THE MICROSCOPE IN MEDICINE /1ND SANITATION. 123 



Water Department of Brooklyn, and other sanitary 

 authorities, is as follows: A measured quantity of water, 

 half a liter or a liter, is filtered through perhaps a quarter 

 of an inch of fine sand on a circle of bolting-cloth sup- . 

 ported by a perforated rubber stopper in a tall funnel. 

 The sand with the organisms collected on it is dropped 

 into a test-tube, shaken up with a small volume of water, 

 and allowed to settle for a moment. The water is then 

 poured off and a second portion of wash-water serves 

 to remove almost aU the organisms from the sand. The 

 two washings mixed together are then made up to some 

 convenient volume, 10 or 15 cc, and thus one has in say 

 10 cc. the organisms originally present in a liter. 



One cubic centimeter of the concentrate is then placed 

 in a cell, i mm. deep and 1000 sq. mm. in area. A special 

 type of ocular micrometer is used with a square ruled 

 upon it, and the lenses and draw- tube are so adjusted 

 that the square shall coincide with i sq. mm. on the stage. 

 Ten or twenty fields in different parts of the cell are 

 then examined and the organisms counted, when a 

 simple calculation enables one to compute the number 

 present in i cc. of the original sample. 



By this method the kind and number of micro-organ- 

 isms present may be easily determined, the cause of 

 existing trouble detected, and the prospect of future diffi- 

 culties predicted. The efhcacy of methods of purifij:a- 

 tion may be tested and the source of trouble located in 

 the particular part of a supply where it exists. 



A few of the most serious odor-producing organisms 

 are figured in Fig. 48. At 4 is Anabcena, the Blue-green 



