and the Rotatory Dispersion Formula. 419 



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could be accurately represented by five figures, consequently 

 we judge of the accuracy with which the formula represents 

 the dispersion by seeing whether the calculated values fall on 

 the curve determined by experiment. The computations are 

 easily made. One has but to determine the value of 



and 



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(tf-»y 



for wave-lengths in the region under investigation. These 

 terms being independent of the vapour- density, can be used 



2 G 2 



