108 THE USE OF THE iMICROSCOFE 



can be used with advantage. The covers should be 0.17 

 miUimeter thick, if possible. 



6. This class of objects includes those fitted for examina- 

 tion with the high-power, oil-immersion objectives with 

 apertures from 1.2 to 1.4, used with a corrected immersion 

 condenser at an aperture of over 1.0. These objects should 

 preferably be mounted in xylol balsam of refractive index 

 approximating 1.52, or, better, in immersion cedar oil. 

 They may also be examined without a cover in the thickened 

 cedar oil used for objectives, the tube length being slightly 

 increased (the amount of increase being shown by the star 

 test). Objects in water, or in media of quite different 

 refrangibilities to balsam (such as ethyl alcohol, or hyrax), 

 may also be correctly viewed, if the layer of liquid is not 

 more than 10 microns thick, and if the tube is properly 

 lengthened or shortened for less or more refraction, and 

 less or more thickness of the layer. These objects may 

 require the highest resolution of the microscope (though less 

 may sometimes serve). The maximum resolution and 

 definition, however, are only to be reached uniformly by 

 one who has attained skill in the scientific use of the 

 microscope. 



Summary. — Objects in watery media are best viewed 

 with a water-immersion objective; unless they are less than 

 about 10 microns from the cover-glass, when oil-immersion 

 objectives may also be used. The examination of objects 

 dry or in water should not be altogether replaced by balsam 

 mounting. The most important mounting media (from 

 the optical point of view) are probably air, water, immersion 

 oil, and monobromide of naphthalin, for the four classes 

 of objectives, respectively. 



Objects may be classified into: those suited for the 

 binocular magnifier with enlargements up to 10 times; those 

 suited for the ordinary Greenough, with magnification to 

 50; those suited to the 16-millimeter objective of the 

 standard microscope, with power up to 100 times or more; 

 those suited to the high di'y objective (not higher than about 

 4 millimeters), with magnification to 400 or more; small 

 objects in water suited to the high-power, water-immersion 



