﻿IX, B, 4 Barber : The Pipette Method 319 



into contact with the cover just outside of the drop. These 

 precautions are taken to avoid crushing the tip against the 

 cover glass. 



Sometimes it may be difficult to see the tip at any distance 

 under the cover glass on account of the refraction of light due 

 to the hanging drops. In this case, focus the low power 2 or 

 3 millimeters below the cover and bring the point up very slowly. 

 It will usually come into view when it approaches the cover, 

 especially if it is first brought up in a deep hanging drop. With 

 the tip held gently against the cover, move the mechanical stage 

 slightly and break off a very small portion of the point. Immerse 

 the tip in the droplet of broth for a few seconds in order that it 

 may become supplied with liquid. This process may be hastened 

 by suction on the rubber tube, which during this part of the 

 process is held in the mouth of the operator. 



If the fields of the high and low powers of the microscope 

 coincide, the tip should now be brought to the very center of 

 the low-power field ; if not, it should be brought to a point, pre- 

 viously determined, which is near the center of the high-power 

 field. It is often helpful to use the micrometer eyepiece in locat- 

 ing this point, especially in changing from the low-power to the 

 oil-immersion lens. Now bring the tip into contact with the 

 cover glass near the edge of the broth drop, and blow out a very 

 small drop with the rubber tube ; then, before the liquid reenters 

 by capillarity, lower the point slightly. If no drop can be blown 

 out, immerse the tip in the broth again and draw in more 

 fluid. If a very small opening is left, it may be necessary to 

 enlarge it somewhat and refill from the sterile drop before it 

 is possible to discharge a droplet. The cover glass should have 

 a film of moisture on it so that the liquid in the pipette will be 

 in contact with liquid on the cover. It is very difficult to dis- 

 charge liquid from a fine point to a dry cover. If necessary, 

 the film of moisture on the cover can be supplied by placing a 

 little slightly warmed water in the bottom of the isolating cham- 

 ber. A little distilled water may be taken into a medicine 

 dropper, heated slightly over the flame, and added gently to the 

 liquid in the bottom of the chamber. The moist air, rising, forms 

 a film on the cover (Plate I, fig. 1). 



With the tip just under the broth droplet, change to the high 

 power. The droplet should be in the field, and when the tip 

 is raised it will come into view. It may now be easily adjusted 

 to the center of the high-power field. It is obvious that care 

 must be taken not to move the mechanical stage or the cover 

 glass during the change to the high power. 



