88 INTERPRETATION OF APPEARANCES. [CH. III. 



section. A striking example will be found in studying mammalian red 

 blood-copuscles on edge. The experiments with the solid glass rods 

 (Fig. 89) furnish excellent and striking examples of optical sections. 



§ 139. Currents in Liquids. — Employ the 16 mm. (fi in.) object- 

 ive, and as object put a few particles of carmine on the middle of a slide, 

 and add a drop of water. Grind the carmine well with a scalpel blade, 

 and then cover it. If the microscope is inclined, a current will be pro- 

 duced in the water, and the particles of carmine will be carried along 

 by it. Note that the particles seem to flow up instead of down— why 

 is this ? 



Lamp-black rubbed in water containing a little mucilage answers well 

 for this experiment. 



§ 140. Velocity Under the Microscope. — In studying currents or 

 the movement of living things under the microscope, one should not 

 forget that the apparent velocity is as unlike the real velocity as the ap- 

 parent size is unlike the real size. If one consults Fig. 37 it will be 

 seen that the actual size of the field of the microscope with the different 

 objectives and oculars is inversely as the magnification. That is, with 

 great magnification only a small area can be seen. The field appears to 

 be large, however, and if any object moves across the field it may ap- 

 pear to move with great rapidity, whereas if one measures the actual 

 distance passed and notes the time, it will be seen that the actual motion 

 is quite slow. One should keep this in mind in studying the circulation 

 of the blood. The truth of what has just been said can be easily dem- 

 onstrated in studying the circulation in the gills of Necturus, or in the 

 frog's foot, by using first a low power in which the field is actually of 

 considerable diameter (Fig. 37, Table, § 47) and then using a high 

 power. With the high power the apparent motion will appear much 

 more rapid. For the form of motion, spiral, serpentine, etc., see Car- 

 penter-Dallinger, p. 375. 



§ 141. Pedesis or Brownian Movement. — Employ the same ob- 

 ject as above, but a 3 mm. ( ^iu.) or higher objective in place of the 16 

 mm. Make the body of the microscope vertical, so that there may be 

 no currents produced. Use a small diaphragm and light the field well. 

 Focus, and there will be seen in the field large motionless masses, and 

 between them small masses in constant motion. This is an indefinite, 

 dancing or oscillating motion. 



This indefinite but continuous motion of small particles in a liquid is 

 called Pe-de'sis or Brownian movement. Also, but improperly, molecu- 

 lar movement, from the smallness of the particles. 



The motion is increased by adding a little gum arabic solution or a 



