Plate VI. 

 Photograph of Hving plankton taken 

 at sea by underwater flash. Only a few 

 organisms can be recognized — 

 3 medusae, Solmaris (cf. Fig. 14; 4) 

 and a salp (cf Fig. 23; 6) — the rest 

 torm an undefmed 'soup' reflecting 

 only highlights mostly out of focus 

 and blurred. About 1/12 natural size. 



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SO that accurately focused organisms appear as bright pin-pricks and the 

 others as round bku's, the size depending on how far they are out of focus. 

 The result is a 'soup' which gives an idea of the abundance of the plankton 

 but only rarely anything identifiable (Plate VI). Sometimes an opaque 

 organism can be seen, if it happens to be in focus. Some can be identified 

 because their shape gives a characteristic series of high-lights — for example 

 the tentacles of a jellyfish which are not simple hair-hkc threads but a series 

 of bulges each of which has its high-light as would a string of beads. Some 

 organisms arc sufficiently opaque to be photographed quite successfully. 



The closed circuit television has special advantages in that it can be 

 controlled in direction, focused, and especially because the actual movement 

 so often gives the organism away. 



Observations from underwater chambers can give valuable information 

 from places otherwise beyond reach; some details of plankton distribution 

 at great depths deduced from the observations made from the bathyscaphe 

 are given in Chapter 8. These methods arc mentioned here, but few indeed 

 will have the opportunity to use them. 



This chapter must end with a short account of preservatives. An un- 

 preserved plankton sample, at the concentration convenient for examination, 

 will not remain alive long, and on death it will encourage a fast and very 

 obnoxious growth of bacteria. The simplest method of preservation and 

 one of the most satisfactory, is by formaldehyde. A concentrated solution 

 is about 40 per cent formaldehyde, and this solution is known as formalin. 

 This is diluted in use to about 4 or 5 per cent formalin (= about 2 per cent 

 formaldehyde), but allowance must be made for the water content of the 



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