l6 ELEMENTARY CHEMICAL MICROSCOPY 



nification employed we cannot bring about a separation of the 

 overlapping rings of the diffraction patterns. The result, there- 

 fore, must be at the best a vague, blurred, uninterpretable 

 image or merely a diffraction pattern. 



If, therefore, our wave theory of light is correct, the most 

 minute particle which we may hope to render distinctly visible 

 by our compound microscopes by transmitted light must have 

 dimensions of at least 0.2 /*. It should not be inferred, however, 

 that the existence of particles many times smaller cannot be 

 indicated, for an invisible particle may yield a large diffraction 

 pattern, a phenomenon which makes ultra-microscopic investi- 

 gations possible; but we must bear in mind that in the case of 

 ultra-microscopic particles we have no picture or image of their 

 shape or structure and that we know of their existence simply 

 through the light diffracted by them and thus have passed far 

 beyond the range of the resolving power of our lenses. Although 

 it is true that the limit of resolving power, 0.2 /u, has been seri- 

 ously questioned by men of recognized authority, it may be 

 accepted as beyond dispute that a moderately skillful micros- 

 copist cannot hope in practical work to carry the resolving 

 power of his instrument beyond this limit. 



In ordinary work a magnification of from 750 to 900 diameters 

 is the upper limit of true usefulness in the study of details of 

 structure. Above this point the worker must be an exceptionally 

 keen and skillful observer in order that he may properly interpret 

 the appearances seen in the images formed. 



It is best, therefore, to make it a rule to work with low mag- 

 nifications. 



