98 ELECTROMAGNETIC RADIATIONS AND MATTER 



For white light, with an average wavelength about 5000 A and a numerical 

 aperture of unity, the resolving power is 10,000 A, or 10" 4 cm, or 1 n. One 

 can use monochromatic blue light to improve this somewhat; and the re- 

 search use of ultraviolet (A = 2537 A from a mercury arc, for example) with 

 fluorescent screens, is an attempt to push the resolution down to 0.1 ;u. In 

 common practice, however, "good" microscopes used in schools and routine 

 examination have a resolving power 5 to 20 /j.. 



The binocular microscope uses two microscopes in parallel, one for each 

 eye. From this double input, one obtains depth perception. 



Phase-contrast and interference features have been superimposed on the 

 simple microscope, broadening its versatility by improving the contrast be- 

 tween different parts of the object under study. Contrast occurs in the 

 normal microscope because of differences in density. In phase and interfer- 

 ence microscopes, used when the density is about the same throughout (soft 

 tissue is ~90 per cent water), advantage is taken of the facts that the speed 

 of light through materials, which determines their refractive index, and the 

 amount to which the plane of polarized light can be rotated, often differ if 

 the molecular composition of the materials is different, even though their 

 density is the same. To take advantage of these facts, two methods are 

 available. Both present a highly contrasted image to the eye, one in inten- 

 sity, one in color. 



The principles are really quite straightforward. The reader is referred to 

 the trade literature for operating detail. Both are extensions of the normal 

 bright-field transmission microscope; only the extensions will be noted here. 

 In the phase microscope, an annular diaphragm is inserted in front of the con- 

 denser lens and therefore before the light falls on the specimen, together 

 with a phase plate composed of a thinly evaporated ring of dielectric on a 

 background of thinly evaporated metal. Thus light passes at different speeds 

 through different parts of the object to be viewed, and the emerging light 

 waves are out of phase. At one point of emergence from the object the phase 

 difference will be such that the waves cancel each other; at another they 

 reinforce each other. The phase plate "fixes" these differences by retard- 

 ing those which pass through the dielectric, and absorbing some of those 

 which pass through the metal. Thus identification and analysis of the struc- 

 ture of (unstained) living cells and tissues, the components of which are so 

 similar in density that discrimination is impossible with the light microscope 

 without killing and staining, is made possible. This instrument, invented by 

 Zernicki in the Netherlands in 1932, is now an indispensible tool in clinical 

 analyses — in bacteriological, histological, and, in particular, pathological 

 studies of tumors and cancerous tissues. Note the contrast in Figure 4-11. 



The interference microscope is a polarizing microscope, adapted so that part 

 of the light passes through the object and part around it, the two then being 



