ILLUMINATING APPARATUS 49 



on the supporting tube below the burner, as shown in Fig. 42, 

 and ignited. This will heat the burner which is inside the 

 hanging incandescent mantle. When the asbestos-covered 

 U-piece has almost burnt out, the bellows should be gently 

 squeezed two or three times, which will drive the spirit from the 

 reservoir to the burner, where it will become volatUised and 

 burn with a steady flame. The bellows may be gently squeezed 

 every five minutes if the light appears to be failing. The handle 

 below the burner regulates the air supply, and should be adjusted 

 tiU the best illumination is obtained. 



The electric arc lamp is useful for photomicrography or 

 projection, but is troublesome for general use. 



