Ch. IV] EXPERIMENTS EOR INTERPRETATION 121 



(2) Saliva. Put a drop of saliva on a slide 1 mm. thick and cover 

 it with a 0.15 mm. cover-glass. Examine as in (1). 



Note the pedetic or Brownian movement of the granules in the 

 rounded salivary corpuscles, the minute granules in the broad oval 

 epithelium, etc. 



(3) Fresh blood. Make a preparation of fresh human blood as 

 follows: Use a clean slide 1 mm. thick and have ready and handy a 

 cover-glass 0.15 mm. thick. Wash the middle finger well with soap 

 and water and wipe it dry with a piece of gauze. Then wipe it again 

 with a piece of gauze wet with 95% alcohol. 



Sterilize a needle by heating it to redness. Make two or three 

 good pricks in the clean finger with the sterile needle. Squeeze the 

 finger well and a drop of blood will run out. Touch this blood to the 

 middle of the slide and cover it immediately. Press the cover down 

 so that there will be a very thin layer of blood. Examine with the 

 dark-ground illumination, using the homogeneous objective with re- 

 duced aperture. Use homogeneous liquid to join the slide and top of 

 the condenser. 



The appearance of a fresh blood preparation with dark-ground 

 illumination will be a revelation to one who has studied blood only 

 with the usual transmitted light. The white corpuscles, or leucocytes 

 are very striking objects, especially the polymorphonuclear ones with 

 granules. These granules show the pedetic or Brownian movement 

 well; and if the room is warm where the work is done the aim el mid 

 movement is very striking. For the blood of 2 individuals studied 

 the leucocytes in one (male) moved 6.8 fx per minute; in the other 

 (female) the movement was 18 fx per minute. 



In addition to the corpuscles and minute granules of various kinds 

 one is almost sure to see the fibrin filaments arranged something like 

 a spider's web. 



§ 212. Difference of appearance due to difference of focus. — 

 If one takes a geometrical pattern like that shown in tig. 73 and looks 

 at it in the ordinary way the appearance is that of white spots on a 

 dark field. If now the head is held closer and closer to the picture an 

 inversion will take place and the appearance is of dark -pots in a white 

 field. This illustrates how difficult it is to determine the real appear- 



