diameter of the field of vision. Thus, the entire sample 

 was distributed linearly within the width of one field of 

 view. The trough was passed slowly under the scope and 

 the catch was identified and counted. This procedure in- 

 creased the accuracy of counting and the speed of analysis. 



60 



TRANSPARENCY 



50 



40 



30 

 DEPTH (FEET) 



20 



Figure 3. Vertical series of filtered material concentrated at the bottom of centrifuge tubes 

 (volume displacement) and related turbidity of water. 



