I oo IN TERPRE TA TION OF A PRE A RA NCES [ CH. Ill 



their independent leaping movements while they are carried along by 

 the current. The pedetic motion makes it difficult to obtain good 

 photographs of milk globules and other small particles. The difficulty 

 may be overcome by mixing the milk with a very weak solution of 

 gelatin and allowing it to cool (see Ch. IX). 



§ 151. Demonstration of Pedesis with the Polarizing Micro- 

 scope (Ch. VI). — The following demonstration shows conclusively 

 that the pedetic motion is real and not illusive. (Ranvier, p. 173.) 



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 

 evaporation. 



Put the preparation under the microscope and examine with, first ; 

 a low then a high power (3 mm. or /s in.). In, the field will be seen 

 multitudes of crystals of carbonate of lime ; the larger crystals 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 by shading the object (§ 109) 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 motion- 

 less 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. 



§ 152. Muscae Volitantes. — These specks or filaments in the 

 eyes due to minute shreds or opacities of the vitreous sometimes appear 

 as part of the object as they are projected into the field of vision. They 

 may be seen by looking into the well lighted microscope when there is 

 no object under the microscope. They may also be seen by looking 

 at brightly illuminated snow or other white surface. By studying 

 them carefully it will be seen that they are somewhat movable and float 



