I ORGANIZATION OF SOLS 9 



reason that its resolving power does not surpass that of the ordinary 

 microscope. It merely reveals the existence of submicroscopic particles. 

 The possibility of ultramicroscopic demonstration is based on the fact 

 that hght incident upon small particles is scattered in all directions. In 

 this way they become radiant (like the dust particles in a dark room 

 where sunlight penetrates through some gap), so that the path of a 

 beam of light in a sol is clearly traced (Tyndall scattering). The lighted 

 sphere surrounding such a dust particle is much larger than the 

 scattering particle itself, and an image of it can be obtained in the 

 microscope if the distance between the colloid particles is not too 

 small. As the objective of the microscope gives an image of planes 

 only, optical cross-sections of the lighted spheres are imaged in the 

 form of deflexion discs. Since the particles in the sol take part in 

 Brownian movement, these scintillating "deflexion discs" oscillate 

 vividly in an irregular manner. It is an impressive sight to watch these 

 luminous spots which, in untiring movement, stand out like bright 

 stars from the pitch-dark background. 



To what extent the size of the "deflexion discs" exceeds that of the 

 particles we do not know; nor can we determine the exact shape of 

 the particles. All the same, the ultramicroscope enables us to draw 

 conclusions as to their circumference in cases of marked deviation 

 from the spherical. Non-spherical particles may be oriented in a field 

 of flow. In that case they scintillate to difterent extents according as the 

 incident ultramicroscopic irradiation is parallel or perpendicular to the 

 direction of flow; they show what is calJed azimuth effect. If the light 

 falls upon the small endplane of submicroscopic rods, they scatter 

 much less than with sideways irradiation. From such diflferences in 

 intensity of the "deflexion discs", depending on the direction of the 

 incident beam, the rod-hke shape of the particles can be inferred. 



Anisodiametric particles are usually birefringent. As they are 

 oriented in a field of flow, sols containing such colloid particles 

 become optically anisotropic in a velocity gradient (Freundlich, 

 Stapelfeldt, and Zocher, 1924). Long rods are oriented at lower 

 rates of shear than shorter ones (Signer and Gross, 1935). From 

 measurements of the birefringence of flow, conclusions can therefore 

 be drawn regarding the rations between length and thickness. 



Si^e of the particles. A clear picture of the world of submicroscopic 

 particles can be obtained with the aid of the methods mentioned. 



