1897J MICROSCOPICAL JOURNAL 343 



been interested more especially in the smaller forms, but 

 chiefly because their operations have been confined to the 

 small quantities of water sent to the laboratories for 

 analysis. During the last decade the old methods of 

 sediment examination have given way to the filtration 

 methods. The 8edgwick-Rafter method, which is most 

 used at the present time in laboratories of water analysis, 

 is carried on as follows : 



A portion of the water to be examined is measured out 

 in a graduate and filtered through a thin layer of quartz 

 sand placed at the bottom of a glass funnel upon a per- 

 foratedrubber stopper, the hole in which is capped with 

 a disc of bolting cloth. When the water has filtered 

 the organisms will be found upon the sand while the fil- 

 tered water will be free from them. The rubber stop- 

 per is then removed and the sand washed into a test tube, 

 with a measured quantity of distilled water delivered 

 from a pipette. Usually 2'0 or 500 c. c. of the sample 

 are filtered and the sand washed with 5 c. c. The test 

 tube is then thoroughly shaken and the water decanted 

 into a second tube; the organisms being lighter than the 

 sand, will pass ofi" with the water, leaving the sand clean 

 upon the walls of the first tube. In this way the organ- 

 isms are concentrated 50 or 100 times. One c. c. of this 

 concentrated fluid is then transferred to a counting cell, 

 which just holds it and which has a su])erficial area of 

 1,000 sq. mm. Alter putting a thin ghiss cover-slip over 

 this cell it is transferred to the st;ige of the microscope 

 for examination. The eye-piece of the microscope is fit- 

 ted with a micrometer in the shape of a ruled square of 

 such a size as to cover one square mm. on the stage, i. e. 

 one thousandth of the entire area of tlie cell. The oroan- 

 isms observed within the limits of the ruled square are 

 then counted and the cell moved until another portion 

 comes in-to view, when another count is made. Thus 10 

 or 20 squares are counted and the number of organisms 



