g2 MICRO-ORGANISMS IN RELATION TO MAN. 
did not make any great progress until recent years, when 
improvements in the manufacture of glass have enabled 
lenses of far greater magnification and definition to be con- 
structed. 
Like all the other groups of Nature’s children, micro-organ- 
isms vary much among themselves with regard to size, shape, 
and even colour. In size we know them from small spherical 
granules less than one twenty-five-thousandth of an inch in 
diameter, and which require a magnification of 1,000 dia- 
meters before they can be microscopically recognized with 
certainty, to organisms like the moulds, which throw out 
processes visible to the naked eye. It is almost impossible 
to form any definite conception of the size of the average 
micro-organism, which closely approximates to one twenty- 
thousandth of an inch in length; and the mere statement 
that such or such a number will, for instance, reach from 
end to end of Burton Bridge, conveys but an indefinite 
impression to the mind. The best attempt to enable us to 
realise the extreme minuteness of organisms of the size just 
mentioned is, I think, that recently given by Prof. Percy 
Frankland, who tells us that no less than four hundred 
millions could be spread in a single layer over one square 
inch. We should thus have a population one hundred times 
as great as that of London settled on an area of a single 
square inch, without any over-crowding, and giving to each 
individual organism one four-hundred-millionth of a square 
inch, which space would be quite sufficient for each member 
of the population to thrive and multiply in. 
The shape or form of micro-organisms is equally diverse, 
ranging from extremely small unicellular spherules to the 
comparatively large multicellular moulds. 
Micro-organisms are divided into three principal groups or 
families, namely, the Moulds, or Hyphomycetes; the Yeasts, 
or Saccharomycetes; and, what are commonly called Bac- 
teria, or Schizomycetes. 
