looking through the eyepiece. When a Nernst lamp or incandescent 

 gas light is used it is necessary to employ a bull's eye lens on 

 a stand, the distance between the source of light and the lens 

 being about 17 cm, that from the lens to the microscope mirror 

 about 40 cm. The pencil of light formed by the lens should 

 after reflection at the mirror give an image of the source of light 

 on the lower face of the dark-ground illuminator. The pencil of 

 light does not fill the entire mirror but rather its upper portion, it 

 being essential that the rays should be parallel to the axis of the 

 microscope and enter through the centre of the illuminator. Before 

 focussing the objective upon the object view the latter direct. A 

 bright spot will be seen upon the object, which, by raising or 

 lowering the illuminator, should be made as small as possible and, 

 by adjusting the position of the mirror, directed into the middle of 

 the upper surface of the illuminator, when its maximum brightness 

 will be attained. The object should now be focussed and viewed 

 with a very low eyepiece, say No. 0. It will generally be found 

 that the brightest point is not situated in the centre of the field. 

 By centreing the sleeve, the point P should be brought into the 

 middle of the field. The low power eyepiece may now be replaced 

 by one of higher power, the compensating eyepiece No. 18 being 

 particularly suitable for this purpose, and, if necessary, the correct- 

 ion collar of the objective should be adjusted and the dark-ground 

 illuminator raised or lowered with the aid of the condenser move- 

 ment, until the bacteria are as bright and the field as dark as 

 possible. 



Observation with dark-ground illumination renders it imperative 

 that the object-slide and cover-glass should be cleaned with the 

 utmost care, otherwise the presence of particles of dust may inter- 

 fere with the observation. The preparation itself should be very 

 thin, i. e. present as little substance as possible, to prevent 

 the particles lying outside the plane of observation from giving rise 

 to disturbing reflections. For similar reasons it is essential to ensure 

 the absence of air bubbles. 



The lower movable iris diaphragm which forms part of the 

 Abbe illuminating apparatus should be thrown out of action during 

 observation with the dark ground illuminator. 



In addition to the Reflecting Condenser just described a simple 

 form of it is made consisting of a plate in which the condenser 



