434 WRIGHT— POLARIZED LIGHT IN THE 



asserts that 0.3 per cent, difference can be detected, but a series of 

 experiments by the writer using a rotating, optically plane-parallel 

 plate and dift'erent sources and intensities of illumination indicate 

 that the figures of Koenig and Brodhun are more nearly correct and 

 that Koenigsberger's statement of the precision attainable is about 

 5 times too great. The difference between different settings, espe- 

 cially for large intensity differences, is of course considerably less 

 than the least perceptible increment but this difference may not be 

 considered to be the least perceptible increment itself. 



In order to render more readily visible the Savart bands and the 

 point of exact compensation Koenigsberger employs a contrast bi- 

 plate of smoky quartz which introduces a difference in intensity 

 between the components (result of pleochroitic absorption) and thus 

 produces a shift of the lines in the halves of the field ; these lines are 

 then shifted by the changes in intensity resulting on reflection from 

 an anisotropic crystal plate. 



In the practical application of this method it is essential that the 

 plate to be examined be well polished and normal to the axis of the 

 microscope ; that the reflecting prism be not too large ; that the 

 rotating glass plate P be mounted with its axis at 45° with the upper 

 nicol plane ; that the Savart plate be accurately constructed and be 

 normal to the microscope axis ; that the telescope be accurately 

 focussed on infinity (in order that convergent light of only very 

 small angular aperture pass through the Savart plate). To measure 

 the degree of anisotropism the glass plate P is rotated until the effect 

 of the crystal plate is exactly compensated and the Savart bands dis- 

 appear or are separated by exactly half a band if the contrast band 

 be used. 



The effect of the tilted plate in compensating intensities normal 

 and parallel with the plane of symmetry can be found from equa- 

 tions 15 simplified for the case of an isotropic body. Thus if D be 

 the amplitude of the transmitted plane-polarized wave and 8 its 

 azimuth, we find 



sin 2r sin 2i 

 D cos i5 = £ cos e —V- ^TT—. — ^ = E cos e • 61, 



sm''(i -f- r) 



sin 2r sin 2i . 



D sm 8 = E sui e-r-. — : :— , — : ,„ = E sin e-Co. 



(sm t cos t -j- sin r cos r)^ 



