86 BACTERIOLOGY. 



carefully turned over on to a slide, and the excess of stain gently 

 and gradually removed by pressure with a strip of filter-paper. 

 This affords a rapid means of demonstration — for example, of a 

 cultivation of Koch's comma bacilli in nutrient gelatine — enabling 

 the microbes to be seen in some parts of the preparation both 

 stained and in active movement. 



Cover-glass Peepaeations. 



Bacteria may be spread out into a thin layer on a cover-glass, 

 and then treated with a dye, or sections of tissues containing bacteria 

 can be stained and then mounted in the usual way. 



The method of making a cover-glass preparation is one which is 

 very commonly employed. In addition to its value as a means of 

 examining bacteria in liquids and solid culture media, it affords 

 the additional advantage of enabling, if necessary, a lar^e number 

 of preparations to be made, which, when dried, can be preserved, 

 stained or unstained, in ordinary cover-glass boxes ; they are 

 then in a convenient form for transport, and can be mounted 

 permanently at leisure. 



The method is as follows : A cover-glass is smeared with the 

 cut surface of an oi-gan or pathological growth, or with sputum ; 

 or a di'op of blood, pus, or culture-fluid is conveyed to it with a 

 looped platinum needle. It is absolutely necessary to spread out 

 the micro-organisms into a sufficiently thin layer, so that the 

 individual bacteiia may be as much as possible in the same plane, 

 otherwise some in the field will be in focus and others out of 

 focus, and it would be impossible to obtain a satisfactory photograph 

 of such a specimen. To overcome this it will be necessary, in the 

 case of cultures on solid media, to diffuse the bacteria in a httle 

 sterilised water ; and even cultures in liquids may sometimes with 

 advantage bs diluted in the same way. By means of another 

 cover-glass the jiiice or fluid is squeezed out between them into a 

 thin layer, and on sliding them apart each cover-glass bears on one 

 side a thin film of the material to be examined ; or a culture is 

 spread out into a thin film by means of a hooked platinum needle. 

 The cover-glass is then placed with the prepared side upwards, and 

 allowed to dry. After a few minutes, it is taken up with a pair of 

 flat-bladed or spring forceps, with the prepared side uppermost, and 

 passed rapidly from above downwards three times through the 

 flame of a spirit lamp or Bunsen burner. Two or three drops of 

 an aqueous solution of fuchsine or methyl violet will be suflicient to 

 cover the film, and after a minute or two the surplus stain is washed 



