WORKING UP MATERIAL 493 



that the position of the microscope objective over the stages may be noted at any time 

 and then later returned to the same spot if required. The compound microscope body 

 may be removed and replaced by a simple lens-holder for viewing larger objects if 

 desired. 



The larger and rarer organisms may be counted over the whole section and its counter- 

 part of plain gauze. Smaller and more numerous organisms may be counted over 

 selected areas and an estimate made of the approximate number on the whole section 

 and its counterpart (see the next section on the interpretation of results), i-in. 

 and |-in. objectives are used for the examination of the majority of the zooplankton 

 and larger phytoplankton forms (f-in. objective only), a |-in. objective being used for 

 the smaller forms. If only parts are selected for examination they should consist either 

 of a complete traverse across the section from side to side (at right angles to the line of 

 motion of the gauze across the water tunnel in the machine) or areas covering repre- 

 sentative parts of such a traverse. This is important because, owing to the slightly 

 asymmetrical path of the water tunnel through the machine, the plankton tends to get 

 thrown in larger numbers towards one side of the gauze than the other (see possible 

 future modification of machine, p. 501). 



It may not always be possible in the time available to carry out a complete analysis, 

 so that only the forms considered to be of importance in a particular investigation can 

 be identified and counted. After examination on the stage, however, it is advisable that 

 the plankton should be removed and stored in tubes numbered to correspond with the 

 respective sections, so that it may be later referred to should it be desired to carry the 

 analysis further in the light of subsequent events. The larger forms may be picked off 

 by forceps or sable brush and the remainder taken off with a safety-razor blade mounted 

 in a handle. The needs of each particular piece of work will dictate the degree to which 

 analysis is carried. For some classes of work it may be desirable to make a full quanti- 

 tative analysis of every second, third or fifth section, and only a cursory examination of 

 the intervening ones to see that no important variation in the plankton density has been 

 missed. 



THE INTERPRETATION AND CHARTING OF RESULTS 



The process of analysis results in a record of the varying numbers of the different 

 organisms caught in each of the consecutive sections of the gauze. The distance of the 

 sea traversed for every section of gauze exposed is obtained by dividing the total dis- 

 tance of the run (known from the reading of ship's automatic log) by the number of 

 sections exposed. If 50 sections have been exposed in a run of a 100 miles, for most 

 practical purposes each section may be said to correspond to a distance of 2 miles of sea ; 

 actually, as will be shown presently, this is not strictly true, for each section will also 

 sample a little of the water immediately preceding and following its corresponding 

 2 miles. But as a rule this effect may be disregarded (see below). 



If one is merely making a study of the irregularity of distribution in different areas 



