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 



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- 



throutrh a paraboloid condenser to J 



show the direction taken by the rays culated 

 of light. (Hatschek.) 



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 amicrdns. The smallest particle 

 of gold observed by Zsigmondy, using bright sunlight illumina- 

 tion, was 1 -O/UL/UL 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 : 



TABLE XII. 



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



