74 



DISPERSE SYSTEMS 



The ultra- microscope is, in principle, just a means of viewing 

 the Tyndall cone through a microscope. A powerful beam of 

 light is thrown horizontally through a small body of fluid placed 

 under a microscope set vertically. The only light entering the 

 * t objective is that diffracted from the 



particles present in and optically dif- 

 ferent from the fluid (Fig. 7). The 

 apparent image bears no relation to the 

 actual size of the particle, but depends 

 on the intensity of the light, and on 

 the indices of refraction of the par- 

 ticle and the dispersant. Neverthe- 

 less, by making certain assumptions, 



FIG. 7. Diagrammatic section the size of the particles may be cal- 

 culated. 



Particles visible under the ordinary 

 microscope are called microns. Smaller particles are termed 

 sub-microns, if they are rendered apparent by the ultra- 

 microscope ; if not, they are amicrons. The smallest particle 

 of gold observed by Zsigmondy, using bright sunlight illumina- 

 tion, was 1 -0/uijUL in diameter. Bearing in mind the large difference 

 in index of refraction between gold and water, this may be 

 considered as the smallest particle ever observed. The follow- 

 ing table (from Zsigmondy) shows the limits of size of the 

 various classes of particles : 



through a paraboloid condenser to 

 show the direction taken by the rays 

 of light. (Hatschek.) 



TABLE XII. 



LOWER LIMITS OF DIAMETERS OF SMALL PARTICLES. 



Visible under 

 Microscope 



MICRONS 

 O-25/i or 

 2-5 x 10~ 5 cm. 



Not visible under microscope 



SUB-MICRONS 



AMICRONS 



Visible by ultra-microscope Not visible by U.M. 



under 1'0/x/z 



Electric arc Strongest 



Sunlight 



15/x/x or 1'0/x/ji or 



15xlO- 7 cm. l-OxlO- 7 cm. 



equals 10~ 3 mm. = 10~ 4 cm., yu/x = 10 7 cm. 



