Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/239

 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.