82 



PRINCIPLES OF GENERAL PHYSIOLOGY 



available illumination, or their refractive index may be too close to that of the 

 surrounding medium. 



In the actual instrument, as supplied by the firm of Zeiss or Winkel, for 

 example, the crater of the positive carbon of the arc is first focussed on to an adjust- 

 able slit, whose aperture can be read off on the graduated head of the adjusting 

 screw. This slit can be rotated through 90 for the purpose of estimation of the 

 actual mass of the particles by the ingenious method of Siedentopf and Zsigmondy 

 (Zsigmondy, 1905, pp. 93-97). The total content of the colloidal matter in a lar-v 

 volume of the solution is first determined by some appropriate chemical method. 

 This solution is then diluted to a known extent, and so far that the particles seen 

 under the ultra-microscope are sufficiently separated to be counted. By aid of a 



micrometer in the 

 ocular, a known 

 area of the field 

 is isolated, and 

 the number of 

 particles in the 

 volume corre- 

 sponding to tlii.x 

 area is counted. 

 The depth of this 

 portion is obtain- 

 ed by rotating 

 the slit through 

 90, when what 

 was previously 

 the depth becomes 

 the width, and 

 can be read off on 

 the ocular micro- 

 meter. A simple 

 calculation then 

 gives the number 

 of particles in 

 unit volume of the 

 original solution, 

 and from this the 

 mass of each is 

 known from the 

 total solid content 

 of the solution. 



Dark Ground 

 Illumination. 

 Another method, 



FIG. 38. ILLUMINATED FIELD AS SEEN*. 



The rays converge to a focus in the centre and then diverge again, a and b. Note 

 that the greatest number of particles is rendered visible in the most brightly 

 illuminated spot c. This is due to the fact that the more intense the illumina- 

 tion, the smaller are the particles that it is possible to observe. The particles 

 which are too small to be seen outside the focus of the beam are obvious under 

 the more brilliant light at this focus. 



sometimes called 



"ultra-microscopic," which is frequently used for examination of bacteria, and is then, 

 of course, not strictly ultra-microscopic, but can also be made to show the presence 

 of structures invisible by the ordinary microscope, is a development of the dark 

 ground illumination by specially constructed sub stage condenser, introduced by 

 Wenham in 1872. The central rays of the illuminating beam are cut out by means 

 of a stop, and the peripheral rays are reflected by a parabolic surface so as to 

 meet at a point in the object under examination ; they cross at such an angle as to 

 pass outside of the field of the objective in use, which only picks up light refracted, 

 or diffracted, from structures in the preparation. The paraboloid form is chiefly 

 used for the investigation of comparatively coarse structures, as in Fig. 6, of 

 Spirogyra. A cardioid surface, as in the apparatus of Siedentopf, made by Zeiss, 

 gives more brilliant illumination, and can be used for the more minute particles of 

 colloidal solutions. This latter instrument has also been fitted, at my suggestion, 

 with an electrical hejting arrangement, so that the changes produced in colloids 

 by heat can be followed by the eye. 



