274 Botanical Microtechnique 



in monochrome with such emulsions. Representative emulsions in this 

 category and Eastman's ortho, process ortho and Verichrome films, 

 D. C. Ortho plates, and Agfa Plenachrome film. 



Noncolor sensitive emulsions such as process plates have not been 

 given adequate attention for photographing such objects as black- 

 stained chromosomes. 



Historically, the best-known emulsion for photomicrography is 

 the Wratten M plate. This is a panchromatic plate having compara- 

 tively coarse grain and slow speed, producing negatives of high con- 

 trast. The more recent fine-grain panchromatic emulsions may well 

 bring about a radical revision of photomicrographic techniques. These 

 emulsions are fast, they have a wide range of color sensitivity, and, 

 because of the fine grain of the negative, enlargements of many diame- 

 ters can be made. This makes possible the use of rclati^ely low 

 microscope magnifications, with greater depth of focus and a large, 

 comparatively flat field. A negative will yield a contact print or lantern 

 slide of a large field, and selected portions of the negative may be 

 greatly enlarged to exhibit finer details of structure. Films in this 

 category are Eastman Panatomic X and Agfa Finopan. The speed 

 ratings of emulsions can be obtained from the manufacturers or from 

 the frequently revised tables of makers of exposure meters. 



The choice between plates and films depends on the size of film, 

 the microscope magnification being used, the type of negati^■e holder, 

 and the focusing method. Large sheet film has considerable concavity, 

 w^hereas a glass plate is flat over its entire area. With the lower mag- 

 nification ranges, up to lOOX. the lack of perfect flatness of the 

 enmlsion does not seriously influence focusing, but, if nujch of the 

 area of a large negative is to be utilized with high magnifications, the 

 use of plates may be necessary. Sheet film holders designed to hold 

 the film along all four sides arc superior to separate adapters that 

 fit into platcholders. Some of these adapters do not hold the emulsion 

 of the film in the same })l;me as when a plate is used in the same 

 holder, therefore the focusing screen or observation tube is not in 

 accurate register with the emulsion, resulting in inaccurate focusing. 

 The forcuoiuQ sources of error sliould he tested for the available 

 apparatus and accessories. 



Roll film is useful only if the conditions are so well standardized 

 that the length of exposure can be estimated accurately. The smaller 

 sizes lie sufficiently flat for moderate magnifications. Pack film has 

 some advantages over roll fihn. Jndi\idual films (an l)e removed from 

 the pack for development, making it possible to establish exposure 



