite nature, 

 as the nature of the polarizing effect would lead us to 

 suppose, the change in the refractive index could not have 

 been greater than in the eighth decimal place, if any. 



A variation of this order would be much smaller than that 

 already observed by Kerr and others in a dielectric under 



