LABORATORY METHODS 2q 



slightly thicker than the rest making squares of i cm. as well as the small squares 

 of 2 mm. It is passed to and fro below the microscope by the Murray long-range 

 mechanical stage until the analysis is complete. Diatoms if present in overwhelming 

 numbers are counted on a restricted number of squares selected at random from 

 different parts of the slide and the quota calculated. Only the common outstanding 

 species of diatoms were identified in the N 70 V samples, since they are better sampled 

 by the N 50 V net in the analysis of which they are treated in greater detail ; important 

 records of diatoms at levels below that at which the N 50 V was fished were, however, 

 obtained by the N 70 V nets. 



Analysis of N 70 H samples 



The N 70 H is made to a similar specification to that of the N 70 V, but was towed 

 for a distance of \ mile (402 m.) through the upper layers of the water which are 

 sampled by hauls with the N 70 V of only 50 m. vertical distance. They therefore caught 

 samples of greater size, and a modification in the method of analysis was made by 

 Mr Andrew Scott. After the larger organisms had been picked out and recorded the 

 sample was washed through a sieve which retained the majority of the catch, including 

 most of the Copepoda, but allowed the very small organisms, diatoms, Protozoa and 

 the smaller Copepoda, to pass through. The volumes of the two portions of plankton so 

 obtained were measured and then each portion sampled with a stempel pipette and 

 analysed as in the N 70 V samples. A fraction of 1/120 was usually taken from the por- 

 tion containing the larger organisms, by taking up 1/60 with the pipette from half its 

 volume. A larger fraction was usually taken from the portion containing the smaller 

 organisms, for the quantity was also usually much smaller. 



Analysis of N 100 H samples 



This net, designed for the capture of the larger plankton organisms only, allows the 

 smaller forms to pass through its meshes. Here the method of analysis is simpler, the 

 examination being carried out usually with a simple microscope. When the sample is 

 of reasonable size the whole is counted, the sample being spread out on a Petri dish 

 placed over a disc divided into alternate black and white octants. With larger samples 

 only half or a quarter would be examined in this way. Catches with this net were some- 

 times very heavy, as when numbers of Euphausians, Amphipods, or Salps were taken, 

 amounting at times to a volume of some 20 litres. In such cases the majority of the 

 organisms would be picked out, counted and recorded on board ship and then thrown 

 away, leaving a small sample for subsequent examination and reference. In instances 

 of very heavy pure catches of one organism, such as occurred with Euphausia superba 

 and Parathemisto gaudichandi, their number would be estimated by displacement 

 methods after counting the number of organisms displacing a given volume of water. 



