6 Fred J. Bates 



straight line that, even on the curve S, the center of the space, 

 no matter what its width, is not on wave-length 589.6 /a/u., which 

 is the point of zero intensity, but at some distance to the left. 

 Really this is the only wave-length that is completely absent, but 

 for several fx-fj. on each side of it the eye can detect no change in 

 the intensity of the blackness and so is compelled to judge by 

 means of points of equal intensity on each side of the central 

 region. The space is thus located with an apparent center to the 

 left of 589.6 fi/M. Assume that the boundary of the black space 

 is defined by the wave-lengths 582 /xfi and 594 ixfi, which are of 

 equal intensity, thus giving it a space with a working width of 

 12 fifi. 



With the transparent solvent in the field it is evident that the 

 position of the black space will be determined as though its 

 center is at C (fig. 3). When the fuchsin solution replaces the 

 solvent, the center, passing to curve F, is found to be at C, or a 

 change of position of 0.7 /x/x. This gives a rotation of 0.6° when 

 there has been no change in the position of the planes of polari- 

 zation. 



The conditions assumed above being very similar to those used 

 by various investigators were easily realized experimentally. An 

 auxiliary cell with the fuchsin solution was thrown into the path 

 of the light and the analyzing nicol rotated until the black space 

 was in its original position. The following data were thus 

 obtained for this part of the spectrum : 



FUCHSIN SOLUTION 



The above is also just as applicable to the clear space method. 



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