206 THE EXAMINATION OF MATERIAL 



The stain most generally used is carbol-thionin (p. 138). The technique 

 is as follows : 



A. Cover-glasses. 1. Hold the cover-glass in a pair of Cornet's or Debrand's 

 forceps and pour on to the film sufficient stain to cover the surface. 



Allow the stain to act for 30-60 seconds. 



2. Wash in distilled water. 



3. Mount the cover-glass on a slide film downwards in a drop of water. 

 Examine with a T V objective and No. 2 ocular. 



4. If the preparation be satisfactory it may be mounted permanently, by 

 drying it in the air or gently heating it and then mounting in Canada balsam. 



To sum up : stain, wash in water, dry, mount in balsam. 



B. Slides. Films made on slides are stained in a similar manner, Hold 

 the slide in the left hand or in a pair of Debrand's forceps ; flood the slide 

 with stain ; wash in water, dry ; place a drop of cedar-wood oil on the film 

 and examine with an immersion lens. The preparation may be mounted by 

 placing a drop of balsam on the film and covering with a cover-glass. 



Dilute carbol-fuchsin, the various carbol-violet stains, Kuhne's or Loeffler's or 

 Roux's blue, etc. may any of them be used in suitable cases in place of carbol- 

 thionin. The particular stain which is most useful for the detection and study of 

 the different species will be referred to in the chapters devoted to those species. 



The disadvantage of the simple stains is that as they stain the groundwork 

 and the organisms the same colour (fig. 153) ; the latter fail to stand out 



v X 'v!%; 



FIG. 153. Simple staining. 

 Scraping from gum stained with dilute carbol-fuchsin (oc. 2, obj. T \.th. Zeiss). 



conspicuously, especially when they are few in number or when the film is 

 thick. The methods of differential staining are adopted to overcome these 

 defects. 



Examination of the blood. In the case of blood-films the necessity for 

 double staining may be avoided by getting rid of the groundwork. Thus 

 if the haemoglobin (which is the only substance in the red cells which takes 

 the stain) be eliminated there remains after staining a colourless groundwork 

 on which the micro-organisms stand out conspicuously. This result may 

 be effected in one of two ways : 



(a) Gunther's method. 1. Dry the film by gently heating it and then 



