786 METHODS OF INVESTIGATING BACTERIA. 



It would of course be best if we could obtain a pure 

 colour picture, i.e., if the Canada balsam and the tissue 

 had an identical refractive index, and as a consequence 

 the tissue were not at all visible, and only the bacteria 

 could be seen by virtue of the stain that is, in them. 

 But usually the various portions of the tissue have a 

 different refractive index to the Canada balsam, and give 

 rise by diffraction of the transmitted rays of light to the 

 structure picture, which consists of lines and shadows, 

 and which obscures the colour picture. We must there- 

 fore try, as far as possible, to obliterate these diffraction 

 appearances and the structure picture, and this can be 

 done by the employment of a suitable illumination appa- 

 ratus. 



Abbe's sub- If we examine a microscopical preparation illuminated at 



stage con- fi rs t w ith a narrow, and then with a constantly but slowly 

 increasing cone of light, we find that the diffraction appear- 

 ances and the structure picture, which were most intense 

 with the narrow diaphragm, gradually disappear ; and to the 

 same degree as the structure picture diminishes, so the colour 

 picture becomes more intense and sharper. Hence where 

 possible a condenser must be employed with so large an angle 

 that the diffraction appearances are completely obliterated. 

 An instrument which attains this object completely is the 

 condenser devised by Abbe, and prepared by Zeiss. This 

 consists of a combination of lenses, the focal point of which 

 is only a few millimetres distant from the front lens. When 

 this lens is placed in the opening of the stage of the micro- 

 scope at a somewhat lower level than that of the stage the 

 focal point falls on the object under examination, and the 

 latter is consequently best illuminated in this position. The 

 angle of aperture of the rays is so great that the most external 

 of them are bent in a layer of water almost 16 towards the 

 axis, and hence the total active cone has an aperture of 120, 

 a broader aperture than that of any other condenser. The 

 rays of light are sent into the system of lenses by means of a 

 mirror which can only rotate around a fixed point in the axis 

 of the microscope. Between the mirror and the lens, and 

 near the focal point of the former, we have the arrangement 

 for diaphragms which can be moved laterally and circularly, 

 so that the cone of rays may be altered in any manner 

 desired. By means of a greater or less diaphragm the 

 aperture of the lens is modified from the smallest to the 

 largest. 



