GERMAN PLANKTON INVESTIGATIONS. 297 



of metal is cut away from the cylinder. The pipette is 

 weighed. The cavity is then filled with mercury, and the 

 pipette re-weighed. The weight of a ccm. of mercury 

 is known, and more and more metal is cut away until the 

 difference between the two weighings becomes equal to 

 this. There are six such pipettes in common use, of 

 capacities OT, 0'2, 0'5, 1, 2*5 and 5 ccm. These pipettes 

 are passed through a strong cork (k), which is fitted into 

 a glass vessel with strong walls. The fluid is poured into 

 the latter vessel, and thoroughly shaken up, so that the 

 plankton becomes evenly distributed throughout the 

 whole volume, and as soon as this is the case the piston 

 is pulled quickly up into the glass tube, so that the space 

 (n) between the metal cylinder and the walls of the tube 

 contains an exactly known volume of liquid. Before this 

 fluid is subsequently emptied out it is as well to smear 

 the under end of the glass tube with a little fat, as other- 

 wise a drop of the fluid might easily hang there. After 

 the volume has been ejected from the pipette, the latter 

 is washed out with a little water, so that no organisms 

 may remain behind. It is impossible to use the ordinary 

 measuring pipettes in this connection, because the open- 

 ing is so small as to become stopped up by some of the 

 larger organisms. 



If the dilution is sufficient for the immediate purpose, 

 the counting may now commence, and for this a special 

 form of microscope (fig. 7) has been devised. This micro- 

 scope has a very large mechanical stage,* which is able to 

 carry a glass plate of 11*5 by 10 cm., and this, by means 

 of two screws, can be moved at will in either of two direc- 

 tions, from or to the observer, or sideways. This glass 



* Mechanical stages are supplied by Zwickert, Optician, Kiel, and are 

 made to fit suitable microscopes, from 60 marks upwards. 



