4 



STUDIES IN GELS 



87 



where d.^ = o. This is not so, however, because empirical data con- 

 cerning the position of the maximum retardation /"(max) in a myelin 

 tube have been mixed up with the optical theory (Schmitt and Bear, 



^937)- 



S2gn of the double refraction. The sign or character of the double re- 

 fraction is a very important datum for the textural analysis of gels. A 

 micellar texture is called optically positive if, as mentioned before, 

 n,, — nj_ has a positive value. If, on the other hand, n„ — nj^ 



is 



smaller than zero, the double refraction is negative. The refractive 

 index n,, always refers to a direction which in some way or other 

 is of a special character : direction of the orientation in the composite 

 l)odies mentioned, direction of growth, direction of pressure or ten- 

 sion, direction of flow, special morphological direction, and so on. 

 In fibres and threads, for example, the fibre axis is the reference axis, 

 in cross-sections of parenchyma cells the tangential direction, in 

 spherites the radial direction. 



The character of the birefringence of gels is indicated by the so- 

 ■caUed index ellipsoid, the long axis of which corresponds to the larger 

 index ny, while the short axis corresponds to the smaller index na. 

 The direction of ny is determined by comparison with a selenite 



o) 



+ 



b) 



\ I 



ii 



I 



-i- 



CI^ 



d) 



+ 



+ 

 f) 



T-ig. 63. Optical character of gels. Reference axis marked by a dotted line, a) Rodlet 

 ■composite body, b) layer composite body, c) thread of gum arable, d) thread of cherry gum, 

 ;) section across a vegetable parenchyma cell (reference axis = tangential direction), 

 /) starch grain (reference axis = radial direction). 



plate (see Ambronn and Frey, 1926). The orientation of the index 

 ■ellipsoid and the direction to which the double refraction refers have 

 heen drawn in Fig. 63, Many gels are isotropic when observed in the 

 direction of the reference axis; they are uniaxial in the crystallographic 



