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 through it.