the JEther cause Double Refraction ? 681 



Observations were made upon bisulphide of carbon in a 

 tube 76 cms. long, and upon water in a tube 73^ cms. long. 

 In neither case could the slightest shift of the band be seen 

 on rotation of the board from the north-south position to 

 the east-west position, whether at noon or at 6 p.m. The 

 time required to pass from one observation to the other did 

 not exceed 15 seconds, and the alternate observations w T ere 

 repeated until it was quite certain that nothing could be 

 detected. 



Of course the significance of this result depends entirely 

 upon the delicacy of the apparatus, and it is worth little 

 without an estimate of the smallest double refraction that 

 would have been detected. It may even be objected that the 

 investigation stands self-condemned. In consequence of the 

 earth's magnetism there must be a rotation of the plane of 

 polarization when the light traverses the bisulphide of carbon 

 in the north and south position ; and this effect, it may be 

 argued, ought to manifest itself upon rotation of the board. 



To take the objection first, it is easy to calculate the 

 rotation of the plane of polarization. For one c.G.s. unit of 

 magnetic potential the rotation in 0S 2 at 18° is '042 minute 

 of angle *. In the present case the length is 76 cms. and 

 the earth's horizontal force is *18 ; so that the whole rotation 

 to be expected f is 



76x-18x-012 = -58'. 



So small a rotation of the plane, which would show itself, if 

 at all. by a fading and not by a displacement of the band, is 

 below the limit of observation. 



The delicacy of the apparatus for its purpose may, indeed, 

 be inferred indirectly from the rotation of the nicol found 

 necessary to engender a marked revival of light at the 

 darkest part of the band. If 6 be this angle, the revived 

 light is sin 2 6. expressed as a fraction of the maximum 

 obtainable with parallel nicols. In the actual observation 

 the nicols remain accurately crossed, and the question is as 

 to the effect of a doable refraction causing e.g. a retardation 

 of vertical vibrations relatively to horizontal ones. If 

 this retardation amounted to i\, A, being the wave-length, 

 the effect would be the same as of a rotation of the nicol 

 through 90°. In general, a retardation of phase e, in place 

 of 7T, gives a revival of light measured by sin 2 (^e). If the 

 revivals of light in the two cases be the same, we may equate 



* Phil. Trans, clxxvi. p. 343 (188o) ; Scientific Papers, vol. it. p 377. 

 t The difference between astronomical and magnetic north is here 

 nejrlected. 



