PHOTOGRAPHIC INTRODUCTION 3 



nant, and after several years' experience we have no 

 hesitation in expressing great satisfaction with its 

 performance. 



Owing to the special colours of the stains em- 

 ployed in bacteriological specimens the use of 

 "colour screens" is absolutely necessary, in order 

 that sufficient contrast between the organism and 

 the background may be obtained, and the resulting 

 photograph be sharp, clean, and crisp. It is in the 

 selection of this "colour screen" that the individu- 

 ality of the photographer becomes apparent, but 

 even after much experience it is not always possible 

 to choose, without trial, the screen which will give 

 the best result. In the selection of these light 

 filters much stress has been laid by some authorities 

 on the advisability of using such screens as give 

 a pure monochromatic light. In the photography of 

 bacteria this appears to be both practically and 

 theoretically wrong. Where the object is to get the 

 best possible photographic resolution of such objects 

 as diatoms, with their so-called secondary markings, 

 whose lines may be resolved into dots many 

 thousands to the inch, then both theory and prac- 

 tice demand that light of the shortest wave length 

 possible be employed. But where contrast alone is 

 ^wanted, as in the photography of bacteria which have 



