no INTERPRETATION OF APPEARANCES [C//. Ill 



164. Demonstration of Pedesis with the Polarizing 

 Microscope. (Ch. VI.) The following demonstration shows con- 

 clusively that the pedetic motion is real and not illusive. (Ranvier, 



P- I73-) 



Open the abdomen of a dead frog (an alcoholic or formalin 

 specimen is satisfactory). Turn the viscera to one side and observe 

 the small, whitish masses at the emergence of the spinal nerves. 

 With fine forceps remove one of these and place it on the middle of 

 a clean slide. Add a drop of water, or of water containing a little 

 gum arabic. Rub the white mass around in the drop of liquid and 

 soon the liquid will have a milky appearance. Remove the white 

 mass, place a cover-glass on the milky liquid and seal the cover by 

 painting a ring of castor oil all around it, half the ring being on the 

 slide and half on the cover-glass. This is to avoid the production 

 of currents by evaporation. 



Put the preparation under the microscope and examine with, first 

 a low power then a high power (3 mm. or y% in.). In the field will 

 be seen multitudes of crystals of carbonate of lime; the larger crys- 

 tals are motionless but the smallest ones exhibit marked pedetic 

 movement. 



Use the micro-polariscope, light with great care and exclude all 

 adventitious light from the microscope b}- shading the object ( 120) 

 and also by shading the eye. Focus sharply and observe the pedetic 

 motion of the small particles, then cross the polarizer and analyzer, 

 that is, turn one or the other until the field is dark. Part of the 

 large motionless crystals will shine continuously and a part will re- 

 main 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. 



165. Muscae Volitantes. These specks or filaments in the 

 eyes due to minute shreds or opacities of the vitreous sometimes ap- 

 pear as part of the object as they are projected into the field of 

 vision. They may be seen by looking into the well lighted micro- 

 scope when there is no object under the microscope. They may also 

 be seen by looking at brightly illuminated snow or other white sur- 

 face. By studying them carefully it will be seen that they are some- 

 what movable and float across the field of vision, and thus do not 

 remain in one position as do the objects under observation. Further- 



