Ch. II] 



DARK-GROUND ILLUMINATION 



69 



§ 121. By lighting from one side as with the mirror only. — Use 

 a 16 mm. or lower objective; for object, a glass slide with dust particles. 

 Open the substage diaphragm to its full extent and reflect the light 

 up through the condenser. The particles will 

 now appear dark on a white field. Make the 

 light unsymmetrical and very oblique by putting 

 the finger in the path of the light on one side 

 (fig. 46) and the field will be dark and the 

 particles bright. 



§ 122. By lighting with a hollow cone. — This 

 lights the- object with a ring of light, all the rays 

 of which are very obUque (fig. 47). 



For this use a 16 mm. objective and a slide 

 with dust upon it as before. Open the substage 

 diaphragm to its full extent, and turn the mirror 

 so that the light is as brilliant as possible. Put 

 a diaphragm with a ring opening in the holder 

 under the substage. If now the object is looked 

 at, the dust particles will shine in a dark field. 



§ 123. Dark-ground illumination by refraction. 

 — In the experiments already given for dark- 

 ground effects the particles of dust have reflected 

 the very oblique light into the microscope objec- 

 tive. The same effect may be produced by minute 

 bodies refracting the very oblique light and thus 

 turning part of it into the objective of the micro- 

 scope. There are two cases: 



(i) Objects whose refraction is less than the 

 mounting medium: For this use air bubbles. 

 Make a preparation by beating on a slide with a 

 knife blade a small drop of gum arable mucilage or other transparent 

 viscid substance like saliva. This will include many air bubbles. 

 Put the preparation under the microscope, using the 16 mm. or 

 lower objective as before. Try an 8x or lox ocular. Use the ring 

 diaphragm and light as well as possible. The air bubbles will shine 

 like globules of silver in a dark field. By focusing carefully the 



Fig. 47. Dark- 

 ground Illumina- 

 tion BY A Hollow 

 Cone of Light. 



A xis The princi- 

 pal optic axis. 



D The central 

 stop (c s) or ring dia- 

 phragm cutting out 

 all but the marginal 

 rays of light, all of 

 which are very ob- • 

 lique to the optic 

 axis and will not 

 enter the objective 

 (06) unless deflected 

 by the object. 



