318 



COMPOUND AND ELECTRON MICROSCOPES 



TABLE VIII-1 



Achromatic Objectives — Huygenian Eyepieces 

 Magnifications and Real Fields 



Microscope tube length 160 mm. Image distance 250 mm. Real fields (italics) in mm. 



Since a 0.35-N.A. objective may not be commercially available (see 

 Table VIII-1), we choose the next larger, 0.50-N.A., having a magnifi- 

 cation number 21 X ; using this in connection with a 7.5 X eyepiece gives 

 a total magnification of 157 diameters. 



It is recommended that the lowest-power eyepiece be used that will 

 disclose the detail desired in order to assure getting the best definition 

 and illumination, and a comfortable high eyepoint. 



Objectives of Numerical Aperture Greater than 1.00 



The higher the numerical aperture, the greater the resolving power of 

 the objective. The definition 



N.A. = n sin a 



includes the index of refraction (ri) of the medium from which the light 

 passes into the objective. If this index is greater than unity, like that 

 of water (1.33), oil of cedar (1.52), or Canada balsam (1.53), an objec- 

 tive having a 1.25 N.A. immersed in oil of cedar (Fig. VIII-4) has an 

 angular aperture of 110.5°. An immersion objective with numerical 

 aperture greater than unity will therefore have an aperture which will 



