214 STANDARDIZATION OF PEPTONES 



The former holds six polarization tubes, which can 

 be brought into the field of observation without 

 opening the heated incubator. In this way, all 

 variations of temperature are avoided during the 

 test. (See Plate.) 



The most important source of error lies in the 

 observer himself. The eye soon becomes tired, and 

 it is impossible to take many readings at a time. One 

 must become so experienced, as to be able to record 

 a reading in at least thirty seconds. So soon as the 

 eyes become weary, the readings become dubious. It 

 is not advisable to resort to the optical method, until 

 one has attained to sufficient certainty in taking 

 readings. 



