MOUNTING 129 



a refractive index of about 1-535. This supposition rests on ob- 

 servations made with tissues fixed in formaldehyde solution and 

 embedded in paraffin. ^^ There is reason to believe that the pro- 

 teins of cells that have been fixed by osmium tetroxide and 

 embedded in butyl methacrylate have a considerably lower index 

 than this. It is desirable that the index of the mounting medium 

 should be somewhat lower again, if undyed sections are to be 

 examined by phase-contrast. The ground cytoplasm will then 

 appear very pale grey by positive contrast, while the mitochondria 

 and certain other cytoplasmic inclusions will be dark grey or 

 black. A mounting medium of refractive index 1-46 has been 

 recommended. ^^^ Media with even lower indices, such as 1-44 or 

 1 -42, seem to give still better results.-^ The refractive index of pure 

 glycerol is 1-474. By mixture with water, any lower refractive 

 index exceeding that of water itself (1-333) can be obtained. The 

 proportions of glycerol and water that give various refractive 

 indices are shown in published tables. ^^" To make a fluid of index 

 1-44, 78 g of glycerol are mixed with 22 ml of distilled water; of 

 index 1-42, 65 g of glycerol with 35 ml of water. 



By the addition of cadmium chloride, the refractive index of 

 glycerol may be increased to 1-54 at saturation. ^^^ The salt im- 

 parts a pale yellow tint to this otherwise colourless medium. 



Farrants's medium is a good example of an adhesive, hydrophil 

 medium, much used in studies of lipids. It is not nearly so good as 

 diluted glycerine for the study of undyed methacrylate sections 

 by phase-contrast. 



Several diff'erent formulae have been published, all containing 

 the same three principal ingredients. The following-^ works well. 



To 40 g of gum arabic Cpreferably powdered), add 40 ml of distilled 

 water. When the gum has dissolved, add 20 g of glycerol. A piece of 

 camphor may be put in]^the medium to act as a disinfectant. 



The pH of this fluid is 4-1, whether camphor be added or not.^* 



True gums are sticky, water-soluble substances that exude 



through cracks in the bark of certain trees. Gum arabic (acacia 



gum) is derived from species of Acacia (Leguminosae), especially 



A. Senegal, a native of the Sudan and other parts of tropical 



K 



