254 



BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



may have entirely different optical properties from those which 

 it possesses in a solid state. 



To avoid this objection several indirect methods based upon 

 the properties of the solid dye were devised. The results ob- 

 tained do not warrant a detailed description, but a bare state- 

 ment of the methods used will be made. Sieben employs a 

 method based on the total reflection of light from the surface of 

 the solid dye. By measuring the absorption of the solid dye for 

 various wave lengths, Wernicke 1 was able to calculate the index 

 of refraction, and for fuchsin his results are given in Table II. 



Table II. 



"Wiedemann, 2 Lundquist, 3 and Merkel 4 made observations to 

 dete mine the elliptic polarization of the light reflected from the 

 surface of the solid substance. The work of these and several 

 othe r observers, although of great interest, is so dependent upon 

 a m. ruber of doubtful assumptions that to show the real value of 

 the results a detailed description must be made. The latter is 

 out-side the limit of a brief outline and must be omitted. Al- 

 thc'igh the refractive index, and therefore, also, the velocity of 

 the light in the substance can be calculated by these indirect 

 methods, still, the direct observation of refraction by a solid 

 prism is by far the simplest and surest procedure. As early as 

 1875 Wernicke 5 determined the indices given in Table III., by 

 means of a prism of solid fuchsin. 



Table III. 



i Wernicke, Pogg. Ann. CLV., p. 87. 



9 Wiedemann, ibid. CLI., p. 1, C1874). 



* Lundquist, ibid. CLIL, pp. 177, 398, 565, (1874). 



« Merkel, Nov. Act. Koy. Soc. Upsala, Ser. III. 



» Wernicke, Pogg. Ann. CLV., p. 93, (1875). 



