40 THE ELEMENTS OF VEGETABLE HISTOLOGY 



The function of the condensing lenses is to con- 

 centrate the light, or bring the light rays to a focus 

 upon the object, thus increasing the illumination of 

 the latter. The Abbe condenser (named after its 

 originator) consists of two or more modified plano- 

 convex lenses (Fig. 21), and is attached to the micro- 

 scope so that the top lens projects through the stage. 

 The condenser mounting in modern instruments is 

 equipped with a focusing device which permits changes 



Fig. 21.- -Abbe Condenser (two lens). 



in the position of the focal point of the light rays 

 passing through the condenser. In fine work, and 

 with high powers, focusing of the condenser is essential. 

 The usual types of Abbe condensers are not corrected 

 for spherical and chromatic aberration. As the func- 

 tion of the condenser is merely concentration of light 

 and not the production of images, these non-corrected 

 lenses serve all ordinary purposes. In using immersion 

 lenses one should place a layer of immersion fluid 

 between the upper lens of the condenser and the lower 



