18 PRACTICAL BACTERIOLOGY 



allowing it to remain from one to three minutes. (The staining 

 process can be hastened and intensified by ' heating/ not ' boiling,"* 

 the cover-glass in the flame until vapour arises.) 



6. Remove excess of stain by washing the specimen with the 

 pipette wash bottle, or directly under the tap. (The specimen can be 

 examined at this stage by laying the preparation side downwards on 

 a slide and removing excess of fluid with filter paper, and if of 

 sufficient interest can be permanently mounted.) 



7. Remove the excess of water by blowing with or without a glass 

 tube, and dry. (A special blower for this purpose can be made from 

 an ordinary atomizer fitted with glass tubes, and about one-third full 

 of calcium chloride.) 



8. Mount in xylol balsam and examine as follows : 



(1.) Place the specimen on the microscope stage so that about the 

 centre of the cover-glass lies in the optical axis. 



(2.) Place a drop of immersion oil on the centre of the cover- 

 glass. 



(3.) Screw down the tube with the coarse adjustment until the 

 point of the lens touches the oil ; the tube is now 

 screwed upwards without breaking the oil connection. 



(4.) Remove the diaphragm below the Abbe condenser, and 

 arrange the flat mirror so that the field is somewhat 

 lighted. 



(5.) Screw the tube carefully down with the coarse adjustment 

 until the preparation comes in view. 



(6.) Regulate the focus with the fine adjustment. 



(7.) Obtain the maximal illumination by manipulating the Abbe 

 and mirror regulator. 



IX. THE CONTACT, OR IMPRESSION SPECIMEN. 



1. Lay a clean cover-glass gently on the top of the desired colony 

 on the plate or dish culture, apply gentle pressure, and lift the cover- 

 glass up by one of its edges without lateral movement. 



2. Air dry. 



3. Fix in the flame, and proceed as with the ordinary cover-glass 

 method, process No. 5, 8. 



The above specimens differ from ordinary cover-glass preparations 

 in that they present an impression of the organisms as they were 

 arranged in the colony from which the preparation was made. It is 

 important to note that ' liquefied-colonies ' cannot be used for the 

 preparation of contact or impression specimens. 



