MICROSCOPY 93 



are molded in a plastic material, using the master grating as a pattern 

 or model. The replica grating may transmit light or it may be silvered 

 for use as a reflection grating. 



Diffraction also occurs in other materials with regions of difference in 

 light transmission or in refractive index, but irregular shapes and spac- 

 ings are much harder to analyze. 



Polarization: Light ordinarily vibrates in all possible planes. Imagine 

 light descending vertically from the sun. It vibrates not only in the north- 

 south plane but also in the east-west plane and at every point of the 

 compass. In certain situations, as by reflection at a certain angle or by 

 passage through special materials, the vibration is restricted to a single 

 plane. Such light is polarized. The ability to polarize a beam, to permit 

 the passage of a beam of polarized light, or to rotate the plane of vibra- 

 tions all depend upon the nature of a material, either the molecular 

 structure or the arrangement of molecules in some larger unit. In ordi- 

 nary microscopy, polarization is of no consequence, but the special tech- 

 niques of polarization microscopy, described in a later section, can give 

 valuable clues to the structure of materials. 



Magnification and resolution 



The magnification of a microscope is stated by the manufacturer. An 

 objective with "43 X" printed on the side might be used with an eye- 

 piece labeled "10 X." The magnification by the objective lens refers to 

 the ratio of the size of the image produced to the size of the object at a 

 specified body tube length. If the body tube is made longer than the 

 customary 160 mm, the magnification by the objective is increased. The 

 magnification by the eyepiece is calculated, assuming the virtual image 

 at 10 in. or 250 mm from the eye point. The total magnification, using 

 160 mm tube length and virtual image at 250 mm, is the product of the 

 two stated figures, or 430 X in this example. 



Magnification is essential in order that images of individual spots 

 on the object will fall on the retina of the eye sufficiently far apart 

 from each other to excite nonadjacent cells. If the magnification is inade- 

 quate, two separate spots will strike the same or closely adjacent retinal 

 cells and will be interpreted as a single spot. In addition, the object to 

 be examined may occupy a very small portion of the field of view. If the 

 magnification is too great the object may appear much larger than the 

 field of view. The "zoom" microscope (Fig. 8-8) offers continuously 



