102 BULLETIN 184, UNITED STATES NATIONAL MUSEUM 



use is troublesome. For meteoric irons about 500 diameters is the 

 practical limit, because the specimens are usually larger than the 

 samples of artificial iron, and it is difficult to get the surface abso- 

 lutely level on the microscope stage so that all parts of the field are 

 in focus. Photography, besides other advantages, is effective at 

 much higher magnification than is practicable in direct examination 

 with a microscope. 



An eye-piece micrometer should be provided for the measurement 

 of octahedral bands and other features. By checking its divisions 

 with a stage micrometer, or even a metric rule, measurements can 

 be made easily. 



Some form of ordmary illuminator should be available for exami- 

 nation at low power with oblique light. 



Getting the surface of the specimen level on the stage is sometimes 

 troublesome. For small specimens a convenient way is to place it, 

 polished side down, on a level surface within a metal ring (a short 

 section of a tube) of suitable height and diameter; place upon the 

 specimen a lump of plasticine higher than the ring; and then press 

 down upon the latter a glass microscope slide until it rests upon 

 the ring. The polished surface is then parallel with the slide, to 

 which it remains attached by the plasticine. 



Photography. — While the microscope is the primary instrument for 

 metallographic study, and is used incessantly in practice, photog- 

 raphy is so important that it may be called indispensable for any 

 extended research. Photographs ofi'er several advantages; they make 

 practical the use of high magnification; they are permanent and 

 permit of exact and repeated comparisons; and— not the least im- 

 portant — their use involves no eyestrain. Photography also may 

 be used with specimens too large to be examined on a microscope 

 stage. A large or irregular specimen can easily be photographed 

 because it lies on the stage of the camera with its polished surface down, 

 all parts of it thus being in focus. 



Photomicrography can hardly be carried on without special equip- 

 ment, such as is used in metallographic laboratories, and this is not 

 always available to those who wish to study meteoric irons. In 

 such cases the microscope must be used. But where photography 

 is available, the chief usefulness of the microscope is in making pre- 

 liminary observations to ascertain the general structure of the sample 

 and to determine what should be photographed. In practice nearly 

 all such microscopic work is done at around 80 to 100 magnification. 



Photomicrography, with the equipment designed for such work, 

 involves much skill and a painstaking technique in exposure, devel- 

 oping, and printing. The subject is treated fully in textbooks on 



