PRACTICAL DETAILS 125 



apparatus for dark-ground illumination consists of a special diaphragm 

 which is placed below the condenser. Slides and cover-glasses of a given 

 thickness, varying with every condenser, are essential. 



B. Special condensers. In these cases the ordinary Abbe condenser is 

 replaced by a prismatic condenser (Cotton and Mouton), a parabolic con- 

 denser (Zeiss) or a spherical condenser (Leitz) arranged in such a way that 

 the rays reflected by the mirror are deviated, so that they pass obliquely 

 through the film of liquid which is placed between the slide and cover-glass, 

 and cannot enter the objective. Under these conditions any particles held 

 in suspension in the preparation on the stage of the microscope are lighted 

 from the sides while the back-ground is obscure. 



In most patterns the dark-ground condenser is placed below the stage 



FIG. 114. Dark-ground illuminator for fixing on the stage. 



in the collar generally used for the Abbe condenser, but instruments are now 

 made to fix on the stage of the microscope. 



These latter are the better, and they can be used either with a dry lens or 

 with an immersion lens. 



3. Method of using the dark-ground illuminator. 



To use dark-ground illumination it is necessary to have : 



1. A powerful source of light ; 



2. A lens to form the image of this source on the mirror ; 



3. A firm microscope stage on which to fix the dark-ground illuminator, an 

 objective and an eyepiece. 



These are all arranged on a rigid table, and it is an advantage to have an 

 optical bench 1 metre long. 



A. The source of light. 



The specific intensity of the light increases the visibility of the objects 

 under the microscope. A Nernst lamp, an arc lamp or an inverted incan- 

 descent gas burner are the sources of light generally used. Electric light is 

 perhaps better, but an Auer burner (inverted incandescent) (p. 118) is quite 

 good enough for most purposes. 



Sometimes it is necessary to use sunlight, and particularly when photo- 

 graphing objects under the ultra-microscope. For this purpose the apparatus 

 is arranged in a dark chamber, and the rays of light falling on an heliostat 

 worked by clock-work pass into the chamber through an opening made in 

 the shutter of the window. 



Whatever the pattern of apparatus used, the rays of light must be 

 condensed by a lens on to the flat surface of the microscope mirror. 



Sometimes it is better to use instead of a lens a large round flask filled 



