4 STUDIES IN GELS TI5 



to Kratky and Platzek (1958), the total double refraction of the 

 swollen gel t-Do amounts to : 



t-Do = t-Do. 



q 



Consequently, if the intrinsic double refraction i-Do of the dry gel 

 is known and the total double refraction t-Do of the swollen gel is 

 measured, the rodlet birefringence r-Do of the swollen gel can be 

 calculated. It is therefore possible to measure rodlet anisotropy in gels 

 capable of swelling, provided the birefringence curves are only plotted 

 from points which result from measurements in imbibition liquids 

 giving rise to similar degrees of swelling. Otherwise one would obtain 

 compHcated kinky curves devoid of regularity, instead of smooth 

 Wiener curves (cf. Fig. 62, p. 84). 



Apart from rodlet double refraction, another form of birefringence 

 mav occur when liquids penetrate between the amorphous chain 

 molecules. This is attributed by Vermaas (1941, 1942) to oriented 

 adsorption of the penetrating molecules. It might also be due, how- 

 ever, to a change in the "intrinsic anisotropy" of the chain molecules 

 caused by the swelling medium, such as that occurring in sols when 

 the refractive index of the dispersing medium is changed (Sadron, 



1937)- 



d. Electron Microscopy 



Electron rays. The electrons which are emitted by a cathode are 

 electrically charged negative particles with a mass of 1/1840 of that 

 of a hydrogen atom. The range of these electrons in air is very short, 

 because they are absorbed or scattered by atoms or molecules which 

 they meet on their path. All investigations with electron rays must 

 therefore be carried out in vacuo. On account of their electric charge 

 they can be made to deviate from their straight trajectory by means of 

 electric or magnetic fields. Bundles of electron rays can therefore be 

 focused by electric coils in much the same way as light rays by lenses. 

 This makes it possible to form images with electron rays according to 

 the laws of geometrical optics (Zworykin and coll., 1945; Burton 

 and Kohl, 1946; Wyckoff, 1949; Frey-Wyssling, 195 i). 



In so far as the electron rays represent a stream of particles, they can 

 hardly be compared with light rays or X-rays. They have the remark- 



