70 DARK-GROUND ILLUMINATION [Cn. II 



from bristol board like that in visiting cards. It is well to blacken 

 the stop with India ink. If such a stop is used all the light 

 which would enter the objective directly is shut out, and only 

 that which is deflected by the object will get into the micro- 

 scope. If the above test is made with different objectives it will be 

 found that the size of the central stop varies directly with the aper- 

 ture, that is, the greater the aperture the larger must be the central 

 stop; and conversely the larger the central stop the narrower will 

 be the open ring allowing the light to pass through the condenser to 

 form the hollow cone of light. It follows from this that with the 

 higher objectives where more light is needed the ring of light is nar- 

 rower; hence a stronger light must be used for a high than for a low 

 objective or the objects will not be lighted brilliantly enough so that 

 they can deflect sufficient light into the microscope to make them 

 satisfactorily visible. 



After inserting the proper central stop, put a slide under the mi- 

 croscope and scatter some flour or starch upon it; then open the 

 iris diaphragm to its full extent, and the particles of flour or starch 

 will shine as if they were self-luminous (see fig. $of, lower half). In 

 case the field looks gray instead of black, either the central stop is 

 not large enough to cut out all the light from the condenser, or the- 

 particles of flour or starch are too numerous, not leaving enough 

 blank space between them. 



123. Dark-ground illumination by refraction. In the ex- 

 periments already given for dark-ground effects the particles of dust 

 have reflected the very oblique light into the microscope objective. 

 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 

 microscope. 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 arabic mucilage or other 

 transparent viscid substance like saliva. This will include many 

 air bubbles. Put the preparation under the microscope, using the 

 i6-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 



