STAINING, PRACTICAL AND THEORETICAL 



ink, but also as a negative stain for spirochaetes, etc., on account 

 of the absence of granules, which occur in Indian ink. It should 

 be noted, however, that for the marking and labelling of laboratory 

 apparatus and specimens, it is not waterproof, and, therefore 

 unsuitable. For such purposes waterproof Indian ink is more satis- 

 factory. Carbochrome ink contains approximately 1 1 % of carbon, 

 the average particle size of which is | to i^, although ther emay be 

 a few aggregates present up to but not exceeding, lO//. 



CEDARWOOD OIL 

 For microscopic immersion lenses 



" Cedarwood oil for microscope immersion lenses " is not 

 the same as " cedarwood oil for clearing." 



Occasionally one sees " cedarwood oil for microscopy " stipu- 

 lated ; such phraseology is vague and confusing as it could imply 

 either type of cedarwood oil, since both are used for microscopy. 

 When specifying cedarwood oil, therefore, it should be made 

 quite clear which type is required by stipulating either " for 

 immersion lenses " or " for clearing " as neither type can be 

 used for the same purpose as the other. 



Cedarwood oil for microscope immersion lenses is prepared by 

 dissolving gums or resins in cedarwood oil " for clearing " to 

 bring it to the required viscosity and refractive index. There 

 are a number of different brands of cedarwood oils for immersion 

 on the market under various proprietary names, which appear to 

 differ slightly from each other in refractive indices, viscosity and 

 depth of colour, usually pale yellowish or pale amber. Although 

 immersion oils with a refractive index of i -520 is recommended 

 by some microscope manufactures, oils possessing a refractive 

 index of i "515 are used by a large number of microscopists. 



In my own laboratories, Optoil, which is a synthetic immersion 

 oil (see page 531) is normally used, and only occasionally cedar- 

 wood oil for immersion lenses. 



A good make of cedarwood immersion oil, such as Cedronol, is 

 excellent for mounting Gram-stained preparations, which often 

 fade when mounted in xylol balsam and most synthetic mountant 

 (but see LPM mountant, page 523 for this purpose). 



526 



