BACTERIA 241 



micrographs of bacteria, etc., and every effort must be 

 made to obtain a sharp, distinct image. A good objective 

 and proper adjustment of the illumination are essential 

 to success. Freedom from diffraction effects is ensured 

 by the use of the largest available aperture ; oil-immersion 

 objectives of T V" and T V are almost always employed, 

 and it is advisable to use an immersion condenser, 

 although an achromatic condenser of N.A. 1-0 usually 

 gives perfectly good results if its back lens is filled with 

 light. A wide condenser aperture also has the effect of 

 eliminating the structure of the more faintly-stained 

 tissues of sections in which bacteria occur, and so en- 

 hancing the contrast of the well-stained bacteria. 



To get proper contrast the preparation must be a good 

 one and sharply stained, but now that several good series 

 of contrast screens are available the colour of the stain 

 used is not of such great consequence as it was before 

 their introduction. A screen with a transmission band 

 complementary to that of the stain works wonders in 

 improving contrast, particularly if its colour is suited to 

 the corrections of the objective, but it is difficult to get 

 even passable results from faded preparations. A table 

 is given on page 267 indicating the most suitable screens 

 for use with various stains. Panchromatic plates are 

 generally to be preferred, and are essential with some 

 screens to obtain the best results and allow of reasonably 

 short exposures, but in the majority of cases ortho- 

 chromatic plates are satisfactory. Slow or medium 

 plates, particularly process plates, should be used 

 when possible in preference to the very fast varie- 

 ties. 



The tricolour process is much more suitable than the 

 screen-plate processes for the colour photography of 

 bacteria, on account of the clearness of the white back- 

 ground and freedom from granularity, but a two-colour 

 process, or a single dyeing of the positive on bichromated 



