The class of organisms known as the filterable viruses or the 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 0.5 mu. 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.  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.  In laboratory uses, denser filters of smaller diameters are used, and the filter is surrounded by the fluid to be tested.  The open end of the filter passes into a vessel from which the air is exhausted and filtration takes place from without inward.  The test of the effectiveness of the filter is made by adding to the filtering fluid some very minute and easily recognizable bacteria and testing the filtrate for their presence.  These filters have been studied microscopically by grinding very thin sections and measuring the diameter of the spaces in the material.  These are very numerous, and from 1/25000 to 1/1000 of an inch in diameter, spaces which would allow bacteria to pass through, but they are held back by the very fine openings between the spaces and by the tortuosity of the intercommunications.  When the coarser of such filters have been long in domestic service in filtering drinking water, bacteria may grow in and through them giving greater bacterial content to the supposed bacteria-free filtrate than in the filtering water.