OBJECTIVES AND OCULARS 19 



A water immersion objective for high powers will be found 

 almost invaluable, since it may be lowered directly into drops 

 of aqueous non-corrosive solutions for the study of suspended 

 matter or material at the bottom of the drop. 



Another convenience consists of a short glass tube just large 

 enough to allow an objective to slip inside. One end of the tube 

 is carefully ground at right angles to the axis and a cover-glass 

 is firmly cemented upon this end. The tube should be short 

 enough to allow the lower lens of the objective to almost touch 

 the cover-glass when the tube is slipped over the objective. 

 When such a tube is placed over an objective it may be forced 

 down into shallow layers of liquids and a study made of material 

 lying at the bottom or masses of matter suspended in a liquid 

 may easily be examined. Since the objective is not itself im- 

 mersed in the liquid it behaves optically exactly as well as when 

 used in the ordinary manner and since it is amply protected, 

 there is no danger of injury even in corrosive liquids. 



