dest sense--was only of service in producing immunity as a preventive
measure against the corresponding organism, but the work of [v.03 p.0172]
Sir Almroth Wright has shown that the use of such vaccines may be of
service even after infection has occurred, especially when the resulting
disease is localized. In this case a general reaction is stimulated by the
vaccine which may aid in the destruction of the invading organisms. In
regulating the administration of such vaccines he has introduced the method
of observing the _opsonic index_, to which reference is made below. Of the
discoveries of new organisms the most important is that of the _Spirochaete
pallida_ in syphilis by Schaudinn and Hoffmann in 1905; and although proof
that it is the cause of the disease is not absolute, the facts that have
been established constitute very strong presumptive evidence in favour of
this being the case. It may be noted, however, that it is still doubtful
whether this organism is to be placed amongst the bacteria or amongst the
protozoa.

[Sidenote: Methods of study.]

The methods employed in studying the relation of bacteria to disease are in
principle comparatively simple, but considerable experience and great care
are necessary in applying them and in interpreting results. In any given
disease there are three chief steps, viz. (1) the discovery of a bacterium
in the affected tissues by means of the microscope; (2) the obtaining of
the bacterium in pure culture; and (3) the production of the disease by
inoculation with a pure culture. By means of microscopic examination more
than one organism may sometimes be observed in the tissues, but one single
organism by its constant presence and special relations to the tissue
changes can usually be selected as the probable cause of the disease, and
attempts towards its cultivation can then be made. Such microscopic
examination requires the use of the finest lenses and the application of
various _staining_ methods. In these latter the basic aniline dyes in
solution are almost exclusively used, on account of their special affinity
for the bacterial protoplasm. The methods vary much in detail, though in
each case the endeavour is to colour the bacteria as deeply, and the
tissues as faintly, as possible. Sometimes a simple watery solution of the
dye is sufficient, but very often the best result is obtained by increasing
the staining power, _e.g._ by addition of weak alkali, application of heat,
&