LIGHT (OPTICAL) MICROSCOPY 



Retinol Imoge 



MecharNcol Tube 

 Length (160 mm) 



Projection Distance 

 (250 mm) 



Fig. 1. The path of light in a microscope. 



solving power. The smallest detail h which 

 can be resolved by an objective is given by 

 the formula h = \/2 N sin U = \/2NA. 



As the above formula indicates, there are 

 three ways in which to decrease the least re- 

 solvable separation h. The first is to decrease 

 X, the wavelength of light; the second is to 

 increase the angle U in the object space; and 

 the third is to increase the refractive index 

 N in the object space. 



The wavelength can be decreased to a 

 rather modest extent by using the blue-violet 

 region, the short wavelength end of the 

 white light spectrum. Still further shorten- 

 ing of the wavelength is possible by using 

 the ultraviolet region. This, however, re- 

 quires a completely different optical system 

 from the light microscope. 



Increasing the angle U in Fig. 2, increases 

 resolving power in proportion to the sine of 

 the angle. Highly complex objective lens 

 systems result in values of sin U as high as 

 about 0.95, and this value represents about 

 the maximum attainable limit for the N.A. 

 of a top-ciuality dry objective. Normally 

 however 0.65 N.A. represents a practical 

 limit for the great majority of microscopes 

 (see Fig. 3). 



To go still further in resolving power "im- 

 mersion objectives" are used, giving a value 

 of N greater than unity in the formula 



h = \/2N sin U 



Immersion objectives, using oil as an im- 

 mersion fluid, result in N.A.'s as high as 1.40, 

 although 1.25 is more common. Monobromo- 



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