DARK-FIELD ILLUMINATOR 99 



small particles and these then stand out as very bright objects on an 

 entirely dark background. As a source of light for the dark-field 

 illuminator we can use an inverted Welsbach light, immediately 

 before the reflecting mirror of the microscope; or still better, a small 

 electric-arc light especially constructed for the purpose. Whichever 

 light is used, its rays must be collected by a condensing lens, so that 

 the mirror receives a very strong bundle or pencil of rays of light. 

 The use of the dark-field illuminator is as follows: 



FIG. 42 



Dark-field illuminator used with Welsbach gas light. 



1. Arrange the dark-field illuminator according to the type of 

 apparatus, either on top of the stage or below it. 



2. Place a drop of the fluid to be examined on a clean slide and 

 cover with a clean No. 1 cover-glass. 



3. Place a drop of immersion oil in the centre of the upper surface 

 of the dark-field illuminator. 



4. Place slide on upper surface of dark-field illuminator. There 

 is now no air between the latter and the slide, since these surfaces 

 have the immersion oil between them. 



5. Place the instrument so that the light of the inverted Welsbach 

 burner or electric arc light is near the refracting mirror of the substage 

 of the microscope, and place between the source of light and the 

 reflector the concave lens which collects the rays of light. When 



