a half to one
`` That , '' he explained to me at length as he worked , `` is what is known as a Berkefeld filter , a little porous cup , made of porcelain . The minute meshes of this filter catch and hold bacteria as if in the meshes of a microscopic sieve , just like an ordinary water filter . It is so fine that it holds back even the tiny bacillus fluorescens liquefaciens which are used to test it . These bacilli measure only from a half to one and one-and-a-half micromillimeters in diameter . In other words 130,000 germs of half a micromillimeter would be necessary to make an inch . '' `` What has it been used for ? '' I ventured .