CHAPTER XXVII 

 PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 



By Francis F. Lucas 



Principles of Photomicrography. Introduction. — Technical microscopy and photo- 

 micrography are widely employed in science and industry. The microscope produces 

 an image which the eye can see; photomicrography is the art of recording the image 

 photographicalljr so that a permanent record is available for reference and study. At 

 its best the microscope will reveal on the photographic plate details of structure which 

 the eye fails to observe. 



Metallography, or that particular branch of it which relates to the microstructure 

 of metals, has grown in the life span of those now living to be one of the most important 

 single control methods of the metal industry. In the paint, pigment, and color indus- 

 tries, improved methods of producing fine pigments have taxed the powers of visible- 

 light microscopes, and to photograph clearly very small pigments, the ultraviolet 

 microscope is the last resort. In the medical and biological sciences the microscope 

 fills an important place. With recent developments in ultraviolet microscopy the 

 resolving powers available for biological investigations have been more than doubled. 

 Since selective absorption to ultraviolet light is manifested by many cells, it is now 

 possible to photograph living cells at extremely high magnifications. By means of 

 the ultraviolet microscope photographs may be taken on optical planes spaced 34^ 

 apart, making it possible to photograph a single living cell, though microscopic in 

 size, on many different planes from the top to the bottom of the cell. The same 

 technique may also be applied to many problems of industrial microscopy. 



Improvements in resolving powers have made it possible to photograph details of 

 structure which measure but 200 atom diameters across. Vision has been pushed 

 downward into the range of colloidal matter so that by ultraviolet microscopy and 

 refined methods of ultramicroscopy the behavior of colloidal dispersions may be 

 studied to better advantage. 



The aim in photomicrography should be a faithful reproduction of the image. The 

 image should be in exact focus. Out-of-focus effects maj^ result in misleading con- 

 clusions. No one should ever attempt to interpret details in terms of structure or 

 composition when the particular details are not in exact focus. Halos, shadows, 

 flares, and other abnormal effects of lighting, photography, or microscopy have no 

 place in scientific or industrial photomicrography. "When photographing a prepara- 

 tion which consists of particles of matter differing in size or when the detail is not all 

 confined to a single focal plane, it is obvious that some of the detail will be above or 

 below the focal plane and therefore out of focus. Such conditions are encountered 

 frequently. The photographic results are of value only as they relate to the details 

 which are in focus in the photomicrograph. 



Principles, Technique, and Optics of Photoniicrogrnphy. — This section' treats of 

 principles, technique, optics, light, and materials. It omits, intentionally, descrip- 

 tive matter of apparatus, information about \\-hi('h is liberally distributed by 

 manufacturers. 



The design trend of scientific apparatus is being changed from time to time. This 

 has resulted in the gradual evolution of better apparatus which is more stable, is made 



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