I] THE USE OF THE MICROSCOPE 7 



not be used. With -|th, or higher dry, or water immersion 

 objectives the definition is much better with a cover-glass the 

 thickness for which the lenses are corrected ; this is sometimes 

 marked on the objective. Some objectives are supplied with a 

 correction collar, so that they can be adjusted to cover-glasses of 

 different thicknesses. With homogeneous immersion lenses, the 

 thickness of the cover-slip within ordinary limits makes no 

 difference to the definition. 



Before using a high objective, see not only that the cover-slip 

 is not abnormally thick, but also that the object and the layer of 

 balsam is thin, otherwise in focussing the deeper parts of the 

 object, the front lens of the objective may be forced down on the 

 slip, and put out of position. 



9. Oil-immersion lenses. Place on the cover-slip and in the 

 centre of the field, a small drop of the oil prepared for the lens. 

 Lower the tube of the microscope slowly until the lens just 

 touches the oil, observing this from the side. Then focus with 

 the fine adjustment. After use, dab the surface of the lens with 

 clean tissue paper to remove most of the oil, touch the surface 

 with a brush dipped in xylol or benzol, and remove this gently 

 with dust-free silk, repeat the cleaning with xylol or benzol and 

 put away the lens. 



10. Measure the thickness of half-a-dozen cover-slips with 

 the 'cover-slip measurer. 7 



