THE COLLOIDAL STATE 97 



The development of the ultramicroscope is due to the Germans 

 Siedentopf and Zsigmondy. The instrument designed by them 

 is a large and expensive affair which others have attempted to 

 simplify. This type of ultramicroscope is sometimes referred to 

 as the slit microscope, the slit being a small aperture correspond- 

 ing to the hole in the shutter of a darkened room into which 

 a beam of light enters and illuminates the motes in the air. The 

 ultramicroscope consists further of a series of lenses and apertures, 

 apart from the microscope proper, which direct a light ray 

 into the colloidal material and thus illuminate it laterally. 

 The Tyndall cone of a colloidal solution, as seen with the 

 naked eye, is the total effect of the scattering of light by many 

 particles, no individual particles being visible. In the ultrami- 

 croscope, the individual particles are "seen" as centers of a 

 burst of light which surrounds each one of them. Thus 

 viewed, a colloidal solution resembles the Milky Way at night 

 but with every "star" dancing about in active Brownian 

 movement. 



The Siedentopf-Zsigmondy ultramicroscope has, because of its 

 inadaptability, given way to the much simpler and more con- 

 venient dark-field condenser. It was invented by F. H. Wenham 

 in 1850. The same fundamental principles underlie this instru- 

 ment as those of the slit type of ultramicroscope, viz., a black 

 background and indirect illumination. The light, however, 

 enters the condenser from below, as in an ordinary microscope. 

 This is possible because the rays strike, and are reflected by, 

 two successive mirrored surfaces which direct and concentrate 

 them at a point in the colloidal solution (Fig. 73). The illumi- 

 nating beam of light which passes beyond this point does not 

 enter the microscope; only the scattered rays from the colloidal 

 particles are visible. 



There are numerous types and modifications of the dark-field 

 condenser, such as the cardioid, the paraboloid, and the change- 

 over condensers. The first two names indicate the nature of the 

 curve of the reflecting surface. The change-over condenser 

 permits going from direct, as in the ordinary microscope, to 

 dark-field illumination, without changing condensers. 



A simple form of dark-field illumination is the central stop 

 diaphragm (Fig. 74). It is used successfully with an Abbe 

 condenser. The central stop excludes all rays within the field of 



