OF MICK.OSCOPIC OBJECTS. 187 



appearing almost black ; yet by transmitted light no colour 

 is more useful, because its distinctness is equally great by 

 artificial light and ordinary daylight. The method of pre- 

 paring this, as given by Dr. Beale, is as follows : 



Glycerine 1 ounce. 



Wood naphtha, or pyroacetic acid 1 drachm. 



Spirits of wine ^ 1 ounce. 



Ferrocyanide (yellow prussiate) of potas- 

 sium 12 grains. 



Tincture of sesquichloride of iron 1 drachm. 



Water 4 ounces. 



Dissolve the ferrocyanide of potassium in one ounce of the 

 water ; add the tincture of sesquichloride of iron to another 

 ounce. Mix these solutions gradually together, shaking the 

 bottle well which contains them it is best to add the iron 

 to the potash solution. When thoroughly mixed, these 

 solutions should produce a dark-blue mixture, perfectly 

 free from any perceptible masses or flocculi. Next mix the 

 naphtha and spirits of wine, and add the glycerine and the 

 remaining two ounces of water. This must now be slowly 

 mixed with the blue liquid, shaking the whole well in a 

 large bottle whilst the two come together. The tincture of 

 sesquichloride of iron is recommended, because it can always 

 be obtained of a uniform strength. 



31. Dr. Turnbull used a mixture slightly different from 

 the above, which is made with the sulphate of iron : 



Purified sulphate of iron 10 grains. 



Ferrocyanide of potassium 32 grains. 



Glycerine 1 ounce. 



Pyroacetic acid 1| drachm. 



Alcohol . 1 ounce. 



Water 4 ounces. 



Dissolve the sulphate of iron in one ounce of the water, 



