Some time ago1 we commented on a new method for the observation of ultra-microscopic particles. The method was first described by two German physicists, Siedentopf and Ziegmondy. The principle is the lateral illumination of the particles by an intense focus of light. With a microscope particles thus illuminated can be seen as small as 0.005 micron and even smaller. The method was first applied to observe minute particles of gold in a piece of glass. It was suggested at that time that it would be of value in the study of colloidal solutions and also of biologic problems.
Raehlmann2 has recently reported some interesting applications of this method. In studying solutions of pigments he finds that there are present numerous very minute particles. Particles of different colored pigments not only vary in color, but are also characterized by different form and movement. This fact may be of great