THE STRUCTURAL FEATURES OF CELLULOSE 87 



where N.A. is the numerical aperture, i.e. the sine of half the angle sub- 

 tended by the objective in the object plane multiplied by the refractive 

 index. When it becomes imperative to observe much finer detail, then 

 either other methods must be used (X-ray analysis being an out- 

 standing example) or radiation of smaller wavelength must be used 

 (this is, of course, the basis of the use of X-rays, but then no image can 

 be seen since X-rays cannot be focused by lenses). This is one of the 

 reasons why so many laboratories throughout the world have been 

 interested in the construction of the ultraviolet microscope, using light 

 of wavelength of the order of 2500 instead of 4000 A. Electron micro- 

 scopes go very much further. We are now familiar with the idea that 

 electrons, though originally formulated as material particles, also have 

 the properties of radiation, and that the wavelength of the radiation 

 concerned is very small indeed, of the order of 0-05 A., dependent on 

 the energy of the electron. The relation between wavelength and the 

 accelerating voltage of an electron was, in fact, shown by de Broglie to 



^® X=himv=\2-21V, 



where V is the accelerating voltage, so that it may be calculated that for 

 an electron accelerated between two plates whose voltages differ by 

 60,000 volts (whose energy therefore is 60,000 electron volts) the 

 corresponding wavelength would be 0-05 A. The resolution for such a 

 wavelength is therefore extremely high even for very imperfect lenses 

 of very low numerical aperture; thus for an aperture of 0-001 the 

 resolving power would be 300 A. and for 0-01, 30 A. The discovery that 

 cylindrically symmetrical magnetic or electric fields can be used as 

 "lenses" for electrons was therefore of the utmost possible significance. 

 The general construction of an electron microscope is very similar to 

 that of the light microscope and we need to notice only a few major 

 differences. Firstly, since the object has to be inserted in the (very high) 

 vacuum system of the microscope then it must be dried very thoroughly. 

 Secondly, since the object has to be transparent to electrons it must be 

 very thin, thicknesses of more than 0-2^ seldom being permissible. 

 Extreme thinness in the object is necessary also to avoid the excessive 

 overheating which would result from electron absorption. Thirdly, 

 the image cannot be seen directly. The image is therefore thrown upon a 

 fluorescent screen for viewing and is subsequently photographed, both 

 the screen and the plate being, of course, within the vacuum system. 

 Lastly, the magnification obtainable in this microscope (50,000 to 

 100,000) takes us down to an order of size hitherto unexplored, in which 

 the detail depends on opacity to electrons and not to visible radiation. 



