CH. ///] INTERPRETATION OF APPEARANCES 101 



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

 the objects 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. 



S 153. In addition to the above experiments it is very strong^' 

 recommended that the student follow the advice of Beale, p. 248, and 

 examine first with a low then with 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 fibers accidentally present 

 have been considered nerve fibers, 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 erro- 

 neous 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 

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

 ciation of Wool Growers, 1875, P- 47°' Proc. Amer. Micr. Soc, 1884, 

 pp. 65-6S. Herzfeld, translated by Salter. — The technical testing of 

 yarns and textile fabrics, London, 1898. 



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

 Jour. Roy, Micr. Soc, 1891, pp. 90-105 ; and for the illusory appear- 

 ances due to diffraction phenomena, see Carpenter-Dallinger, p. 434. 

 Mercer. Trans. Amer. Micr. Soc., pp. 321-396. 



If it is necessary to see all sides of an ordinary gross object, and 

 to observe it with varying illumination and under various conditions of 

 temperature, moisture, etc., in order to obtain a fairly accurate and 

 satisfactory knowledge of it, so much the more is it necessary not to be 

 satisfied in microscopical observation until every means of investigation 

 and verification has been called into service, and then of the image 

 that falls upon the retina, only such details will be noted as the brain 

 behind the eye is ready to appreciate. 



