MAQNUS80N — ANOMALOUS DISPERSION OF CYANIN. 253 



is placed in the second position, the crossed prism spectrum will 

 be broken into two bands as is seen in (c), Fig. 2. The red is 

 deviated very much and the band turns up sharply as the orange 

 is approached ; the yellow is absorbed, and hence, absent, leav- 

 ing the spectrum in two parts ; the green is bent least, while the 

 deviation rapidly increases as the violet is approached. This 

 method is very striking to the eye and gives an easy means of 

 determining roughly the nature of a spectrum. 



With the dye in solution and placed in a common hollow 

 prism, the deviation produced by the solvent is superimposed 

 upon that made by the dye itself. To separate the two, Soret 1 

 made a glass trough with parallel sides, having a diagonal glass 

 partition. On one side he poured the solution and on the other 

 the pure solvent. Consequently, any deviation in the light was 

 due to the dissolved substance onlv and not to the solvent. Still, 

 it must be noted that a substance when dissolved may not have 

 the same optional properties as when in the solid state. For 

 the same purpose DeKlercker 2 used two hollow prisms of the 

 same angle and placed in opposite directions, the one being Slled 

 with the solution and the other with the solvent. 



Quantitative measurements, by the ordinary spectron ieter 

 method, for determining the refractive index for various v:ave 

 lengths have been carried out by several physicists. Sie;>en 3 

 and Ketteler 4 in particular made a large number of observations 

 upon various substances, in several solvents, and at different 

 temperatures and concentrations. Among the solvents used 

 were alcohol, chloroform, and water. Their aim was to make 

 experimental observations upon which they could base a theory 

 that would explain this strange phenomenon. This experi- 

 mental data was afterwards used bv Ketteler in his theoretical 

 deductions and some important results were reached. Still, it 

 cannot be said that he succeeded in establishing the theory ad- 

 vanced, inasmuch as it is evident that a substance in solution 



i Soret, Pogg. Ann. CXLIIL, p. 325, (1871). 



* DeKlercker, Comp. Rend. LXXXIX. (1879). 



8 Sieben, Wied. Ann. VIII., p. 137 ; XXIII., p. 312. 



* Ketteler, ibid. XII., pp. 363, 481. 



