microscopy to form the image are: (1) absorption, (2) refractive index 

 and thickness, (3) mass differences, (4) polarization, and (5) fluores- 

 cence. The ordinary light microscope is used almost exclusively for the 

 examination of fixed and stained materials where perception of the image 

 is based upon the differential absorption of light by the different stained 

 parts of the specimen. As a result, the light making up the image shows 

 variations in intensity or color to give contrast between the different 

 parts of the image. When cell structures show selective absorption of 

 light of specific wavelengths such as the ultraviolet, they are examined 

 with the ultraviolet microscope. Living cells, which are highly trans- 

 parent and absorb virtually no light, must be examined with the phase- 

 contrast microscope, which converts phase changes induced by differ- 

 ences in refractive index and thickness of different parts of the cell into 

 light intensity changes. Differences in the mass of substances containing 

 atoms of carbon, oxygen, and nitrogen in different parts of the cell will 

 cause scattering of an electron beam and are responsible for image 

 formation in the electron microscope. Because some cell structures are 

 anisotropic they will alter the natural path of polarized light passing 

 through them and can be examined with the polarization microscope. 

 The presence in cells of substances which fluoresce or emit visible light 

 when illuminated with shorter wavelength radiations makes it possible 

 to examine cells with the fluorescence microscope. 



Optical and Electron Microscopy 



In the electron microscope and the light microscope, the arrange- 

 ment and function of the different parts are essentially the same. To 

 emphasize this similarity in basic design, the two microscopes will be 

 described together in this section. 



The essential parts of the conventional light microscope are: (1) 

 a light source, (2) a condensing lens system to collect and focus light 

 on the specimen, (3) an objective lens to form and magnify the image 

 of the specimen, and (4) an eyepiece lens to enlarge further the image 

 formed by the objective and to project this image to the retina of the 

 eye or a photographic film (Figures 11-8 and 11-9). 



The electron microscope is unique in that it is the only instrument 

 currently available which permits the examination of structures with 

 dimensions as small as 1 m// (10 A). This is quite remarkable when 

 one considers that the intramolecular spacing of atoms is of the order of 

 1 to 5 A. The high resolving power of the electron microscope is due 

 simply to the fact that electrons of exceedingly short wavelengths are 

 used to illuminate the specimen. The wavelength of an electron beam is 



SURVEY OF CYTOLOGICAL TECHNIQUES / 225 



