GEOLOGIC PHOTOGRAPHY 803 



Thin-section Technique. — Photography of rock and mineral material in thin section 

 may be done with either unpolarized incident or reflected light, or with polarized 

 light between crossed Nicol prisms or polaroid plates. When using thin sections with 

 unpolarized light, the photographic problem is to secure correct definition and ade- 

 quate contrast of grain boundaries and microscopic structure. Definition is obtained 

 by routine focusing of an adequately equipped microscope and in extreme cases by 

 employing monochromatic light and suitable filters; contrast is obtained largely by 

 altering the illumination intensity and angle of incidence until the desired results show 

 on the focusing panel. Many workers prefer a clear-glass focusing panel to ground 

 glass, because the critical focus point is sharper. The practice of cementing a thin 

 cover glass on a ground-glass plate to secure a clear spot is not recommended because 

 the focal plane is shifted thereby. 



Photomicrographs made through crossed Nicol prisms should be exposed on 

 panchromatic plates to gain correct color rendering. Here apochromatic objectives 

 and oculars are desirable for the best work since they bring nearly all wavelengths into 

 focus on a single plane. More light is required when working with crossed Nicols, 

 although the usual laboratory microscope lamp will generally prove strong enough. 

 Very dense, deeply colored sections may require a stronger source such as a laboratory 

 arc illuminator. 



Relief-object Technique. — When photographing geologic specimens, other than 

 thin sections, more or less surface relief is present and must be photographed sharply. 

 This involves mastery of two details: securing depth of focus, and control of lighting. 

 For megascopic negatives sufficient depth of focus is usually attained with a small 

 diaphragm opening. With small relief objects, whether photographed through a 

 compound microscope or through the Micro-Tessar or Micro-Summar lens in a view- 

 type camera, the securing of sufficient depth of focus is often the most serious problem. 

 The two approaches are through stopping down the objective and guarding against 

 vibration during the lengthened exposure and, secondly, employing lenses with as 

 short a focal length as possible. When the resulting negative is of the very highest 

 quality, it is possible to obtain some additional magnification by ordinary darkroom 

 enlargement without loss of definition, this method, however, is not a substitute for 

 lens quality and skill in making the original negative. 



Illumination of relief objects evolves into a trial-and-error technique employing 

 the few comparativeljr simple devices mentioned above, i.e., shadow contrast is 

 secured by low-angle illumination and detail in shadows by a somewhat weaker high- 

 angle light. Methodical experiments leading to a standardized technique for a given 

 laboratory is the best solution of the lighting problem. Among special devices which 

 will be useful at times are the time-tested ammonium chloride coating blown on fossils 

 when a matte white surface will show detail better; the ingenious use of water for 

 "painting" outlines of obscure details such as footprints and plant impressions; and 

 the cloth or tissue paper "tent" separating the light sources from the camera lens and 

 object when it is necessary to prevent point reflections from crystal faces or highly 

 polished surfaces into the camera. Polaroid screens are valuable for the latter pur- 

 pose also. 



Color Technique. — Natural-color photographs in the geologic laboratory are 

 valuable chiefly for recording details of minerals and petrographic thin sections 

 between crossed Nicol prisms. The indoor technique resembles that of field color 

 photography, except for the use of artificial light and accompanying filters, and follows 

 standard indoor color procedure. Except for lantern-slide illustrations for mineralogy 

 and petrography, color photography does not appear to be practical or necessary for 

 indoor geologic photography. 



