682 Lord Rayleigh : Does Motion through 



6 to -Je. Hence i£ we find that rotation 6 produces a sensible 

 effect in lessening the darkness at the darkest place, we may 

 infer that there is delicacy sufficient to detect a relative 

 retardation of 26 due to double refraction. This comparison 

 would apply if the test for double refraction were made by 

 simple observation of the revival of light. As actually 

 carried out by location of the band, the test must be many 

 times more delicate. 



It was found that a marked fading of the band attended 

 a rotation of the nicol through 4'. According to this e would 



be 450 ; or since a retardation of -J A corresponds to e = 7r, a 



retardation amounting to — ^- x ^A should be perceptible 



many times over, regard being paid to the superior delicacy 

 of the method in which a band is displaced relatively to fixed 

 marks. 



Another and perhaps more satisfactory method of deter- 

 mining the sensitiveness was by introducing a thin upright strip 

 of glass which could be compressed in the direction of its length 

 by small loads. These loads were applied symmetrically in 

 such a manner as to cause no flexure. The double refraction 

 due to the loads is of exactly the character to be tested for, 

 and accordingly this method affords a very direct check. 

 If the load be given, the effect is independent of the length 

 of the strip and of its thickness along the line of vision, but 

 is inversely as the width. The strip actually employed had 

 a width of 15 mm. ; and the application (or removal) of a 

 total of 50 gins, caused a marked shifting of the band, while 



o m o 



25 gms. was just perceptible with certainty. 



To interpret this we may employ some results of Wertheim 

 (Mascart's Traite oV Optique, t. ii. p. 232), who found that it 

 requires a load of 10 kilograms per millimetre of width to 

 give a relative retardation of ^\, so that with the actual strip 

 the load would need to be 150 kilograms. The retardation 

 just perceptible is accordingly ■JXh-OOOO. This may be con- 

 sidered to agree well with what was expected from the effect 

 of rotating the nicol. 



We have now only to compare the relative retardation which 

 would be detected with the whole retardation incurred in 

 traversing the 76 cm. of bisulphide of carbon. In this 

 length there are contained 1,200,000 wave-lengths of yellow 

 light, or 2,400,000 half wave-lengths. The retardation due 

 to the refraction maybe reckoned at 6 of this, or 1,410,000 

 half wave-lengths. Thus the double refraction that might 

 be detected, estimated as a fraction of the whole refraction, 



