40 



Scientific Proceedings (102). 



in which 



A — the corrected reading for the first volume of the suspen- 

 sion = (a — constant) 

 a = first observed reading, 

 vol a = first volume of the suspension on which reading a is made. 



b = second observed reading, 

 vol b = second volume of the suspension (diluted), on which 



reading b is made. 

 A concrete example will illustrate the method. In 4 c.c. (vol a) 

 of a given suspension the loop disappears 1.2 cm. below the menis- 

 cus (reading a). The suspension is diluted to 10 c.c. (vol b). 

 The loop now disappears 2.7 cm. below the meniscus (reading b). 

 Then the corrected reading for the opacity of the suspension (^4), 



. 4(2.7 - 1.2) 



is — or 1.0 cm. 



10—4 



Such a corrected reading may be directly compared with cor- 

 rected readings on other suspensions of the same organism, since 

 the corrected readings on such suspensions stand in inverse ratio 

 to their bacterial concentrations. Thus, if two suspensions of 

 the same organism are to be compared and the corrected reading 

 for one is half that for the other, the first suspension contains 

 twice as many bacteria as the second. If the actual bacterial 

 count is required, a standard for the given organism must first 

 be established by the correlation of several corrected readings 

 with the corresponding counts, obtained by the usual methods. 

 Thereafter the concentration of the organism per cubic centimeter 

 in suspensions under examination is obtained by inverse propor- 

 tion: 



The required count The standard corrected opacity 

 The standard count The given corrected opacity 

 The simplest possible instrument for making the determinations 

 consists of a piece of 18 gauge nichrome, chromel, or black iron 

 wire about 20 cm. long, bent into a small loop at right angles to 

 one end, and with the other end thrust through a cork, near one 

 side, so that a view may be obtained past the cork into an ordinary 

 test tube, 1.6 X 16 cm. on whose lip it rests. The sterile test 

 tube is partly filled with a measured quantity of the suspension, 

 and the wire loop raised or lowered through the cork until the 



