AND CONTENTS OF THE MOUTH. 85 



To thin the preparation as much as possible, we must place 

 upon it the cover-glass, well centred, and let it act by its own 

 pressure. In this way the mucus becomes thinned, and spreads 

 slowly without altering materially the elements involved in it. 

 When the preparation has become partially thinned, press down 

 lightly and gently with the rod and leave it undisturbed, and so 

 on until the very slender film of the coloured matter has in every 

 direction neared the edges of the cover-glass ; care being taken in 

 these handlings to strain off the redundant liquid without wetting 

 the upper surface of the cover-glass. 



The preparations so mounted, in water, are in all parts very 

 transparent, clear, and bright ; they do not exhibit superposition 

 of elements or corpuscles, and even in hot weather, keep well under 

 the microscope for several days, if care be taken to wet now and 

 then one of the edges, as we suggested in the first Memoir. Even 

 the bacteria in the centre of the particle of sputum which were 

 not at first reached by the colouring medium become in time gra- 

 dually coloured, and these weak colourings are often very useful 

 for examining minute details. 



In order to obtain fructifications of Leptothrix from sputa, it is 

 necessary to take the particle of sputum, not from the bottom, but 

 from that side of the tube where, perhaps accidentally, a vertical 

 streak of the mucus, slender and adhesive, has been left, as we 

 shall see later. 



For researches on fructifications, powerful immersion objectives 

 are required, so as to be able to detect all details. We have used 

 a No. II objective (i/i8th homogeneous immersion) of Hartnack. 



Collection and Preparation of the Contents of the Mouth. 



We shall especially deal with t\iQ. patina dentaria (surface of the 

 enamel, tartar), etc., saliva and patina of the tongue. 



Patina Dentaria. — The deep layers of this patina contain almost 

 exclusively bud-growths and knots of large filaments which take a 

 brilUant colour with the solution of iodine. The fructifications, 

 on the contrary, are simply found on the superficial layers, so that 

 for their investigation we must possibly remove the upper surface 

 of the patina. By scraping the tooth with a small instrument or the 



