98 MICROSCOPY 



same as taking a very thin slice out of the center of the cell. By succes- 

 sive examinations of various optical sections within the cell, a fairly 

 complete three-dimensional picture can be built up. 



Aberrations 



High quality optical components for microscopes must incorporate 

 corrections for certain "aberrations" if the image is to represent the object. 

 Otherwise several aberrations inherent in optical systems can be ex- 

 tremely annoying. 



Spherical Aherration: Lens elements with spherical curvature do not 

 focus at a point because the edges produce relatively greater refraction 

 than the center. An object cannot be brought to focus at a point but 

 rather is spread over some distance along the optical axis. This spherical 

 aberration could be corrected by using lenses with nonspherical curvature, 

 but microsope components are so small that the sphere is about the only 

 curvature that can be made practically. More commonly, spherical aber- 

 ration is corrected by combining lens elements in such a manner that one 

 lens compensates for the spherical aberration of another. 



Chromatic Aherration: Because refraction depends on the wavelength 

 of the light, a single lens does not focus all colors at the same point. The 

 object seems to be surrounded by halos of different colors. This color 

 discrepancy is called chromatic aberration. It can be corrected by using 

 combinations of lens elements of different refractive index. Achromatic 

 objectives composed of different kinds of glass achieve adequate correc- 

 tion for ordinary purposes. High-resolution microscopy demands more 

 correction, however, and apochromatic objectives made up of several ele- 

 ments of glass and the mineral fluorite are used. Achromatic and apo- 

 chromatic objectives correct for spherical as well as chromatic aberra- 

 tions. 



Corrections for the other aberrations (astigmatism, coma, curvature of 

 field) are of little concern to the microscopist since the manufacturers 

 have nearly eliminated these problems. Microscopes corrected for spheri- 

 cal and chromatic aberration are magnificent instruments, but they must 

 be used properly. For example, correction for spherical aberration was 

 designed for use with a cover glass 0.18 mm thick, and any other thick- 

 ness introduces spherical aberrations. 



