AND HOW TO USE IT. 191 



A good blue stain (tending to purple) is also given by the 

 substance termed indigo carmine ; it is particularly good for 

 sections of brain and spinal cord that have been hardened in 

 chromic acid. A saturated solution of the powder in distilled 

 water having been prepared, this may either be used with the 

 addition of about 4 per cent, of oxalic acid, or if an alcoholic 

 fluid be preferred, methylated spirit may be added to the aqueous 

 solution ; the mixture being filtered to remove any colouring 

 matter that may have be^n precipitated. If sections thus stained 

 have an excess of colour, it may be removed by the action of a 

 saturated solution of oxalic acid in alcohol. 



A beautiful green hue is given by treating with a saturated 

 solution of picric acid in water, sections previously stained with 

 anihne blue ; or the two agents may be used together, four or five 

 parts of a saturated solution of the latter being added to a 

 saturated aqueous solution of the former. This picro-aniline, it 

 is believed, may be relied on for permanence ; and it acts well in 

 double staining with picro-carmine. 



Double Staining — A good example is seen in double staining 

 the frond of a fern with logwood and aniline blue 3 the sori taking 

 the latter, and standing out brilliantly on the general surface tinged 

 by the former. If a section of stem be stained throughout by a 

 solution of eosin (2 grains to the ounce), and be then placed, after 

 washing in strong alcohol, in a J grain solution of Nicholson's blue 

 made neutral, the blue will, in a short time, entirely drive out the red; 

 but by carefully watching the process, it will be seen that the 

 different tissues will change colour at different times, softer cells 

 giving up their red and taking in the blue more quickly than the 

 harder ; so that, by stopping the process at the right point (which 

 must be determined by taking out a section and dipping it in 

 alcohol, and examining it under the microscope), the two kinds of 

 cells are beautifully differentiated by their colouring. The best 

 effects are usually produced by carmine and indigo-carmine, 

 logwood and picro-carmine, carmine or logwood and aniline 

 blue or green. 



Osmic Acid. — Various degrees of dilution of a i per cent, 

 solution. This agent is very serviceable in the preparation of 



