REVERSED AND NON-REVERSED SPECTRA. 



101 



the filled trough are similarly compared. Data of this kind for a dilute solu- 

 tion of mercury-potassic iodide and two kinds of glass are given in table 25. 

 A/a is the micrometer reading at M for the half-silvered plate alone. In case 

 of the solution, it is to include the glass plate of the trough. NN a = AN. 



TABLE 25. Dispersion constants. 



The computed values ni-{-K=(NN a )/e = AN/e are given in the graph 

 figure 67 (series numbered), from which the characteristic difference in the 

 dispersion of the solution and of the glass is apparent. 



If, now, the value of B is computed by equation (4), between successive 

 Fraunhofer lines BE, Cb, DF, the results come out as in table 26 and 

 the coefficient B should be correct to about i per cent. For instance, in the 

 case of the solution (N N a )\(NN a )\ f = dN = 0.0201, 0.0200, 0.0229, 



respectively. 



TABLE 26. 



