8 THE PREPARATION OF MICROSCOPICAL OBJECTS 



2. Mullet's Fluid : a solution of bichromate of potash with 

 a little sodic sulphate in water. 



D. Staining. 



Various reagents are employed for the purpose of staining 

 preparations ; some of these merely dye the whole preparation 

 more or less uniformly, but the most useful ones are those which 

 stain certain parts of the cells only, or at any rate stain these 

 much more strongly than the other parts (selective stains). The 

 most important are the following : 



1. Haematoxylin. There are various preparations of haema- 

 toxylin, or logwood, used in microscopical work : the best is that 

 proposed by Delafield. It is prepared thus : dissolve 4 grammes 

 of crystallised haematoxylin in 25 cubic centimetres of strong 

 alcohol ; add this solution to 400 c.c. of a saturated solution of 

 ammonia alum, and expose to the light in an unstoppered bottle 

 for 3 to 4 days. Filter, add 100 c.c. glycerine and 100 c.c. of 90 

 per cent, alcohol. 



The specimens, which must be perfectly free from all trace 

 of acid, should be cut into small pieces, and passed through 

 weak spirit to water. They should then be left in the haema- 

 toxylin in a covered vessel or stoppered bottle for from one to 

 twelve hours, according to the size of the specimen and the 

 depth of staining desired, and then brought up through water 

 and weak spirit, and left in strong spirit for some hours before 

 mounting. Haematoxylin stains the nuclei of cells much more 

 strongly than the other parts. 



2. Borax-Carmine. This, which is perhaps the most gener- 

 ally useful of all the stains in ordinary use, is prepared as 

 follows. Dissolve 2 parts of carmine and 4 parts of borax in 

 100 parts of water : add an equal volume of 70 per cent, 

 alcohol ; let the mixture stand for a couple of days, and then 

 filter. 



Specimens may be left in borax-carmine for from one to 

 twenty-four hours, or even for two or three days : on removal 

 they should be placed in acid-alcohol i.e., 70 per cent, alcohol 

 to which a few drops of hydrochloric acid have been added 

 until they become a bright scarlet colour, when they should be 

 transferred to 70, and then to 90 per cent, alcohol, in which 

 latter they may be left till required. The time of immersion in 



