80 BACTERIOLOGY. 



the medium in which the section is mounted, nothing would be 

 visible under the microscope. As, however, the cells and their nuclei, 

 and the tissue fibres do differ in this respect, the rays which pass 

 through them are diffracted, and an image of lines and shadows is 

 developed. If in such a tissue there were minute coloured objects, 

 and if it were possible to mount the tissue in a medium of exactly 

 the same refractive power, the tissue being then invisible, the 

 detection of the coloured objects would be much more easy. This 

 is exactly what is required in dealing with bacteria which have been 

 stained with aniline dyes, and the desired result can be obtained 

 by the use of the sub-stage condenser. 



If we use the full aperture of the condenser the greatly converged 

 rays play on the component parts of the tissue, light enters from 



FIG. 24. RAMSDEX MICROMETER EYE-PIECE. 



all sides, the shadows disappear, and the structure picture is lost. 

 If now a diaphragm is inserted, so that we are practically only 

 dealing with parallel rays, the structure picture reappears. As the 

 diaphragm is gradually increased in size the structure picture 

 gradually becomes less and less distinct, while the colour picture, 

 the image of the stained bacteria, becomes more and more intense. 

 When, therefore, bacteria in the living condition and unstained tissues 

 are examined a diaphragm must be used, and when attention is 

 to be concentrated upon the stained bacteria in a section or in a 

 cover-glass preparation, the diaphragm must be removed and the 

 field flooded with light. 



Micrometer. For the measurement of bacteria a stage micro- 

 meter may be used with a camera lucida. The stage micrometer 

 consists of a slip of thin glass ruled with a scale consisting of tenths 

 and hundredths of a millimetre. The image of this can be projected 



