200 BACTERIOLOGICAL DIAGNOSIS. 



5. Wash thoroughly in tap water, continuing the 

 washing until the sections have a decidedly blue tinge. 

 The haematoxylin compounds are very much like litmus, 

 being red in presence of acids and blue in presence of 

 alkalies ; the sections are to be coloured blue, and the 

 necessary alkali is contained in the tap-water. It will 

 hasten the process to rinse them in a very dilute solu- 

 tion of ammonia. 



6. Stain in watery eosin for a minute or so. This is 

 the counter stain. The hsematin will stain all nuclei blue, 

 but will scarcely tinge anything else ; the eosin is added 

 to stain other structures a pale pink and thus make 

 them more visible. It stains almost instantaneously. 



7. Wash off the eosin under the tap. 



The sections are now stained. But they are opaque 

 and not in a suitable condition to be examined under 

 the microscope, and are to be rendered transparent by- 

 being mounted in balsam. Now this cannot be done in 

 the same way as was used in the mounting of films, for 

 the drying would cause the sections to shrivel and 

 obscure their structure. The water is to be removed, 

 it is true, but by the use of absolute alcohol ; at least 

 two lots should be used, and the slide rocked from time 

 to time. Then the alcohol (which will not mix with 

 balsam) is to be removed by the use of xylol, balsam 

 added, and the section covered with a cover-glass. 

 The remaining steps are therefore : 



8. Absolute alcohol, two lots (to dehydrate). 



g. Xylol, two lots (to render the section permeable to 

 balsam). 



10. Balsam and a cover-glass. 



The last three steps are practically the same as the 

 first three, but in the reversed order, and similar pheno- 

 mena are seen. The section is opaque whilst wetted 



