LIGHT (OPTICAL) MICROSCOPY 



focus. The visible effect is a loss of contrast, jectionable in photomicrography where eye- 

 Well made objectives are normally quite well accommodation and re-focusing in scanning 

 corrected for spherical aberration. High the field are not possible. Special fiat-field 

 N.A. dry objectives, however, are quite sen- objectives are sometimes used here, and also 

 sitive to the effect of cover-glass thickness on special negative lens systems can sometimes 

 spherical aberration. Most objectives are de- by used to replace the eyepiece, to improve 

 signed to work with 0.18 mm cover-glass the performance of regular objectives, 

 thickness, and even small departures from (5) Distortion is the lens aberration in 

 this nominal value will result in loss of con- which straight lines are imaged as curved 

 trast in the image in high N.A. dry objec- lines. This aberration is generally under good 

 tives. The experienced microscopist uses control in a microscope, 

 cover-glasses very close to 0.18 mm for opti- (6) Chromatic aberration is the defect 

 mum image quality. Immersion objectives which causes light of different wavelengths 

 are much less sensitive to cover-glass thick- to be brought to different foci. Chromatic 

 ness, since immersion oil is quite close to the aberration is normally under good control 

 glass cover slip in refractive index and forms in a microscope, particularly in the highly 

 essentially a homogeneous optical medium complex "apochromatic" objectives (see 

 between specimen and objective. "Objective Types")- 



(2) Astigmatism is the defect whereby the (7) Lateral color results in light of one 

 image of a point is drawn out into two sepa- color being imaged at a greater magnification 

 rate line images at 90° to each other. In a than light of another color, causing the image 

 well made microscope, astigmatism will of an off-axis point to be spread out into a 

 sometimes be present to a minor extent near tiny spectrum. This defect is greatest in the 

 the margin of the field, but not at the center, higher-power objectives. It can be compen- 



(3) Coma is the lens defect in which dif- sated by proper choice of eyepiece, and 

 ferent circular concentric zones of the lens again the manufacturer's recommendations 

 system have different magnifications. It re- should be followed. 



suits in a comet-shaped image of a point 



object. Like astigmatism, in a well made Objective lypes 



microscope it will sometimes be present to a Three types of objectives are normally 



minor extent near the margin of the field, but made available by microscope manufactur- 



not at the center. ers. These are called "achromats", "semi- 



(4) Curvature of field is the defect in apochromats", and "apochromats", named 

 which a flat object is imaged as a curved in order of increasing excellence and com- 

 surface. This defect is the most difficult one plexity (see Fig. 3). The achromats are the 

 to deal with in a microscope design. The simplest and least expensive. For most pur- 

 high-power objectives in particular tend to poses achromats do an adequate job, and 

 have strongly curved fields, so that when consequently they are popularly used on 

 the central image is in sharp focus the mar- most medical and laboratory microscopes, 

 ginal image is out of focus, and vice versa. The achromat is corrected for chromatic 

 A certain amount of compensation for the aberration at two wavelengths, one in the 

 objective curvature of field is possible by red and one in the blue, and is fully corrected 

 optimum choice of eyepiece, and in this re- for spherical aberration at one wavelength 

 gard the microscopist is well-advised to fol- in the yellow-green. At other wavelengths 

 low the manufacturer's recommendations on in the visible spectrum the correction is good, 

 objective and eyepiece combinations. but not complete. 



Normally, curvature of field is more ob- By combining fluorite lenses with glass 



448 



