New York Agricultural Experiment Station. 105 



yji 



G 



€ 



Capillary Pipettes Used in the 

 Microscopic Method of Counting 

 Bacteria in Milk. 



Form of pipette. — Three differ- 

 ent forms of pipettes * were tried 

 but the one which gave the most 

 satisfactory results was the 

 straight-bored, square-tipped 

 pipette shown in Fig. 2. 



This pipette is' slightly different 

 from the one originally figured 

 by Breed, which was similar to 

 Fig. 1. The objection to the 

 latter pipette is that the beveled 

 tip causes the milk to run back 

 on the outside of the pipette in 

 such a way as to make it difficult 

 to deposit the whole drop. This 

 does not happen if the tip is 

 squared as in Figs. 2 and 3. 

 Fig. 3 shows another form in 

 which the bore is slightly widened 

 near the graduation mark. This 

 feature saves time because it is 

 difficult to stop the meniscus in 

 a straight bore at the desired 

 point. This form of pipette has 

 one. objection, however, in that 

 the wider the bore the greater 

 the chance of error in measure- 

 ment, but, on the whole, this 

 pipette is almost if not quite as 

 satisfactory as the one shown in 

 Fig. 2. 



Error in measurement of 

 sample. — It is difficult to measure 

 quantities as small as 0.01 of 

 a cubic centimeter accurately 

 with capillary pipettes. The 



* Since the above was written a new form of pipette has been devised by Dr. 

 Breed which is more satisfactory than any of those figured. It can be obtained of 

 Bausch and Lomb Optical Co. 



