CH. //] LIGHTING AND FOCUSING 59 



utilized. To accomplish this, Siedentopf and Beck employ a para- 

 bolic reflector instead of a condenser of the usual type. Others 

 used condensers specially modified. That of Reichert is conical and 

 silvered on the conical surface ; that of Leitz makes use of two 

 internal reflections. By all these pieces of apparatus a hollow cone 

 of light of an aperture greater than i.oo N. A. is concentrated upon 

 the field, hence high powers as well as low ones can be used pro- 

 vided a sufficiently brilliant source of light is employed (sunlight, 

 arc lamp, etc.). 



Ultramicroscopy . In 1903 Siedentopf and Zsigmondy published 

 a method by which a further evolution of dark-ground illumination 

 was attained according to the general principle just considered. By 

 their method the field is illuminated by a very brilliant cone or 

 wedge of light from the side, z. e., at right angles to the axis of the 

 microscope. It is evident that none of the rays can 'enter the micro- 

 scope with even the widest apertured objectives unless the light is de- 

 flected by something in the field. The brilliant light so used renders 

 minute particles luminous something as sunlight entering a small 

 hole in a darkened room renders particles of dust luminous. As 

 this method of lighting rendered particles luminous and therefore 

 visible that were invisible with the microscope as ordinarily used, 

 the use of the microscope with this lighting has come to be called 

 Ultra m icroscopy . 



(2) The second method was used by Toppler, 1867, and has 

 been revived by Gordon, (J. R. M. S. 1906) and others. In this 

 method the object is lighted by a solid cone of light from the con- 

 denser as usual, but the aperture of the condenser must only fill the 

 middle part of the aperture of the objective. In the first method 

 the aperture of the condenser must be great and that of the objective 

 moderate, while in this the reverse is the case, and the objective 

 should have a large aperture and the condenser a moderate aperture. 

 The solid cone of light used for illumination has some of its rays 

 deflected by objects in the field so that they enter the marginal 

 zones of the objective. To secure dark-ground illumination in this 

 manner only these marginal rays are utilized for the image, and the 

 central, solid cone of light entering the objective must be eliminated. 

 This is accomplished by placing a diaphragm or stop on the back 

 lens of the objective of just the right size to cut out the central solid 

 cone and allow the marginal rays to pass on to form the image. 



