34 INTERPRE TA TION OF APPEARANCES. 



will be in focus and a different optical section obtained. The most 

 satisfactory optical sections are obtained with high objectives having 

 large aperture (§ 18). 



Nearly all the transparent objects studied may be viewed in optical 

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

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

 (§§ 89. 9 1 ) furnish excellent and striking examples of optical sections 

 (PI. Ill, Fig. 24-26). 



§ 93. Currents in Liquids. — Employ the 18 mm. (^ in.) objective, 

 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 (§§ 3, 34) ? 



Lamp-black rubbed in water containing a little mucilage answers 

 well for this experiment. 



§ 94. Pedesis or Brownian Movement. — Employ the same object 

 as above, but a 5 mm. (A in.) or higher objective in place of the 18 mm. 

 Make the body of the microscope vertical, so that there may be no cur- 

 rents 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 Pedesis or Brozvnian movement. Also, but improperly, molecular 

 movement, from the smallness of the particles. 



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

 slight amount of silicate of soda or of soap ; sulphuric acid and various 

 saline compounds retard or check the motion. One of the best objects 

 is pumice stone ground finely. In this the movement is so active that 

 it is difficult to follow the course of single particles. Pedesis is exhibited 

 by all solid matter if finel}- enough divided and in a suitable liquid. 

 No adequate explanation of this phenomenon has yet been offered. 

 See Carpenter 182-183, Beale 195, Jevons in Quart. Jour. Science, new 

 series, Vol. VIII, (1878), p. 167. 



Compare the pedetic motion with that of a' current by slightly inclin- 

 ing the body of the microscope. The small particles will continue their 

 independent leaping movements while they are carried along by the 

 current. 



§ 95. Demonstration of Pedesis with the Polarizing Micro- 

 scope. — The following demonstration shows conclusively that the pe- 

 detic motion is real and not illusive. (Ranvier, p. 173). 



