306 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



0*005 of the first dilution. On plate G we have used the 

 same proportion of the first dilution, but we have taken 

 half a centimetre for the purpose of counting, that is, five 

 times as much as for plate 1, and consequently the true 

 volume is 0*025 cm. 



This true volume is important for the estimation of the 

 coefficient. Under the heading "calculation" this is set 

 out. Under the heading " true volume " for the first 

 plate we get the number 0*005. The number of times 

 that this is contained in the original volume (first dilu- 

 tion) is naturally 50/*005, that is, 10,000, so that one ten- 

 thousandth part of the total number of organisms have 

 been enumerated. For example, the number of Melosira 

 reckoned for the first plate is 43. This multiplied by the 

 coefficient 10,000, gives us the total number of Melosira 

 in the catch, that is, 430,000. This number is of course 

 approximate, and is corrected by the numbers obtained on 

 the succeeding plates. 



Suppose the individuals of a certain species have been 

 counted on several plates, then it becomes necessary to 

 determine the coefficient for the plates taken together 

 (see heading " Coeff." on right-hand side of the table). 



With this object the numbers given under the heading 

 " true volume " for all the plates enumerated for a given 

 organism are added together, and the coefficient is calcu- 

 lated exactly as described above for a single glass plate. 

 For instance, for Melosira three plates in all have been 

 counted, those numbered 1 to 3. The numbers respectively 

 noted are 43, 38 and 48. Total 129. The sum of the true 

 volume for the three plates is 3 x *005 = 0*015 ccm. 

 Then the coefficient is 50/0*015, that is, 3,333. The total 

 number of individuals counted, 129, is now multiplied by 

 the coefficient, and that gives us the total number of 

 Melosira in the whole catch, viz., 429,957. 



