COLLOIDS IN PROTOPLASM 



therefore, is very suitable to demonstrate the 

 existence of solid particles in colloidal solutions. 



About ten years ago Zsigmondy, in Jena, very 

 ingeniously used the principle of Tyndall's pheno- 

 menon to show the single particles themselves 

 in colloid solutions by means of the microscope. 

 Whilst microscopical objects are usually illumi- 

 nated by rays of light so directed that they are 

 parallel to the axis of the microscope, Zsigmondy's 

 microscope was arranged in such a manner that a 

 very thin and strong ray of electric light was 

 thrown through the microscopical preparation 

 from the side, vertical to the axis of the micro- 

 scope. Consequently the microscopical field of 

 vision remained dark. The suspended particles, 

 when illuminated from the side, reflect the light 

 and become visible, appearing like small stars 

 on the dark sky. The strong dispersion of light 

 does not permit us to recognise the size and shape 

 of the single particles. But they can be counted 

 exactly. In this way the particles of platinum or 

 of gold-sols were made visible, and even their size 

 could be indirectly determined. An arrangement 

 was even made for studying living cells and 

 protoplasm by means of the ultramicroscope. 

 It was clearly shown that numerous particles in 

 protoplasm are made visible by this method which 

 could not be seen by the ordinary microscope. * 



Ordinary microscopical observation with the 

 strongest lenses can show particles of about 250 /x/x 

 25 



