90 INTERPRETATION OF APPEARANCES. [CH. III. 



ventitious light from the microscope by shading the object (§ 102) 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 remain dark, but the 

 small crystals between the large ones will shine for an instant, then dis- 

 appear, only to appear again the next instant. This demonstration is 

 believed to furnish absolute proof that the pedetic movement is real and 

 not illusory. 



§ 143. 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 ob- 

 ject under the microscope. They may also be seen by looking at the 

 brightly illuminated snow or other white surface. By studying them 

 carefully it will be seen that the}' are somewhat movable and float across 

 the field of vision, and thus do not remain in one position as do the ob- 

 jects under observation. Furthermore, one may, by taking a little 

 pains, familiarize himself with the special forms in his own eyes so that 

 the more conspicuous, at least, may be instantly recognized. 



§ 144. In addition to the above experiments it is very strongly rec- 

 ommended that the student follow the advice of Beale, p. 248, and ex- 

 amine 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 (Beale, 288). Fibers of cotton, 

 linen and silk. Textile fibres accidentally present have been considered 

 nerve fibres, etc. Human and animal hairs. Study with especial care 

 hairs from various parts of the body of the animals used for dissection 

 in the laboratory where you work. These are liable to be present in 

 histological preparations, and unless their character is understood there 

 is chance for much confusion and erroneous interpretation. 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 and places not frequently dusted. 

 In the last will be found a regular museum of objects. 



For figures (photo-micrographs, etc.) of the various forms of starch, 

 see Bulletin No. 13 of the Chemical Division of the U. S. Department 

 of Agriculture. For Hair and Wool, see Bulletin of the National Asso- 



