METHODS OF OCEANOGRAPHICAL RESEARCH 261 



on the amount of salt contained, on the alkalinity or 

 acidity, and on the presence of minute organic substances 

 in solution. 



A further physical ol)servation that has to be made is 

 the penetration of light into the sea. It is interesting to 

 know the strength and colour of the light at different 

 depths. A rough and ready way of noting the trans- 

 parence of the water from place to place is to lower a 

 circular, white disc to the depth at which it disappears. 

 This method of course gives little information beyond a 

 mere rough comparison of the transparency of different 

 regions. Special cameras have been used which can be 

 exposed for a definite time at any required depth, but they 

 have yielded little information beyond recording the 

 actual presence and colour of the light. Recently the light 

 has been measured electrically by means of photo-electric 

 cells and it is to be hoped that by this means an advance 

 will soon be made in our knowledge of the light under the 

 sea. 



This completes a short outline of the methods used in 

 studying the actual physical and chemical conditions 

 in the sea — the depth, the currents, the temperature and so 

 on. It yet remains to be said how knowledge is obtained 

 of the living world present therein. All the various 

 animals and plants that live either creeping on the bottom, 

 or swimming or drifting in the water above the bottom, 

 have to be captured and the methods of catching have to 

 be well thought out because not only are the animals 

 wanted in order that their structure may be examined, 

 but also that numbers may be obtained showing their 

 actual abundance from place to place and from season to 

 season. 



One of the first instruments to be used for the capture 

 of animals from the sea bottom was the " naturalist's 



