XI. WHEN TO USE SPECIAL MICROSCOPES 



345 



detail revealed by brightfield methods depends in part on the wave- 

 length of the radiation used. Visual microscopy is limited to light of 

 about 380 to 740 m^u because of the sensitivity of the human eye. 

 Longer and shorter wavelength radiation can be used with photo- 

 graphic and other recording mechanisms. 



The image formed by a lens of a small point of light is a central disc 

 surrounded by a series of dark and bright rings. A good lens concen- 

 trates most of the light into the central disc as shown by the plot of 

 the Airy disc image in Figure IB. As two such points in the object 

 approach and gradually merge, their images overlap and separate 



ABC 



Fig. 1. Light distributions in the object (A), the image 

 (B), and for the resolution of two objects (C). 



identity is lost when there is no dip, as in Figure IC, between the fus- 

 ing energy curves. The limit of resolution of the microscope is thus 

 determined in terms of distance. The image of the specimen is the 

 sum of the image discs from all the points of light from the specimen. 

 Enough energy must be present in each disc to stimulate the eye or 

 recording mechanism, otherwise it wall not be perceived. This is 

 important when the energy distribution is quite different in adjacent 

 Airy discs in the image. Unless the energies in both Airy discs are 

 adequate, methods for increasing resolution {16) should be used with 

 care to prevent incorrect observations. 



Resolution, or the minimum separation of two small objects that 

 still permits them to be observed as two, depends on the wavelength 

 (X) of the radiation used and the angle of the cone of light from the 

 specimen accepted by the front lens of the objective. The light-gath- 

 ering power of an objective is expressed as numerical aperture (N.A.) 

 which is the sine of the half-angle (JJ), of the above cone, multiplied 



