CHAPTER XIII 

 THE WATER-IMMERSION OBJECTIVE 



Advantages of High-power Water -immersion Objec- 

 tives. — The high-power water-immersion objective may 

 well be used whenever the objects are in water, or in an 

 aqueous medium of approximately the same refractive 

 index as water; such as, blood serum, normal salt solution, 

 plant sap, dilute sugar solution, sea water, diluted acetic 

 acid, or watery jellies. Under these circumstances, except 

 with objects in optical contact with the cover-glass, or 

 within a few microns of it, a good water-immersion objec- 

 tive, properly adjusted, gives markedly harder, less glassy, 

 and more realistic images than those given by an equally 

 good oil-immersion objective; the difference increasing from 

 zero with increase in the depth below the coverglass of the 

 watery liquid focused through. Of course, for objects in 

 balsam or immersion oil, or for objects in optical contact 

 with the cover-glass, the oil-immersion objective is unri- 

 valed, if the tube length is correct. The oil-immersion 

 objective, however, sometimes gives slightly impaired 

 figures because of wrong cover-glass thickness. This may 

 be compensated for by altering the tube length (which can 

 only be done on the binocular by pulling out the eye- 

 pieces partially). In such cases the water-immersion 

 objective, with its correction collar, may be superior to the 

 oil-immersion objective. However, a water-immersion 

 objective is troublesome to use on balsam-mounted objects, 

 when the depth of the object below the cover is variable; 

 because the correction collar must be altered continually. 

 But for balsam preparations spread in a plane, especially 

 if on the cover-glass (such as bacterial and blood smears), 

 the water-immersion objective may give good images. 

 Thus Prof. W. J. Schmidt (113) says: 



131 



