120 DARK-GROUND ILLUMINATION [Ch. IV 



of the large motionless crystals will shine continuously and a part 

 will remain dark, but small crystals between the large ones will shine 

 for an instant, then disappear, only to appear again the next instant. 

 This demonstration is believed to furnish absolute proof that the 

 pedetic movement is real and not illusory. 



For the help given by the micro-spectroscope see Ch. VIII. 



§ 210. Use of dark-ground illumination for interpreting appearances. 

 — Dark-ground illumination is almost invaluable for bringing out 

 details of structure and for showing movement in living things. The 

 granules and different parts in living cells and minute organisms are 

 so nearly the same refractive index that it is exceedingly difficult to 

 differentiate them with the ordinary methods of illumination. On 

 the other hand, with dark-ground illumination the different structures 

 stand out with the greatest clearness. 



§ 211. Specimens to use for dark-ground illumination. — 



(i) Organisms from hay infusion. Use for the infusion a small 

 fruit jar or other glass dish. Go to a stream or pond and from a shal- 

 low stagnant pool along the edge take some of the surface of the mud 

 and put it into the jar with some of the water. Add some of the dead 

 grass found along the edge of the pond, cut up into short pieces. Set 

 in a warm dimly lighted or dark place for a day or longer. This 

 should soon be alive with all sorts of minute living things. 



If it is not easy to get the water, mud, and dead grass, fairly good re- 

 sults are obtained by putting some ordinary hay in water of any kind. 



With fine forceps take a leaf or piece of stem of the dead grass and 

 put it on a slide i mm. thick. Move it around and press it down so 

 that a good drop of liquid and debris will be on the slide. Remove 

 the grass and cover the liquid with a 0.15 mm. cover-glass. This 

 should be studied fresh with a 4 mm. objective, 5X or iox ocular, and 

 transmitted light. Now put in place the dark-ground illuminator, 

 center it (§ 126), and add some distilled water or some homogeneous 

 liquid to the top of the condenser and run it up till the liquid is in 

 contact with the under side of the slide. 



Put a drop of homogeneous liquid on the cover-glass and use a 

 homogeneous immersion objective in which the aperture has been 

 cut down to 0.95 or less. Examine as directed in § 127-128. 



