GERMAN PLANKTON INVESTIGATIONS. 305 



heading the amount of the dilution is recorded. 2"5 : 50 

 means that 2' 5 ccm. of the original volume has been 

 diluted with 47'5 ccm. of water, so that the total volume 

 becomes 50 ccm. It is always better to dilute to either 

 100, 50 or 25 ccm., because flasks of these capacities can 

 be easily obtained, the exact volume being indicated by a 

 mark on the neck of the flask, and so any required dilu- 

 tion can be readily prepared for use. For the last plates, 

 from 7 to 14, the original volume is taken, but from it 

 only the larger species are enumerated. The next space 

 is headed " number," and refers to the number of the glass 

 plate used for the counting. As a rule ten plates are 

 found sufficient. The form appended gives, however, 

 fourteen. On the right of the form these successive 

 numbers are used as head numbers. With reference to 

 the "volume used," the numbers in this column signify 

 exactly what volume of water has been used on each 

 individual plate. For instance, on the first plate one- 

 tenth of a ccm. was used, and this was from the dilution 

 2'5 :50. This volume of liquid would be measured out by 

 means of the " Stempelpipette described above, and then 

 brought on to the plate. As seen by reference to the 

 form, several different volumes are used. Returning 

 now to the rows on the left, we come to the heading " true 

 volume." This differs from the volume used, and it 

 gives us the proportion of the original volume that is 

 actually investigated, whereas the volume used is the 

 volume actually placed on the plate. The true volume 

 is calculated as follows : — First of all 2*5 ccm. of the first 

 dilution have been subsequently diluted to 50 ccm., and 

 of this one-tenth of a ccm. has been taken. Now each 

 of the 50 ccm. of the second dilution contains one-fiftieth 

 of 2*5 ccm., that is 01)5 ccm. of the first dilution, and 

 one-tenth of a ccm., the volume actually counted, contains 

 u 



