60 THE MICROSCOPE 



about 10 ohms. A current of 2\ amperes is needed, 

 and is conducted to the coils in the stage through the 

 rheostat. With the help of the ammeter any desired 

 temperature can be obtained and maintained, up to 

 about 200 C. If immersion oil contact is made 

 between the top lens of the condenser and the lower 

 surface of the bulb, this stage works very well indeed 

 with the J^-inch oil immersion lens. 



FIG. 53. Lorrain Smith's warm stage. 



Dark Ground or Paraboloid Condenser. This is an 

 immersion substage condenser of high aperture by 

 means of which unstained objects such as bacteria 

 can be shown as bright white particles upon a 

 dense black background. The central rays of light 

 are blocked out by means of an opaque stop while the 

 peripheral rays are reflected from the paraboloidal sides 

 of the condenser and refracted by the object viewed. 

 To obtain the best results with this type of condenser 

 a powerful illuminant such as a small arc lamp or an 

 incandescent gas lamp is needed, together with 

 picked slides of a certain thickness (specified for the 



