Ch. II] 



1 ) ARK-GROUND ILLUMINATION 



67 



Xow make the light oblique, either by making the diaphragm open- 

 ing eccentric or, if simply a mirror is used, by swinging the mirror side- 

 wise. On focusing up and down, the lines will sway from side to side. 

 What is the direction of apparent movement in focusing down with 

 reference to the illuminating 

 ray? What in focusing up? If 

 one understands the experiment 

 it may sometimes save a great 

 deal of confusion. (See under 

 testing the microscope for sway- 

 ing with central light, § 166.) 



Dark-ground Illumination 



§ 117. Dark-ground illumi- 

 nation. — By this is meant that 

 form of illumination in which 

 the object appears light and the 

 background dark. The appear- 

 ance is something like a series 

 of bright objects in a dark night. 



In order to be available for 

 dark-ground illumination ob- 

 jects must be in a refracting 

 medium different from them- 

 selves, and must have either 

 strongly refracting or reflecting 

 qualities. 



The optical arrangements for 

 this form of illumination must 

 be such that the object is lighted 



by a beam of light which cannot get into the objective either because 

 the rays are so oblique, or because they are cut out before reaching 

 the eye. In either case the light on the background never reaches the 

 eye. Only that which is reflected, refracted, or diffracted by the 

 object reaches the eye. Consequently the appearance is of a bright 

 object in a dark field. 



Fig. 46. Oblique Light with a 

 Condenser. 



(From Chamot). 



The iris diaphragm is opened com- 

 pletely and the light from one side is 

 blocked out by inserting the finger; this 

 gives unsymmetrical light and all of it is 

 oblique to the optic axis. 



