252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



of science seeks first an accurate description of the gross appearance 

 and behavior of its segment of Nature ; and this has customarily been 

 followed by the attempt to analyze the matter that is present into its 

 components, and to ascertain significant relations between these com- 

 ponents and the gross properties with which the original description 

 was so largely concerned. In chemistry, this analysis has been 

 achieved through the interpretation and correlation of elaborate series 

 of chemical reactions. For certain other sciences, the microscope has 

 been a much more direct analytical tool. Thus, the analytical stage 

 for biology may be said to have begun when compound microscopes 

 made accessible for study the individual cells that are the unit struc- 

 tures of living matter. 



Since then, there has been a steady improvement in techniques of 

 microscopy which, though providing very little extension in vision, 

 have yielded a steady flow of new knowledge about the cellular level 

 of organization. In our day, the electron microscope is of such ex- 

 ceptional importance because it does give such an extension, and one 

 that allows us not merely to see smaller detail but also to reach in 

 biology the deeper and more fundamental level of organization where 

 molecules are the units. For the past 25 years we have known that the 

 proteins and many other substances extracted from living matter have 

 very large molecular particles. A number of these particles have 

 already been seen under the electron microscope, and the fact of this 

 visibility takes us beyond chemistry's conventional concern with sta- 

 tistical molecular aggregates to observations on separate molecules 

 and their interactions. It is the principal object of this discourse to 

 illustrate some of the more immediate applications we have made 

 of this ability to see macromolecules. 



Before doing this, however, I must give some idea of the great ex- 

 tension in vision already achieved with this new microscope. The 

 limiting resolving power of any optical system defines its ability to 

 portray small objects: it depends on the wavelength of the illumina- 

 tion we use. Optical theory has demonstrated that no microscope can 

 possibly reveal the true shape of an object smaller than about half 

 this wavelength. Accordingly, visible light cannot delineate objects 

 smaller than about 0.2 micron (about a hundred-thousandth of an 

 inch) ; with ultraviolet light we can do little better. Electron micro- 

 scopes, though only about 15 years old as scientific instruments, have 

 already portrayed objects more than a hundred times smaller than 

 this in linear dimensions and hence more than a million times smaller 

 in volume and weight. Such tiny particles are only 5 to 10 atoms 

 across. The newly visible world having such relatively small mole- 

 cules at its lower limit is about as extensive, and as full of unknown 

 things, as that aspect of Nature which the optical microscope first 



