4 STUDIES IN GELS 85 



ally the optical effects occurring in these systems. In this calculation 

 one must assume greatly idealized textures with, for instance, parallel 

 circular cjdinders or parallel planes (Fig. 6oa and b). Such aggregates, 

 meeting the mathematical requirements, are designated as "composite 

 bodies" (German: Mischkorper). As the structural units (cylinders or 

 planes) are not bound together, they do not possess a micellar structure 

 in our sense. One can imagine, however, that a gel is formed out of 

 such an idealized composite body if the structtiral units are somehow 

 anastomosed with each other. This does not affect the general character 

 of the optical effects, but it is obvious that quantitative calculations 

 according to Wiener's formulae cannot give very accurate results for 

 gels with a micellar structure, since the geometrical conditions for an 

 accurate mathematical treatment of the problem are not satisfied. 



The rods or layers of the composite body must be supposed to be 

 optically isotropic. It then follows from theory that the behaviour 

 of the composite body with respect to polarized light depends on the 

 direction of its vibration, i.e., such a body is anisotropic, provided 

 the diameter of the cyHnders and the distances between the cylinders 

 or layers are small compared with the wavelength of the light. It should 

 be borne in mind that by "small" we do not mean arbitrarily small, as 

 the structural units should possess true phase boundaries. Single chain 

 molecules, for instance, cannot act as structural units in a composite 

 body. 



Optical anisotropy can manifest itself in three different ways : 



1 . Birefringence. The refractive power (n^y) for directions parallel to 

 the axis of the composite body is different from that perpendicular 

 to it (nj^) (Fig. 60a, b) so that, in polarized light, interference colours 

 occur as in doubly refracting crystals. 



2. Anisotropic absorption (dichroism). In coloured composite bodies, 

 absorption is different parallel (k^^) and perpendicular (kj^) to the axis ; 

 they therefore show different colours depending on their position with 

 respect to the plane of oscillation of linearly polarized light (Fig. 6i). 



3. Anisotropic diffraction. Transmitted light is differently diffracted in 

 different directions; the typical gloss of silk, for instance, must be 

 attributed to this effect. 



The composite bodies possess a very typical characteristic: their 

 anisotropy is not constant but is a function of the properties of the sub- 

 stance enclosed between the particles, which in microscopy we de- 



