MAQNUSSON — ANOMALOUS DISPERSION OF CYANIN. 283 



these experiments. Therefore, it is desirable to secure a ratio 

 between the indices for the different wave lengths and find the 

 index for some one wave length by another method. 



Let u equal index of refraction for A. 



Let Mi equal index of refraction for A x . 



Let n equal displacement of fringes for wave length A in terms of that 

 same wave length. 



Let n, equal displacement of fringes for wave length A t in terms of that 

 same wave length. 



Then: n A t : n A t :: u — 1: ji. — 1. 



Solving for // 1 : 



(H — 1) n x A x 

 *i-l + n~A (34). 



This method is advantageous when the indices for portions of 

 the spectrum, as in the absorption band for cyanin, are difficult 

 to measure by other methods. The displacements n on the 

 photographs were measured by means of a filar micrometer at- 

 tached to an eye-piece magnifying about twenty diameters. 



Column a in Tables IX-XII gives the width of a fringe, 

 for the given wave length, on the photograph in terms of divi- 

 sions on the micrometer screw. The width of the displacement 

 in the same units is given in column b. Hence i equals n, 

 the displacement in terms of fringes of the wave length used. 



