PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 781 



fine details which existed in the structure were not developed and were oftentimes 

 obliterated. 



The development of high-power metallography led to the utilization of the full 

 potential resolving ability of existing optical systems. In order to make use of this 

 resolving power, it was necessary to learn how to prepare metal specimens to better 

 advantage for microscopic examination. 



The trend in the design of metallographic apparatus has been entirely toward the 

 inverted type of microscope. Many faults of the early inverted microscope have been 

 overcome. These instruments are more convenient to use than any other type, and 

 they are more compact so that the illuminating train, microscope, and camera can be 

 rigidly mounted as a unit and insulated by spring suspensions, or otherwise, from 

 outside disturbances. A great deal of care and attention has been devoted by the 

 manufacturers to the optical and mechanical design of the apparatus. It has been 

 made largely foolproof, so that the average worker of limited experience can achieve 

 remarkable results. Twenty years ago magnifications of 100 X to 500 X were the rule 

 with an occasional photograph at 1000 X, but common practice today has increased 

 magnifications to 3000 X with good definition and a high order of resolution. 



The optical systems of higher numerical aperture are coming into common use, 

 and with increase in numerical aperture of objectives more skill and experience are 

 required on the part of the user. The monobromnaphthalene objective of 1.60 

 N.A. is now available for metallographic work but because of certain inherent char- 

 acteristics of this lens it is used only with considerable difficulty. 



Preparation of Specimens. — The preparation of specimens for metallographic 

 examination is a laborious and somewhat tedious job but can be systematized so as to 

 take from the task much of the drudgery. The vast majority of specimens are alloys, 

 which usually have more than one phase present. Most specimens contain solid 

 nonmetallic inclusions. Some specimens are cold-worked in whole or in part, and 

 some contain fractures, cracks, or discontinuities which it is desired to study. Some 

 specimens are hard and some are so soft that any attempt to polish a surface results 

 in a myriad of scratches which are difficult to remove. When etched deeply enough 

 to get below the scratches, these soft specimens generally have their fine details of 

 structure and often the coarser ones obliterated by the very operation which was 

 intended to disclose them. When more than one constituent is present in a specimen, 

 differences in solubility of the constituents are disclosed by etching "and, if care and 

 understanding are not exercised, misleading conclusions are apt to occur. The use of 

 oblique illumination, dark field, or polarized light cannot compensate for faulty 

 workmanship in the preparation of a metallographic specimen. The detail must be 

 developed in the specimen, and then the order of resolving power and magnification 

 in the optical system must be sufficient to reveal the detail clearly. Since photog- 

 raphy in the final analysis is rendering in black and white the contrasts and gradations 

 in tone and color value which exist in the object, it follows that, if one is to achieve crisp, 

 brilliant images photographically, the subject must have extremely well-developed 

 detail, otherwise we could not photograph it at all at very high magnifications. 



Soft metals such as lead cable-sheath alloys, precious metals, aluminum, etc., 

 can best be prepared by cutting a specimen with a very rugged microtome. A very 

 satisfactory one is the Jung wood-sectioning microtome. The feed of this instrument 

 can be set for any thickness of section from 2 to 50 n. The specimen is first roughly 

 cut to a flat surface by setting the feed to cut off slices about 10m thick. This thickness 

 is graduallj^ reduced to the minimum cut of 2 m. The sections or slices are not used 

 in the subsequent metallographic examination. After the block or specimen has 

 been removed from the microtome, the cut surface is etched without any further 

 treatment. 



