ultra-microscopic or the invisible organisms have a special interest
in many ways. The limitation in the power of the microscope for the
study of minute objects is due not to a defect in the instrument but
to the length of the wave of light. It is impossible to see clearly
under the microscope using white light, objects which are smaller in
diameter than the length of the wave which gives a limit of 0.5µ. or
1/125,000 of an inch. By using waves of shorter length, as the
ultra-violet light, objects of 0.1µ. 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. The objects are not visible as such,
but the dispersion of the light by their presence is seen.

The demonstration that infectious diseases were produced by organisms
so small as to be beyond demonstration with the best microscopes was
made possible by showing, that some fluid from a diseased animal was
infectious; and capable of producing the disease when inoculated into
a susceptible animal. The fluid was then filtered through porcelain
filters which were known to hold back all objects of the size of the
smallest bacteria and the disease produced by inoculating with the
clear filtrate. There are a number of such filters of different
degrees of porosity manufactured, and they are often used to procure
pure water for drinking, for which use they are more or less,
generally however, less efficacious. The filter has the form of a
hollow cylinder and the liquid to be filtered is forced through it
under pressure. For domestic use the filter is attached by its open
end to the water tap and the pressure from the mains forces the water