Photomicrography 215 



time with one or more trial exposures, developing the films ai once. 

 Subsequent exposures under similar conditions can then be made 

 in rapid succession. In the larger film-pack sizes the film has con- 

 siderable curvature along the edges, but the central portions are 

 adequately flat. 



Processing of negatives will not be discussed. The worker who 

 is not proficient in the processing of negatives, or who does not have 

 available the services of an expert, would be rash to undertake 

 photomicrography. 



The Setting Up and Operation of the Apparatus 



Before outlining the procedure used in taking photomicrographs, 

 some suggestions are offered concerning the choice of objectives and 

 oculars for any given subject. The ultimate aim of the photographer 

 is a finished print on paper, or a lantern-slide (transparency) image 

 on a screen. The image should convey to the observer the intent of 

 the photographer: a low-power survey of a large area, with little 

 emphasis on cell detail; a rendering of texture and tone in black and 

 white, without much cell detail; an accurate reproduction of details 

 within a cell or within a minute object; or the sharp outlining of 

 an object against a contrasting background, without detail within 

 the object. The worker may have other aims and may combine them, 

 with emphasis placed where needed. 



When using the standard oculars that are used for visual work, 

 the best results are obtained with oculars of moderate magnification, 

 8 to 12X- For use with optical-bench outfits, special photographic 

 oculars, the Homals of Zeiss or the Amphiplans of Bausch and Lomb 

 may be used. These oculars produce a flat field on even the largest 

 negative. 



The objective to use is one that covers the desired area of the 

 object generously, especially when using visual oculars, so that the 

 important area will be in focus simultaneously and the out-of-focus 

 marginal region can be masked out in the finished product. In addi- 

 tion to adequate coverage, the objective should have adequate re- 

 solving power to show the necessary detail. Keep in mind that, as the 

 magnification and resolving power increase, depth of focus decreases. 

 It may be advantageous to obtain a sharp negative covering the 

 necessary area and depth of the object — but having relatively low 

 magnification — and to enlarge a few diameters in making the positive. 

 However, the positive must show the detail that the photographer 

 intended to show. Some workers prefer to keep the negative image 



