XII. ELECTRON MICROSCOPY 407 



find, the treatment he has given the organism, while satisfactory for 

 the hght microscope, has introduced many fine scale artifacts that can 

 be detected in the electron microscope, he may then develop new 

 techniques such as that of growing the organisms directly on the 

 plastic mpmlirane or of removing the organisms intact from the surface 

 of a culture. Having developed the techniques so that he is satisfied 

 with the authenticity^ of the resulting images, he returns to his 

 original research problem. 



6. Analysis of Electron Microscope Results 



Research results obtained from electron micrographs take two 

 forms. The most obvious and direct of these are measurements of 

 size and shape of the particles and structures recorded in the image. 

 Of greater importance are the qualitative examinations of the micro- 

 graphs that provide information regarding the results of an experi- 

 ment in the form of visual impressions. 



Direct Measurements. Direct measurements that can be made 

 with the electron microscope are limited to those of size and shape, 

 in other words, to the simple measurements of lengths and angles. 

 Very often size is an important criterion in an experiment and in 

 that case the electron microscope is invaluable. The actual measure- 

 ment, of course, is made with some type of linear scale on the electron 

 micrograph, the magnification of that micrograph being used to 

 convert the measurements into the actual dimensions of the original 

 object. It is, of course, necessary to know the magnification. Us- 

 ually the electron microscope has been calibrated at a previous time 

 by the research worker or by the electron microscopist so that the 

 calibration is available. Furthermore, even if the accuracy of the 

 calibration is unknown, the magnification can be depended upon 

 to remain constant over a relatively long period. Hence, relative 

 measurements can be made with high accuracy. Absolute measure- 

 ments, when necessary, can be made to within 2%. The methods 

 for obtaining accurate calibrations are described in the literature (2). 

 When it is necessary to make measurements to an accuracy that 

 approaches the resolving power demonstrated in the electron micro- 

 graph, it is desirable that the worker have some understanding of 

 those image phenomena that may influence his measurements. ^Vhile 

 it is not possible to describe these in detail hei-e, it is desirable to call 

 attention to the fact that the image contour phenomena that exist 



