When the proper point of correction is reached the 

 expansion of outline is the same in both directions. 



Immersion Objectives. As has been stated 

 before, immersion contact between the objective and 

 cover glass is made with either water or homogeneous 

 fluid. With the former distilled water only should be 

 used and kept in a suitable bottle. .Cedar oil is used 

 for the homogeneous fluid but is specially prepared, the 

 commercial cedar oil being too thin and volatile and 

 not of the proper refractive index. A small bottle is 

 generally supplied with each oil immersion objective. 

 Great care should be used in keeping it free from dust, 

 as it often happens that an objective fails to give satis- 

 faction because of a small particle of dust which may 

 float in the fluid in front of the hemisphere. Great 

 care should also be exercised in applying oil to the 

 front lens and after the application it is strongly 

 recommended to examine it with a magnifier, that 

 there may be no air bubbles present. Air bubbles as 

 well as dust seriously interfere with obtaining satis- 

 factory results. If bubbles are present the oil should 

 be removed and a fresh quantity applied. An air 

 bubble will entirely destroy the clear definition of a lens 

 even when not directly in front of the lens itself. Care 

 should also be taken not to apply too great a quantity. 

 After the stopper has been withdrawn from the oil, 

 allow the oil to run down the rod or brush until the 



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