CLEARING 31 



Staining is best performed by placing a few drops of the 

 staining fluid in a watch-glass, and immersing the sections in it. 

 The exact strength of the fluid, and the time of exposure of the 

 sections to its action varies in each case, and must be ascer- 

 tained by preliminary trials. As a rule, when differentiated 

 staining is desired, the best results are obtained by using a 

 dilute solution-, and by exposing the sections for a long time to 

 its action ; after staining and before mounting for observation 

 it is as a rule necessary to wash the sections in order to remove 

 the superfluous staining fluid ; when the staining substance is 

 dissolved in alcohol, the sections are to be washed out with 

 alcohol ; when dissolved in water they are to be washed with 

 water : in the case of iodine staining this need not be done, 

 as these colourings fade rapidly when the staining fluid is 

 removed. 



Clearing the Preparations. If it is not desired to observe the 

 details of structure of the protoplasm or of the nucleus, the best 

 clearing agent for ordinary use is a solution of potash, either in 

 water or alcohol. 



The clearing action of potash is due to the swelling of various 

 parts of the cells and their contents, so that they become more 

 transparent ; at the same time it dissolves many of the granules 

 in the protoplasm, and saponifies the oil-drops. The swelling 

 caused by the action of the solution in water is often too great, 

 especially when it is desired to see the cell-walls distinctly ; this 

 difficulty may be got over by the use of the alcoholic solution. 



After treatment with the aqueous solution of potash, the sec- 

 tions should be washed in distilled water, and after treatment 

 with the alcoholic solution in dilute alcohol ; the sections, in 

 either case, may be mounted in glycerine : or the sections may 

 be treated at once with a mixture of potash solution and glycerine, 

 but in any case the potash must be washed out before mounting 

 as a permanent object. 



If treatment with potash solution does not readily make 

 the tissues transparent, the action of the reagent may be 

 accelerated and intensified by warming over a spirit-lamp. If 

 the action be too strong, and the tissues become too transparent, 

 this may be corrected by neutralizing with acetic acid. 



