DARK GROUND ILLUMINATION 179 



An oil-immersion condenser or an Abbe oiled to the 

 slip should always be used when experiments are to be 

 made with oblique light, in order that rays of sufficient 

 obliquity may reach the objective. 



Dark Ground Illumination. Opaque objects are easily 

 mounted and illuminated in such a way that they are 

 exhibited on a dark ground, but the method described 

 in this chapter is that employed to display transparent 

 preparations in a similar manner. By using a hollow cone 

 of light, whose apex is in the plane of the object, the 

 particles of the preparation are made to appear as if self- 

 luminous in a black field. To ensure a completely black 

 field, and sufficient contrast between it and the object, 

 no light must be allowed to pass directly from the con- 

 denser to the objective, the light forming the image being 

 diffused from the object itself. The effects produced are 

 in many cases very beautiful, but. until recently the 

 scientific interest attached to this method of illumination 

 was small. Since the introduction of new dark ground 

 illuminators, suitable for use with high-power objectives, 

 its usefulness has greatly increased, particularly in 

 the domain of bacteriology. Organisms and flagella, 

 ordinarily almost invisible, are rendered distinctly on 

 a black field, and can be detected and photographed in 

 a living state, whereas, heretofore, difficult methods of 

 preparation and staining were essential. 



Dark Ground Illumination by Central Stops. -The spot 

 lens and Wenham's paraboloid were formerly employed 

 to produce dark ground illumination, and from them the 

 new illuminators have been evolved. The earlier forms 

 did not prove satisfactory on account of the chromatic 

 aberrations they introduced, and because the use of any 

 one was limited to low-power objectives of a N.A. to 

 which the amount of central light it cut off was suited. 

 However, good pictures can be obtained when they are 

 employed, though a much better method, for use with 



