35 INTERPRETATION OF APPEARANCES. 



Open the abdomen of a dead frog (an alcoholic specimen will do) ; 

 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 miroscope and examine with first a low 

 then a higher power (3 mm. or l /s in.). In the field will be seen mul- 

 titudes of crystals of carbonate of lime, the larger crystals are motion- 

 less but the smallest ones exhibit marked pedetic movement. 



Use the micro-polariscope (see Ch. IV), light with great care and ex- 

 clude all adventitious light from the microscope by shading the object 

 (§ 66) and also by shading the eye. Focus sharply and observe the 

 pedetic motion of the small particles, then cross the polarizer and anal- 

 yzer, 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 remain 

 dark, but the 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. 



§ 96. In addition to the above experiments it is very strongly recom- 

 mended that the student follow the advice of Beale, p. 248, and exam- 

 ine first with a low then a higher power mounted dry, then in water, 

 lighted with reflected light, then with transmitted light, the following : 

 Potato, wheat, rice, and corn starch, easily obtained by scraping the 

 potato and the grains mentioned ; bread crumbs ; portions of feather. 

 Portions of feather accidentally present in histological preparations 

 have been mistaken for lymphatic vessels (B. 248). Fibers of cotton, 

 linen and silk. Textile fibers accidentally present have been consid- 

 ered nerve fibers, etc. Human and animal hairs, especially cat hairs. 

 These are very liable to be present in preparations made in this labora- 

 tory. The scales of butterflies and moths, especially the common 

 clothes moth. The dust swept from carpeted and wood floors. Tea 

 leaves and coffee grounds. Dust found in living rooms in places not 

 frequently dusted. In the last will be found a regular museum of ob- 

 jects. 



For different appearances due to the illuminator see Nelson, in Jour. 

 Roy. Micr. Soc, 1891, pp. 90-105. 



