802 HANDBOOK OF PHOTOGRAPHY 



accuracy. For fine microscopic work an accurately threaded metal flange should be 

 employed as the lens mount. (The writer does not advocate the laboratory use of 

 ultra-small cameras because the purchase price and operational efi^ort, if expended on 

 standard-size equipment will yield better results. Obviouslj^ portability and even 

 low operational expense are not of prime importance in the laboratory.) 



Lenses should be of the highest possible quality; a rapid rectilinear, stopped down, 

 will not equal a good anastigmat, even in copying. In scientific laboratorj'' photog- 

 raphy the qualitj^ standards are properly set so high that only the best lenses available 

 should be considered. One other generalization is possible: one lens will not be 

 sufficient. A geologic photographic laboratory should possess objectives of the 

 Micro-Tessar or Micro-Summar class for photographing opaque relief objects of 

 almost microscopic size, and well-corrected anastigmats of 4- and 8-in. focal length for 

 routine megascopic work. Obviously the size and relief of the specimen, together 

 with the desired image size, will determine the proper lens and in many cases will 

 allow little leeway. Shutters cease to be important in most geologic laboratorj^ 

 photography. Usually shutters are unnecessary, although for shorter exposures than 

 1 sec. a between-the-lens shutter will be required. Focal-plane shutters are unneces- 

 sary and unwise because of vibration. A lens shade is a valuable accessory. 



Camera supports should be of generous proportions and good design. A solid-leg 

 wooden tripod with revolving and tilting head is the best general support commercially 

 available for megascopic work; its size should be adequate for the camera employed. 

 Provision against slippage of the tripod feet should be made, such as soft rubber tips 

 or a light chain loop placed around the feet. When makeshift supports are employed 

 occasionally for megascopic work, it is imperative that the assembly of camera and 

 support be solid and vibration be eliminated. Positive connection of parts by screws, 

 thumb nuts, or C clamps is necessary, and the insertion of a vibration trap, beneath 

 the camera, consisting of sponge rubber mat or four tennis balls, will improve long- 

 exposure negatives. 



Object supports are a great convenience in megascopic work and frequently are 

 necessities. Small- and medium-sized specimens are supported adequately by a piece 

 of plasticene modeling clay covered with wax paper to protect the back of the speci- 

 men; large rocks may be held for photographing by blocking up with wood or lead 

 blocks. Control of background is best gained bj^ employing a deep box of sufficient 

 size, lined with black flannel or velvet. The specimen should be supported in the 

 plane of the open top on glass or a stiff pillar from the box bottom.; for white back- 

 grounds a glass-topped box, covered with smooth tracing cloth (or ground glass) and 

 lighted evenly from inside, is used to support the specimen. 



Illumination is best provided by strong — 500 watt or more — prefocused-type 

 incandescent lamps, mounted in parabolic reflectors and held by metal floor stands 

 whose arms permit three-way adjustment. At least three units, consisting of lamps, 

 reflector, and floor stand, should be provided. Heavy ring stands may be substituted 

 for floor stands in table-top work. Photoflood bulbs are less satisfactory, although 

 cheaper, for permanent equipment, because of their short life and rapid change in 

 the quantity and spectrum of their emissions. 



The microscope and accessories employed in thin-section technique should be the 

 best quality petrographic microscope obtainable. Especial attention should be given 

 the objectives used for photographic work; they should be apochromatic if possible. 

 Accessories in keeping with the microscope need no further comment, except for the 

 camera itself. For any small amount of work a temporary setup of view camera may 

 prove satisfactory. A lighttight sleeve around the ocular tube, convenience in 

 focusing and changing plates, and absolute rigidity are essential. For any consider- 

 able amount of thin-section photograph}^ it is advisable to purchase a standard 

 photomicrographic camera. 



