280 Modern Microscopy 



placed over each will keep out dust and prevent evapo- 

 ration, as the fluids in each will serve for many sets of 

 slides. The 3 inches by 1 inch glass troughs are also 

 very handy, especially for certain staining processes, as 

 they may be placed under the microscope for examination 

 during operations ; they will also hold several slides of 

 sections, in which case the sections should be downwards, 

 and a small strip of glass placed between the slides at one 

 end. If your object has been stained in toto previous to 

 embedding, then, after the xylol and 90 per cent, alcohol 

 bath it only remains to clear in xylol-phenol and enclose 

 in balsam in the usual way. If you pass sections stained 

 in toto from the paraf&n dissolving xylol-bath into balsam, 

 without the intervention of the alcohol-bath, failure will 

 almost certainly result as a consequence, first, from a trace 

 of paraf&n, and, secondly, from the Mayer's albumen being 

 carried into the balsam and causing cloudiness. 



It will be generally found by the beginner that staining 

 sections upon the slide is more under his control than 

 staining in bulk, and will yield him more information 

 respecting staining in its scientific meaning as distinguished 

 from mere dyeing. In scientific staining we aim, not at 

 the mere colouring of tissues to render them more beautiful, 

 but to differentiate structure by colouring the various ele- 

 ments differently. The aesthetic microtomist will aim at a 

 combination of the scientific with the artistic, in so far as 

 there be no departure from the erudite rendering of nature. 



In brief, the theory of staining is to colour certain ele- 

 ments of a tissue — otherwise, perhaps, difficult of observa- 

 tion — of a different colour to surrounding elements, by 

 taking advantage of the afi&nity of certain elements for 

 certain stains, bearing in mind the knowledge gained by 

 a previous study of the subject, and knowing exactly what 

 it is wished to differentiate. Sections that are to be stained 

 upon the slide may be stained in alcoholic solutions of 

 stains of varying alcoholic strengths, or in aqueous solu- 

 tions, or in both. 



