122 



COLLOIDS IN BIOLOGY AND MEDICINE 



In this dark field illumination the form of the objects are not 

 given, but every point appears as a small bright disc, which under 

 some circumstances may be surrounded by one or several rings of 

 light. The ultramicroscope is especially suited for the recognition of 

 inhomogeneities in a medium. 



Apart from the recognition of form, the field of application of the 

 microscope was enormously extended by the invention of the ultrami- 

 croscope. 



At about seven hundred diameters' magnification, the limit of the 

 available microscopical magnification is reached theoretically and 

 practically, i.e., revelation of new details ceases. The ability to make 

 particles visible in the ultramicroscope is almost unlimited, provided 

 only a sufficiently strong source of light is available. Practically, 

 the limits of visibility in our latitude with the best sunlight is about 

 10 MM (1 MM = 1 millionth part of a millimeter). [ZSIGMONDY gives 

 5 MM. TrJ 



For our purposes, two types must be distinguished: (a) Ultra- 

 microscopes for the study of colloids. They permit the observation 

 of objects or inhomogeneities down to 10 MM and require very bright 

 sources of light sunlight reflected from a heliostat, or electric arc 

 lights. (6) Ultramicroscopes for the study of organized materials 

 (microorganisms, animal and plant cells), suitable for the study of 

 objects no smaller than 0.1 M- Welsbach or Nernst lights in com- 

 bination with suitable lenses furnish sufficient illumination. 



Ultramicroscopy for the Study of Colloidal Solutions. 



The original slit-ultramicroscope constructed by H. SIEDENTOPF 

 and R. ZSIGMONDY with rectangular arrangement of the optical axes 



FIG. 3 la. Slit-ultramicroscope. 



is nowadays only employed for the study of solid objects (glasses) 

 and on this account may be omitted from consideration in bio- 

 colloid investigations. Recently ZSIGMONDY has adapted the original 

 slit-ultramicroscope to immersion. A large proportion of the light 



