﻿IX, B, 4 Barber: The Pipette Method 325 



methods, and placed on the stage of the microscope with the 

 open end to the right. The pipette is made as usual, though 

 it is advantageous to have the turned portion rather short and 

 brought to an angle of about 50 degrees instead of at right 

 angles. If the low power alone is to be used, it is focused 

 on the edge of a drop of sterile broth and then lowered some- 

 what. If the high power is to be used, there is no preliminary- 

 focusing with the low power, but the lens is focused on the 

 cover glass in the neighborhood of the edge of a sterile drop. 



The pipette is held with the thumb and index finger of the 

 right hand and steadied by pressing the other fingers against 

 and beneath the stage of the microscope. The shank is held 

 horizontally and the tip brought into the chamber past the 

 objective. It is then held far enough from the cover glass to 

 avoid any drop and moved back and forth until the capillary 

 can be seen, often only as a shadow in the field. The pipette 

 is then slowly withdrawn until the tip appears. This is then 

 kept in view while the objective is focused on the edge of the 

 drop of broth. After the tip is filled from the drop, the chamber 

 is moved and the bacteria brought into the field. The pipette 

 may be filled from a test tube before introducing it into the 

 isolating chamber. If it is necessary to break off the point, 

 scratch it very gently on the side of the tube. The rapidity 

 with which the broth rises is a good index of the size of the 

 opening. 



The screws governing the mechanical stage and the fine adjust- 

 ment of the microscope are manipulated by the thumb and second 

 finger of the left hand, while the index finger is bent and braced 

 against the pillar. The isolation is carried out as in the other 

 methods. It is necessary to keep the point of the pipette con- 

 tinually in view during the process. 



The advantages of this method are its simplicity and the 

 rapidity with which the pipette may be adjusted into place 

 and manipulated. The chief disadvantage is, obviously, the 

 difficulty of properly governing the movements of the pipette 

 with the fingers alone, especially under the higher powers. It 

 is surprising, however, how steady the pipette can be held 

 and how accurately it can be moved when the hand is well 

 supported against the stage of the microscope. The isolation 

 of organisms distinguishable under the low power is compara- 

 tively easy, and the difficulty of isolating cocci and other smaller 

 bacteria under a i- or ^-inch objective is not great after some 

 practice. This method is not adapted to microscopes in which 

 the fine adjustment and the screws of the mechanical stage 



