100 ELECTROMAGNETIC RADIATIONS AND MATTER 



the early 1950's, it has been used for quantitative studies of proteins in living 

 muscle, growth rates of cells and parts of cells, and similar problems on 

 living tissue which can be studied only with a nondestructive tool. 



Electron Microscope 



This development of the last twenty years has added a new dimension to 

 the depth to which tissues can be viewed. After fixing and staining (e.g., 

 permanganate, phosphotungstic acid, osmium oxide), a very thin cut to be 

 examined is placed in high vacuum, and bombarded from below by electrons 

 (from a hot filament) which have been accelerated through a small aperture. 

 Some of the electrons hit dense parts of the object and are scattered and 

 absorbed — the principle is the same as for X rays (Figure 4-10 (a)); others 

 pass on through less dense parts and fall upon a fluorescent screen or 

 photographic plate. Proper alignment permits, in today's machines, ampli- 

 fications of 500 to 100,000 x , with resolution of a few angstroms. 



One instrument, which can be considered typical for biological work,** 

 gives a 15- A resolving power; 600 to 120,000x magnification; and accelera- 

 tion voltages of 100, 75, or 50 kv, to give electron beams of equivalent wave- 

 lengths of 0.037, 0.043, and 0.054 A. The "lenses" are electric voltages be- 

 tween charged plates. The amplification can be increased to over 1 ,000,000 x 

 by photographing the screen, and enlarging the photograph. 



Others 



The ultraviolet microscope and fluorescence microscope have been used 

 and improved since the early 1900's. They have some specialized uses in 

 biological research. X-ray microscopy is useful when the sections to be 

 studied are opaque to visible and ultraviolet light. For example, in histo- 

 logical sections on bone, soft (~5 kvp) X rays are absorbed by the mineral 

 component, passed by the organic component. 



Reflection microscopy, especially the slowly developing infrared reflection 

 techniques, may find limited use in future studies on biological material. 



PROBLEMS 



4-1 : Draw the shapes of sigma and pi bonds. 



4-2: If all 10 28 atoms in a human being were lined up side by side, how long would 



be the line, in miles? 

 4-3: It costs an input of about 105 kcal/mole to pull the first hydrogen off a water 



molecule. "Light" of what wavelength will blast it off? (calculate it). 

 4-4: Sketch intensity vs distance for the penetration of electromagnetic radiation 



into tissue, presuming concentration of absorbent of 0. 1 moles/1 and molecular 



extinction coefficients of 0.1, 1.0, and 10.0. 



**The limitations should be realized: the tissue sample is dead, dry, and thin while being 

 viewed in the electron microscope. 



