STAINING METHODS. 275 



parts distilled water. There they should remain from one to three 

 minutes. As a result of the precipitation from the iodine solu- 

 tion the sections are dark purple. After transfer to alcohol the 

 sections are decolorised ; then they are placed in oil of cloves, and 

 then in Canada balsam. Somewhat of a modification of Gram's 

 method, known as the Gram-Giinther method, is now specially 

 used. For decolorising, not only alcohol but also 3 per cent, 

 hydrochloric acid alcohol is used. The sections, treated with 

 iodine, are placed for half a minute in alcohol, about ten seconds 

 in 3 per cent, hydrochloric acid alcohol, for several minutes again 

 in fresh alcohol, and repeatedly in further quantities of clean 

 alcohol till the maximum of decolorisation ; ultimately, when no 

 more colour comes out of the section, place it in xylol, and thence 

 in Canada balsam dissolved in xylol. Certain bacteria, as well as 

 the cell-nucleus will be deprived of their colour by this process. 

 The staining relations in such a case can therefore serve for the 

 distinguishing of bacteria by a kind of " differential diagnosis ". 

 Gram's method has also been found of use for cover-glass pre- 

 parations. 



In searching for bacteria in tissues, after staining, Abbe's sub- 

 stage condenser (or ^illuminating apparatus) can be used with 

 great advantage, and in a quite special fashion. After focussing 

 the preparation, the iris diaphragm is opened quite wide, so that 

 the object is flooded by a cone of illumination of the full aperture 

 of the objective. All unstained structures now disappear more or 

 less completely, while the stained light-absorbing bodies remain 

 visible. We may call this "isolation of the coloured image". 

 Approximately similar effects are obtained with other substage 

 condensers. 



After these generalised considerations we will endeavour to 

 study more closely the morphological characteristics of certain 

 definite bacteria which are not difficult to procure. 



Bacteria in the "fur " of Teeth. We draw first upon a source 

 which provides for us simultaneously pretty well all the charac- 

 teristic bacterial forms the white " fur " of the teeth. Numerous 

 species of bacteria dwell therein, and we can reckon almost with 

 certainty to obtain therefrom spherical bacteria, rodlets, threads 

 and spirals (Fig. 102). If a small quantity of the " fur" is diffused 

 in a drop of water, and examined with the highest possible power, 

 we first see thicker threads in parallel bundles, and thinner ones 



