MICROSCOPICAL PREPARATIONS. 7 



and is then ready for examination. It should be noted that 

 all motile bacteria do not show their flagella when they are 

 treated with a mordant of the above composition. One must 

 proceed experimentally, for some bacteria require a mordant 

 to which a few drops of a 10 per cent, soda solution have 

 been added, whilst others require an addition of sulphuric or 

 acetic acid in place of soda. Loffler found that several of 

 the acid-forming bacteria require an alkaline mordant, whereas 

 a number of alkali-forming organisms require an acid mordant. 

 By such careful means beautiful pictures have been obtained, 

 which show that bacteria are supplied with these organs of 

 movement arranged in various ways ; they often cover the 

 whole surface of the cell. A similar treatment brings out 

 clearly the coating of slime which, for instance, surrounds the 

 cells of acetic acid bacteria, but is invisible in an ordinary 

 microscopical examination. 



In a few cases staining has proved of value in determining 

 the species ; this is the case with the acetic acid bacteria 

 investigated by E. C. Hansen, Bacterium aceli, B. Pasteurianum, 

 and B. Kiitzingianum. Staining is most readily done by 

 treating a vigorous growth with an aqueous solution of iodine 

 in potass, iodide, or an alcoholic solution of iodine. The 

 slime surrounding the cells of B. aceti is coloured yellow, 

 whereas that of the other two species is coloured blue. The 

 latter reaction is brought out more clearly when the slime is 

 forced out to the sides by pressure on the cover-glass. Blue 

 coloured flecks are thus formed, while the cells themselves 

 are either colourless or yellow. A similar blue coloration of 

 the spores of Saccharomyces mellacei with iodine has also been 

 demonstrated by J. C. Holm. 



Photographs of micro-organisms are now frequently em- 

 ployed. The preparation may be first stained, for instance, 

 by Loffler 's method, to bring out the characteristics more 

 sharply, and to remove foreign bodies. Whilst the usual 

 sketches of microscopical preparations are always more or 

 less diagrammatic, omitting everything except the special 

 characters it is desired to emphasise, micro-photography 

 provides a more correct representation of the object, and 

 has a further incidental advantage. It is well known that 



