80 



DISPERSE SYSTEMS 



>^ \ / ,' 



water, but ncj cone of light is seen in the water itself when viewed 

 at right angles to the direction of the light and against a dark 

 background. If now a colloid be dispersed through the water, 



light will be diffracted from the particles 

 in the water and the beam will appear 

 in the solution as a diffuse cone of light. 

 This diffracted light is plane polarised 

 (p. 126), and is always produced when 

 light passes through any medium con- 

 taining particles whose diameter is 

 small in comparison with the wave- 

 length of light. 



(c) The ultra-microscope is, in prin- 

 ciple, 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 

 different from the fluid (Fig. 15). 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 

 particle and the dispersant. Nevertheless, by making certain 

 assumptions, the size of the particles may be calculated. The 

 essential featin*e of the ultra-microscope is not that it is a more 

 powerful kind of microscope, but a new method of illumination, so 



Fig. 15. — Diagrammatic section 

 tlirousli a Wenham jsaraboloid con- 

 denser to show the direction taken by 

 the rays of light. (Hatscheli.) 



TABLE XII 



LOWER LIMITS OF DIAMETERS OF SMALL PARTICLES. 



Visible under 

 Microscope 



Microns 

 0-2;Li or 

 2-5 X 10^^ cm. 



Sub-microns 

 (photographed 



by U.V. light) 



100/XjU, or 

 1-0 X 10 5 cm. 



Not visible under microscope 



8UB-MICRONS 



Visible by ultra-microscope 



Amicrons 

 Not visible by U.M. 

 under l-Oju/x 



Electric arc 



\b\i\x or 



15 X 10-' cm. 



Strongest 

 Sunlight 

 TOju,^ or 

 1-0 X 10-7 cm. 



/i equals 10 ^ mm. = 10 * cm., jliju, = 10 ' cm. 



