502 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Leitz' Dark-ground Illuminator for the Examination of Living 

 Bacteria.* — This dark-ground illuminator (fig. 180) is mainly intended 

 for examining living and unstained bacteria under the Microscope. 

 The method involved depends upon the contrast produced between the 

 intensely illuminated bacteria and their dark surroundings. Two reflect- 

 ing surfaces, one internal, the other external (see figure) are so shaped 

 as to almost completely unite the rays in a point P, so that by the dimi- 

 nution of the astigmatism to its lowest limits an intense illumination of 

 the bacteria is obtained. Since the apertures of the extreme rays «P 

 and b P lie within the limits 1 ' 1 and 1 - 45, it follows that a considerable 

 amount of light is collected at P. When dry lenses are used all the rays 

 which enter from below and converge towards P go to illuminate the 

 bacteria (shown by lines and dots), and are totally reflected at the surface 

 of the cover-glass. The light diffused by the bacteria (represented by 

 dotted lines) enters the objective, and thus produces an image of the 

 bacteria, which under these act as self-luminous bodies. As the rays are 



Fig. 130. 



united at P by reflection instead of by refraction, there is no chromatic 

 dispersion, and the annular illumination of the bacteria obviates diffrac- 

 tion. The optical portion of the dark-ground illuminator is contained 

 in a mount provided with a centring arrangement, and slips from below 

 into the sleeve which usually carries the Abbe condenser. Since the 

 point P should lie within the preparation, it is necessary to use slides of 

 uniform thickness, the proper thickness being 1*0 mm. The requisite 

 correction is effected by raising or lowering the dark -ground illuminator 

 by means of the movement forming part of the illuminating apparatus. 

 It should in this connection be noted that the space below the object- 

 slide Q should always be filled with oil. A Nernst lamp or incandescent 

 gas lamp may be used, but the best source of light is a small arc-lamp. 

 The Wetzlar firm have devised a special model, similar to that used for 

 the Edinger apparatus, requiring a current of four amperes, and capable 

 of attachment to any existing house supply. Immersion lenses may be 



* Special Circular, English version, E. Leitz, London. 



