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HANDBOOK OF PHOTOGRAPHY 



and carefully wiped. Several different swabs should be used in succession, especially 

 if any immersion fluid has dried on the lens mounts or surfaces. All of this must be 

 removed. The lens surfaces are inspected with magnifiers. Finally a small hand 

 syringe is used to blow loose dust particles and cotton linters from the lens surfaces. 



Microscopy of Metals. Development of Metallography. — Metallography is that 

 branch of science which deals with the anatomy of metals and teaches how the struc- 

 ture changes under the influence of mechanical and thermal treatments. It is the 

 means by which good, poor, or indifferent physical properties are explained on the 

 basis of structure. With control of structure comes control of physical properties. 



Two decades ago optical systems were available which theory indicated were 

 capable of resolving some 140,000 lines per inch, and others could be developed with 

 considerably greater resolving powers. Yet photomicrographs of metal structures 



Fig. 12. — Precision high-power metallographic apparatus. 



were at low magnifications, and the lens systems employed had low resolving powers. 

 Immersion objectives of high numerical aperture probably were seldom utilized, and, 

 if so, the resulting photomicrographs were of doubtful scientific or industrial value. 

 Such interest as they attracted was due more to, the large magnification employed 

 rather than to any real information disclosed by the photographs. High-magnifica- 

 tion images were not crisp or brilliant but were the exact opposite. Resolution 

 probably was of the order of a few thousand lines per inch. The best optical systems 

 were not used to their full potential resolving abilitj^, and except in rare cases it 

 appears likely that objectives of 1.40 N.A. were not employed in the microscopy of 

 metals. 



The preparation of metal specimens was crude and in the course tjf evolutionary 

 development. Better and more uniformly graded abrasives were needed to replace 

 the ones which had been adopted from the metal-polishing and buffing trades. Speci- 

 mens were left in a scratched and pitted condition, and, when subsequently etched, any 



