CHAPTER XII 



OPAQUE OBJECTS METALLOGRAPHY 



THE methods of photomicrography as applied to opaque 

 objects may be conveniently considered under two heads, 

 those used when only low magnifications are required, 

 and those employed for high powers. The former find 

 considerable application for natural history objects and 

 the like, and the latter in metallography. 



Natural History and other Objects Requiring Low 

 Powers, No microscope is required in many cases, and the 

 methods given in Chapter VII apply ; but w r hen somewhat 

 higher magnifications are desirable, and the microscope is 

 employed, more intense illumination is necessary. The 

 method adopted to take the photographs reproduced on 

 Plate 26 was to place the radiant in front of the micro- 

 scope stage, and to direct a converging beam on to the 

 object by means of a bull's-eye condenser. The beam 

 should impinge on the object as nearly vertically as 

 possible, so as to obtain even illumination, and to obviate 

 as much as possible the production of excessive shadows. 

 But when working with objectives of \" or J" focal 

 length, the beam of light will of necessity have to be 

 oblique, otherwise a shadow of the objective will be cast 

 on the object itself. This method will be found un- 

 satisfactory for objectives higher than |". 



By placing a light and condenser on each side of the 

 object, more even illumination may be obtained in some 

 cases and shadows may be eliminated to some extent, 

 though the presence of shadows, if not too intense, is 



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