MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 35 



so good. If a very great dilution is required, distilled water 

 can be added, after the addition of one to two volumes of 

 dilute sulphuric acid. 



When the different volumes of liquid are measured with 

 accuracy, and particular care taken that the cells are 

 thoroughly distributed by vigorous and prolonged shaking, 

 the determination can be made with great accuracy. Two 

 similar dilutions must always be made, and samples taken 

 from each for counting. As a matter of course, experiments 

 must also be made in order to determine the number of the 

 small squares whose cell contents must be counted in order 

 to arrive at a true average. Such a counting and determina- 

 tion of the average numbers is continued until the number 

 finally obtained is found to have no further influence on the 

 average value. The number of countings necessary, and the 

 accuracy generally, depend on the experience and care of the 

 observer. Hcinsen found that, as a general rule, it was 

 sufficient to count the cells in 48 to 64 small squares. 



