74 DARK-GROUND ILLUMINATION WITH HIGH POWERS [Ch. II 



This will bring the drop of homogeneous liquid on the lower side of 

 the slide on the top of the condenser and will thus bring it in homo- 

 geneous contact. Now run the homogeneous immersion objective 

 with aperture not above i.oo down into the drop of liquid on the 

 top of the cover. 



§ 128. Small arc lamp as radiant. — If the small arc lamp is used, 

 arrange its condenser (fig. 49) so that when the lamp is 25 to 50 cm. 

 away the beam of light will almost cover the mirror. Use the plane 

 mirror. Reflect the light up through the condenser. When the 

 mirror is properly arranged the specimen will be brilliantly illuminated. 

 Focus as usual with the fine adjustment. The objects should be of 

 almost dazzling brilliancy in a dark field. Sometimes a slight read- 

 justment of the mirror or of the centering will improve the appearance; 

 and sometimes a slight elevation or depression of the condenser helps. 



For continuous study it is less trying to the eyes if a piece of polished 

 daylight glass is placed over the top of the ocular or held in some way 

 in the path of the light on its way to the mirror. It renders the light 

 bluish, however. 



After using the dark-ground illuminator one will see the importance 

 of using clean, perfect slides and cover-glasses. If any dirt or scratches 

 are present they show with painful distinctness. 



§ 129. Mazda lamp as radiant. — If the small arc lamp is not 

 available, an incandescent, nitrogen-filled lamp of 100 or more watts 

 makes a fairly good substitute. The best arrangement is to have the 

 lamp in one of the lanterns for daylight glass (fig. 37-38). The 

 frosted daylight glass should be removed and either no glass at all or 

 polished daylight glass be used, as the ground-glass dims the light 

 too much. Have the lantern close to the microscope, and reflect 

 the light up through the condenser with the concave mirror as in 

 ordinary observation (§ 96). This light serves very well for the 

 homogeneous immersion objective and of course also for all the dry 

 objectives. 



§ 130. Water immersion dark-ground condenser. — For dry 

 objectives and also for much of the work with the homogeneous im- 

 mersion objective it suffices to use distilled water for connecting the 

 slide with the condenser. This admits of an angle of 61°+ striking 



