PROPERTIES USED IN CONCENTRATION*. 13 



animation. At the same time it increases the likelihood of finding 

 parasites and their eggs when these are present. 



The justification for thus shifting the greater amount of time from 

 that of microscopic examination to that of preliminary treatment is 

 readily found. Microscopic examination of feces for parasites and 

 their eggs is one which calls for some degree of mental concentration. 

 The judgment is constantly exercised in passing upon various objects 

 which catch the eye and by a superficial resemblance in size, shape, 

 color or refraction to parasites and eggs call for a determination of 

 their spurious character. The determination of some of these forms 

 requires the use of the higher powers of the microscope and some 

 little time in examination. In addition to the fact that microscopic 

 examination makes considerable mental demands, is the fact that 

 microscopic work is eyestrain a matter of considerable importance 

 to anyone who has much work of the kind to do and the saving of 

 eyestrain by eliminating part of the objects which must be picked up 

 by the eye, only to be rejected as not being the thing sought for, is 

 greatly to be desired. 



On the other hand, the treatment of feces preliminary to micro- 

 scopic examination calls for little mental concentration and no eye- 

 strain, and so leaves the mind free for other things and prolongs the 

 period of usefulness of the eyes. Moreover, the fact that this pre- 

 liminary treatment can largely be left to the operation of natural 

 laws and to power machinery also saves to the operator the time when 

 things can be left unattended. 



PROPERTIES USED IN CONCENTRATION. 



Concentration, the object of all improvements on the smear method, 

 is obtained by eliminating as much as possible of the nonparasitic 

 material by taking advantage of differences in physical, chemical, and 

 biological properties between such material and the parasites. These 

 differences are in most cases those of specific gravity, of sixe, of 

 physical and chemical solubility, of adhesiveness, and of capacity for 

 growth and development. 



PHYSICAL PROPERTIES. 



The elimination of matter having a specific gravity less than that 

 of the parasites is accomplished by sedimenting in water and decant- 

 ing material which floats or remains suspended, and by centrifuging 

 in water, and is based on the general truth that parasites are heavier 

 than water and will settle to the bottom. Dock and Bass (1910) lay 

 emphasis on the fact that- 

 Most * * * food particles arc irregular in outline and shape, making 

 their surface per giveii weight greater than that of the round, oval eggs. 



