_Effect of the Sub-stage Condenser._--The sub-stage condenser gives the most powerful illumination when it has been racked up until it almost touches the specimen. It produces a cone of rays of very short focus, and the apex of the cone should correspond with the particular bacterium or group of bacterias under observation. The effect of the condenser without a diaphragm is to obliterate what Koch has termed a _structure picture_. If the component parts of a tissue section were colourless and of the same refractive power as the medium in which the section is mounted, nothing would be visible under the microscope. As, however, the cells and their nuclei and the tissues do not 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 facilitated. This is exactly what is required in dealing with bacteria which has 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 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 the 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--(Crookshank). The wide-angle condenser, it will be understood, consists of a combination of lenses, which concentrate all the light entering them to a small point, and the condenser must be so accurately focussed that this brilliant cone of light, when it emerges from the upper lens of the condenser, falls upon the object from all directions, forming a wide-angle cone of light, at the apex of which the object must be placed (see Fig. 149). That is to say, the object is illuminated by a cone of rays passing through it in all directions. [Illustration: Fig. 149.--Front Lens of Condenser.]