XII. ELECTRON MICROSCOPY 391 



In general, however, there has been no device available to convert 

 that data into a form intelligible to the human senses. Instead, the 

 research worker has had to content himself with making small num- 

 bers of measurements on systems that he considers to contain iai'ge 

 numbers of only a few dif!"erent types of identical particles. Such 

 methods have been ineffectual as soon as those particles become organ- 

 ized in complex heterogeneous structures. It is in this range that 

 the electron microscope provides much of the required information in 

 the form of a visual image. 



We can also use the television analogy to illustrate the funda- 

 mental research problem of electron microscopy. On the television 

 screen we see images representing scenes with which we have some 

 familiarit3\ Within certain limitations they are presented quite 

 faithfully. If this is not so, the observer's familiarity with everyday 

 scenes enables him to realize the change immediately and at least 

 mentally compensate for the deficiencies of the presentation. With 

 the electron microscope, however, the situation is not the same. The 

 images presented are of things initially totally unfamiliar to the 

 research worker. Moreover, he has no experience or information 

 by which he may judge their accuracy. To obtain that experience 

 he must then carry on by varying his technique of preparation, 

 performing other experiments, observing the results, and accumulat- 

 ing the necessary experience as he progresses. In fact, most of the 

 preparatory work involved in applying the electron microscope to a 

 particular problem is concerned with the development of a method 

 of preparing the specimen in such a way that the images produced 

 can be considered reliable. Once the research worker has convinced 

 himself that his images are true representations of the specimen, he 

 is ready to use his instrument as a means of visually observing the 

 results of experiments that concern his original research, and electron 

 microscopy per se plays no further role in his work. 



Two of the ideas implicit in the above discussion, which might be 

 emphasized further, concern the precise definition of the nature 

 of the information obtained with the electron microscope. It should 

 be realized that visual information obtained from a monochromatic 

 image involves only the geometry of an object. As mentioned above, 

 the intensity differences in an electron microscope image are the result 

 of differences in scattering power of the individual points of the speci- 

 men, so that there is some correlation between the intensity of an 

 image point and the physical properties of the corresponding speci- 



