SEA-FISHERIES LABORATORY. 191 



other allied organisms, but in the case of these minuter 

 forms several dilutions may be required before all the 

 species can be estimated. The scarce forms are taken 

 first, and after one or two further dilutions the numbers 

 of the more abundant species are arrived at. In dealing 

 with samples containing Rhizosolenia it was found 

 necessary to dilute the collection to 5,000 cubic centi- 

 metres before the numbers in J c.c. could be counted. 

 The examination of the Diatoms, &c, is carried out, under 

 the microscope, with either a § inch English objective or 

 a No. 3 Leitz objective (higher powers being used when 

 necessary for the critical examination of any species), 

 the 4 c.c. sample being passed very slowly backwards 

 and forwards across the stage. In front of the worker 

 there is a tray containing a number of empty glasses 

 with names corresponding to the species that are 

 usually found. A vessel containing small shot is 

 placed close to the hand, and as the sample is passed 

 under the objective of the microscope, each organism seen 

 is recorded by placing one of the shot into the corre- 

 sponding glass. When the examination is completed the 

 shot in each glass is counted and the estimation made. 

 This entire operation is repeated with at least five distinct 

 dips from each gathering. This method of estimation was 

 tested both against several dilutions of the one mixture, 

 and also against actual countings of the organisms 

 present, and the variation in all cases was found to be of 

 practically no account. The bottle which originally 

 contained the sample, and which was marked to show the 

 height occupied by the organisms, is now placed under a 

 graduated burette filled with water. The stopcock is 

 opened and the water allowed to run into the bottle up 

 to the mark exactly. The loss from the burette is read off 

 and this represents the volume of the catch in cubic 

 centimetres. 



