344 OSCAR W. RICHARDS 



A. PROPERTIES OF MATERIALS ESSENTIAL 

 FOR MICROSCOPY 



By means of the microscope, the identification of a material or an 

 impurity can be accomphshed without damage to the material itself 

 and by the use of an extremely minute sample. Since the nature and 

 structure of materials can be discovered so advantageously with it, 

 the microscope is considered a necessary experimental tool in almost 

 every laboratory. Actually, microscopy comprises many kinds of 

 instruments and techniques. Since some of these require consider- 

 able equipment, skill, and training they will be discussed at greater 

 length, but first some general principles will be established. 



The simple microscope, or magnifying glass, may be any spherical 

 lens thicker at the center than at the edge. Magnifiers are most use- 

 ful when the desired detail may be seen with 15 diameters (15 X) 

 magnification or less. For greater magnification the compound mi- 

 croscope is used. Two lens systems are used in the compound mi- 

 croscope, one of which (ocular or eyepiece) magnifies the image pro- 

 duced by the other (objective) and the total magnification is the 

 product of the two magnifications referred to a plane 10 inches from 

 the eyepoint. While the image might be magnified further by more 

 and more lens systems, actually some light is lost and the image de- 

 teriorated due to residual defects at each lens system. Hence, no 

 practical gain is derived from a combination of more than two lens 

 systems. 



1. Image Formation — Possibilities and Limitations 



The information obtained from the use of a microscope must 

 come from the examination of the enlarged image formed by it of 

 the specimen. Therefore it is advantageous to give some considera- 

 tion to image formation and to the limitations of microscopy in gen- 

 eral before evaluating the different instruments and techniques. 

 The limitations of the microscope derive from : the available probing 

 radiation, the nature of the materials used in its construction, the dif- 

 ficulties in preparing specimens for examination, the sensitivity of 

 the observer's eye or other recording procedure, and the difficulty 

 of interpreting a greatly enlarged image of a specimen. 



Electromagnetic radiation is used ordinarily for microscopy, but 

 electrons behave similarly and may be used {cf. Chapter XII) and 

 even a proton microscope has been described {15). The fineness of 



