MAGNUSSON — ANOMALOUS DISPERSION OF CYANIN. 



255 



It appears that only one prism was made and that was far 

 from being satisfactory, for in speaking of the results Wernicke 

 says that the A and B lines were "plain," the rest "uncertain." 

 The D, E, and F lines could not be determined as the prism ab- 

 sorbed all the light in this part of the spectrum. 



It was almost twenty years later that Pfliiger 1 succeeded in 

 making solid prisms of several of the aniline dyes. The method 

 of making the prisms is as follows : A piece of glass tubing is 

 laid upon a piece of plate glass, and in the prismatic opening 

 between the two an alcoholic solution of the dye is placed, drop 

 by drop, and allowed to evaporate. The solid dye will thus fill 

 up the prismatic space between the tube and the plate. When 

 the deposit is of the desired thickness, the process is stopped, 

 the glass tube removed by a sharp blow, and a small bi-prism is 

 thus left on the glass plate. This is simple in theory but tedious 

 in practice. To secure three good prisms Pfliiger 2 made over 

 three hundred attempts, and still even the best had optically 

 poor surfaces. Since the angles of the prisms used by Pfliiger 

 were very small, the largest being 2' 9". 6, the irregularities in 

 the surface became serious sources of error, inasmuch as a small 

 error in measuring the angle of the prism produces a large per- 

 centage error in the results. With solid prisms of several of 

 the aniline dyes, formed in this manner, Pfliiger determined 

 the refractive indices, for the visible part of the spectrum, by 

 the usual direct spectrometer method. 



Fuchsin and cyanin received the most attention and the aver- 

 age results obtained by four prisms of the latter are given in 

 Table IV. 3 



Table IV. 



With cyanin he extended his experiments into the ultra vio- 

 let. To do this he used an iron arc spectrum, dispersed by a 



i Pfliiger, Wied. Ann. LVI., p. 412, (1895) ; LXV., pp. 173, 214, 225, (1898). 

 a Pfliiger, ibid. LXV., p. 205, (1898). 

 s Pfliiger, ibid., p. 206. 



