MICROLITEE DIVER TECHNIQUE 359 



inside diameter of the tip of the pipette should be about 0.07-0.12 

 mm., and the outside diameter about 0.15-0.25 mm., depending on 

 the size of the diver employed. The delivery end of the pipette should 

 be strictly cylindrical for about 1 cm. from the end, or, even better, 

 slightly trumpet shaped at the tip. This form minimizes the tend- 

 ency of the liquid to creep up on the outside. 



Type 2 Pipette. This pipette resembles the type 1 ; however, it 

 is intended for viscous liquids, particularly for paraffin oil. The 

 orifice at the tip must have a diameter no less than 0.15-0.18 mm. 

 and the glass wall thickness must be about 0.04 mm. In making the 

 pipette, the ratio of wall thickness to diameter of the bulb {A, Fig. 

 122) should be about 1:4, and the pipette is drawn to a tip with an 

 outside diameter of 0.2-0.25 mm. Paraffin oil has no tendency to 

 creep so that the exact shape of the tip is not important. 



Type 3 Pipette.* This form of pipette is known as a "braking 

 pipette," and is intended for the transfer of cells and pieces of tissue 

 together with known amounts of liquid. The pipette (Fig. 123) is 

 drawn out at its upper end to a hair-thin tip so that the rate of 

 filling and emptying will be determined by the rate at which air 

 can pass through the very fine tube or ''brake" (E) . The outside 

 diameter of the pipette {A, Fig. 123) is 0.3-0.5 mm. The cork (B) 

 is smoothly cut in half lengthwise and a fine longitudinal groove 

 is made in the cut surface to hold the pipette. The rubber tubing 

 (D) is used for the operation of the -pipette by mouth. C in the 

 figure indicates the jacket tube. 



In preparing the pipette, the upper end is drawn out long and 

 with a gradual tapering. Small portions are broken off with a fine 

 forceps until the rate of filling and discharge of the pipette is suit- 

 able. To test this, dip the mouth of the pipette into water and 

 observe the rate at which the water rises by capillarity. A rate of 

 about 1-5 mm./sec. has been found to be about right. The dimen- 

 sions of the delivery end of the pipette will be determined primarily 

 by the size of the object to be transferred. Whenever possible, taper 

 the mouth of the pipette, since it is difficult to reproduce the size 

 of the last droplet remaining in the mouth after delivery. 



When in operation, should the braking tip of the pipette become 

 filled with condensed water which cannot be dislodged by sucking 

 or blowing, apply a slight excess pressure to the jacket tube and 



*See Bibliography Appendix, Ref. 47. 



