CLEARING 49 



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 ascertained 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 

 ot 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 dis- 

 tinctly ; this difficulty may be got over by the use of 

 the alcoholic solution. 



After treatment with the aqueous solution of potash, 

 the sections should be washed in distilled water, and 



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