ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



769 



other appliances designed for the same purpose may be regarded as 

 objectives which form an image of the source of light in the plane of 

 the particles, but solely through the instrumentality of rays prevented 

 from passing directly into the objective. The observer, therefore, sees 

 diffused light only at those points which are occupied by particles 



Fig. 168. 



differing optically from the surrounding medium, whereas the rest of 

 the field remains dark. Those rays which go to form an image of 

 the source of light occupy within the condenser the space bounded by 

 two cones having their apices and axes in common. The aperture of 

 the inner cone is slightly greater than the angle subtended by the object 



Fig. 1G9. 



under observation, so as to make sure that no direct rays enter the 

 objective. In order to secure an increased illumination when the 

 magnitude of the particles is diminished, three conditions are necessary : 

 (1) the difference between the apertures of the inner and outer pencils 

 should be as great as possible ; (2) the image of the source formed at 

 the apex of the pencil should be as well defined as possible ; (3) the 



