or 1/125,000 of an inch.

By using waves of shorter length, as the ultra-violet light, objects of 0.1 mu.

or 1/250000 of an inch can be seen; but as these methods depend upon photography for the demonstration of the object the study is difficult.

The presence of objects still smaller than 0.1 m.

can be detected in a fluid by the use of the dark field illumination and the ultra-microscope, the principle of which is the direction of a powerful oblique ray of light into the field of the microscope.