BACTERIOLOGICAL METHODS 



the Rhode Island Shellfish Commission. This standard may be 

 graphically represented as follows (Example C) : 



EXAMPLE C 



It sometimes happens in laboratory practice that the smaller 

 quantities of shell water from a number of oysters show positive 

 results, whereas larger amounts of liquor from an equal number of 

 oysters show negative results. In such cases it is customary to 

 give the next lower numerical value to the positive results in the 

 high dilutions, and such positive results shall be considered as 

 being transferred to a lower dilution giving negative results in 

 another oyster. This recession of assigned values shall, however, 

 not be carried beyond the point where the number of such reces- 

 sions is greater than the number of instances where other oysters 

 in the series of five failed to give positive results. This may be 

 illustrated as follows (Examples D and E) : 



EXAMPLE D 



