ELECTRON MICROSCOPE IN BIOLOGY — WYCKOFF 253 



brought to light. It will take a long time to discern all that is here, 

 and still longer fully to understand what we are seeing. In the 

 meantime, we have all the excitement of seeing what no man expected 

 to see, and of using much of the new knowledge thus gained to ap- 

 proach with clearer understanding many of biology's essential 

 problems. 



An indication of the relationship of the electron microscope to the 

 familiar optical microscope may help in interpreting the images it 

 produces. Seventy-five years ago, in 1879, Sir William Crookes dem- 

 onstrated in the Royal Institution various forms of his new cathode- 

 ray tube. The electron microscope is such a tube in greatly elaborated 

 form. Like the Crookes tube, it has a source of electrons and a fluores- 

 cent screen to show their pattern ; a magnetic field is commonly used 

 to change their direction. In the electron microscope there are sev- 

 eral of these fields, and they are so arranged that the electrons follow 

 paths through the tube that imitate the path of light through the glass 

 lenses of an optical microscope. 



From a consideration of what an electron microscope is, it will be 

 realized that there is no compelling technical reason why we should 

 not have had some form of this instrument many years ago. Probably 

 it was not built because its advantages become obvious only when our 

 realization of the wave qualities of the electron started people thinking 

 about it as an illuminant. Here is, in fact, a rather striking example 

 of how our scientific imaginations may be bound by the ideas current 

 at any time. 



In science, as in most other fields of human endeavor, we rarely get 

 everything we would like, and accordingly, it should not be a surprise 

 that there are important limitations to what can be looked at under an 

 electron microscope. For the most part these arise from the relative 

 impenetrability of matter by electrons ; therefore, the object must be 

 extremely thin and, after thorough drying, it must be examined under 

 the vacuum of the tube. This is not an overwhelming disadvantage, 

 but it makes necessary new methods of specimen preparation that 

 will preserve the structures we are interested in, down to their finest 

 molecular details. 



To delineate very small objects we must look at their very highly 

 magnified images ; this involves magnifications far greater than those 

 useful in optical microscopy. The detail seen in the best optical micro- 

 scopes becomes increasingly "fuzzy," indistinct, and meaningless when 

 viewed at magnifications above about 1,500 X ; but we can now have 

 electron micrographs in which detail is sharp at 150,000 X. Occa- 

 sional electron micrographs are worth studying at twice this magni- 

 fication. These magnifications are so vastly in excess of any in our 

 previous experience that most of us find it difficult to have a vivid 

 realization of how very small are the objects we are seeing. 



