AGf~ICULTURAL RESEARCH, PUSA.

4,/)4, PRACl'.lCAL BACTERIOLOGY
thcre is a distinct biological separation of the viruses
and the ordinary bacteria. It may be said that much
of our recent knowledge of the filterable viruses has
been contributed by British and American workers.
As explained on p. 51, the limit of visibility for
il1dividual partides is about 0'25/" when direct illumination
by transmitted light is uscd, and is governed
by the wave-length of the light employed. By darkground
illumination howevel' (q.v.) a VCl'Y minute
object may be rendered visible owing to its refraction
of light, and is seen as a luminous spot. No resolution
is obtained; that is, no details of its form or structUl'C
are revealed, but its presence merely is noted.
Resolution is determined by thc \vave-length of the
light used, the shorter the wave-length the smaller the
object that can be resolved. With ordinary transmitted
light the limit is 0'25/1.. If, however, short-wave ultraviolet
light be used much smaller objects can be resolved,
but as the eye is insensitive to such short waves
photographic methods must be employed. For this
work a special microscope having an optical system
eOllstructed entirely of quartz (since glass absorbs
ultra-violet rays) must be employed, while the technical
diITicultics are great. Under these circumstances
Barnard has succeeded in resolving nnd photographing
objects O'075/L in diameter, and in the future it is
probable that even smaller particles will be observed,
so that the term "ultramicroscopic " is bccoming H
misnomer.
The evidence regarding viruses which has accumulated
during the past few years indicates that those
of animal origin are definitely organised living struchues
comparable with ordinary bacteria, but very
much smaller in size. They propngate, infect tissues,
incite immunity, etc., like the visible bacteria, and
there is no rcason why the range of micro-organisms
in nature should be bounded by the limit of human
vision with ordinary light .• lust as the visible pathogenic
organisms vary in size from that of the anthrax
bacillus down to the influenza baeillus and the pleuru-