MICROSCOPE OBJECTIVE CORRECTIONS 



321 



(fl) (b) (c) 



Fig. VIII-6. (a) 10 X (16 mm) Achromat, 0.25 N.A. 



(b) 43 X (4 mm) Achromat, 0.65 N.A. 



(c) 97 X (1.8 mm) Achromat, 1.25 N.A. 



Note precision of construction and relative position of lenses. (Courtesy Bausch 

 and Lomb Optical Company, Rochester, New York.) 



Curvature of field depends upon the focal length of the objective, its 

 numerical aperture, and its spherical correction. The long-focal-length 

 objectives, such as the 48-mm and 32-mm objectives, have relatively- 

 flat image fields, because of their simple construction and small numeri- 

 cal aperture. The 16-mm objective (Fig. VIII-6a) produces a slight 

 curvature of the image field, noticeable only when the object under 

 examination is very thin. The high-power objectives have still greater 

 curvature of field because of their complex lens system. As seen in 

 Figs. VIII-6 and 7, they are formed of several extremely short-focus 

 lenses. 



Chromatism in a microscope objective means that the image of an 

 axial point source of white light has colored rings around it, due to the 

 fact that the focal length of a lens varies with wavelength. This 

 variation of the focal length because of dispersion of a lens is called 

 chromatic aberration. Its existence can be shown by arranging a lens 

 as in Fig. VIII-8, where light from a small white source S falls on a lens 

 of tolerably wide aperture and is received on a white paper screen, first 

 in position A, then in position B. In position A the outer edge of the 

 image will be red; in position B, blue. 



Because of this defect it is impossible to obtain colorless images with 

 simple spherical lenses. Since various kinds of glass have different 

 dispersive powers, it is possible with the proper choice of combinations of 

 lens shapes and indices of refraction to obtain an achromatic lens train 

 whose focal length is the same for all colors. 



