OPTICAL THEORY OF LIGHT MICROSCOPE 





U-6" 54' 



UM4-29 



Fig. 2. The dependence of resolving power upon numerical aperture. The same object is here photo- 

 graphed with two different objectives. The picture at left was obtained with a 0.12 N.A. objective. The 

 much sharper and better resolved picture at right was taken with a 0.25 N.A. objective. 



naphthalene immersion objectives with 

 N.A.'s of 1.60 have been made, but they are 

 very uncommon, and are difficult to use. 



Depth of Focus 



At any given focus setting of a microscope 

 only a limited thickness of the specimen ap- 

 pears in sharp focus. This thickness is called 

 the "depth of focus" and, like resolving 

 power is largely dependent on the objective 

 design. The depth of focus is (approximately) 

 inversely proportional to the scjuare of the 

 N.A. of the optical system, and is extremely 

 small for high N.A. systems, e.g., a 1.40 

 N.A. objective has a depth of focus of about 

 3^ wavelength of light (0.00025 mm). 



Aberrations 



A first-class microscope is remarkably free 

 from aberrations, or image defects, at least 

 in the central portions of the field. However, 

 some understanding of the seven different 

 basic lens aberrations is essential to the 



Fig. 3. Objective types. The objective at 

 the right is a 4 mm, 0.65 N.A. Achromat hav- 

 ing 5 glass elements, and no fluorite. The central 

 objective is a 4 mm, 0.85 N.A. semi-apochromat, 

 having 5 glass lenses and 1 fluorite lens. The ob- 

 jective at the left is a 4 mm, 0.95 N.A. apochromat, 

 having 5 glass elements and 2 fluorite lenses. 



proper selection and intelligent use of a 

 microscope and its accessories; hence a brief 

 discussion of the aberrations as they apply 

 to a microscope is given here. 



(1) Spherical aberration exists where light 

 from a single object point on the axis is more 

 strongly refracted by either the inner or the 

 outer portion of the lens aperture, resulting 

 in failure of the light to come to a common 



447 



