METHODS OF EXAMINATION 19 



be applied to a microscope fitted with a mechanical 

 stage. The methods used for different purposes 

 may be classified as follows : 



METHOD A. EXAMINATION IN A SOLUTION OF 

 CONSTANT STRENGTH 



It was not so easy as at first expected to keep 

 amoeba* or their cysts in a solution of constant 

 strength under conditions suitable for continuous 

 observation under the microscope, because the 

 layer of liquid plus the thickness of the cover-glass 

 must not be so great as to prevent the use of high- 

 power objectives. The cover-glass must not rest 

 on the preparations, as amoebae are thereby gradually 

 killed and cysts will not excyst under the pressure. 

 Again, the stagnation which occurs in the usual 

 forms of live cells on microscope slides is harmful 

 to amoebas, as they appear to require more oxygen 

 than they can then obtain. These difficulties were 

 obviated in two different ways, viz. : 



Method A, 1. A sheet of thin glass about three 

 inches long by two inches wide is placed on the micro- 

 scope stage, and over this is spread a piece of very 

 thin fine-meshed muslin (chiffon). On moistening 

 this it will spread out quite flat. Amoeba? or 

 amoeba-cysts are dropped on to the cloth, and can 

 be seen with the microscope lying between the 

 meshes on the surface of the glass plate (PL I., A). 

 One end of the muslin falls into a bottle containing 

 water or the solution to be tested, in the latter case 

 evaporation being prevented by covering the whole 

 apparatus over with a shallow dish. It was noticed 

 that, if a cover-glass be placed over the cysts, 



