512 HEKSEY AND BACKUS [CHAP. 13 



C. Television 



Underwater television is often proposed as a tool for scattering-layer re- 

 search. The chief objection to its use that has been raised is the necessity for 

 illuminating the animals to be viewed whereas the vertical movements of 

 these organisms appear to be controlled by light itself. The reality of this 

 objection was confirmed by Backus and Barnes (1957), who made night-time 

 observations with a surface echo-sounder of the effect of a television camera 

 suspended in a deep scattering layer located about 35 m below the sea surface. 

 The lights of the television camera were alternately turned on and off. The 

 repellent effect of the lights on the scatterers was large. Nevertheless, counts of 

 small plankters appearing on the television screen were made in this layer, and 

 at several levels above and below it, using the lights only long enough to make 

 the counts. These observations were repeated at the same depths during day- 

 light hours when the scattering layer had migrated to a level well below. The 

 similarity of the day and night counts indicated no relationship between the 

 micro-plankters observed (mostly copepods) and the scattering layer. In other 

 experiments an echo-sounder transducer was mounted on the gantry of the 

 television camera and simultaneous observations of mid- water sound-scatterers 

 were made with the two tools. A high correlation between the appearance of 

 small fishes on the television screen and the appearance of strong echo -sequences 

 on the echo-sounder record was observed, supporting the earlier observations 

 of Johnson et al. (1956) made with suspended echo-sounder and camera. 



D. Direct Observation 



Direct observation in deep scattering layers is an approach of considerable 

 dromise. For instance, Peres (1958) in dives in the bathyscaph "F.N.R.S. 3" 

 off the south of France was able to make generalities concerning the vertical 

 distribution of various sorts of bathypelagic fishes with some confidence. The 

 point penetrations of scattering layers which these deep-diving vehicles now 

 make, however, permit little to be seen of animals in strata of low population. 

 However deep-going, sonar-equipped submarines of the near future which can 

 cruise widely at depth should contribute materially to the solution of scattering- 

 layer problems. 



4. Sound-Scattering Theory 



We know from the discussions of Chapter 12 that the fraction of energy 

 reflected specularly at a plane surface is a function both of the angle of in- 

 cidence and of the acoustic impedances of the two media. (Acoustic impedance 

 is the product of density and sound velocity or pc.) The greater the contrast 

 between the acoustic impedances of the two media, the more intense the 

 reflected sound when the sound strikes the boundary at nearly normal in- 

 cidence. The same generality holds for sound scattered by discrete objects. 

 However, the shape, size and orientation of the scatterer affect the intensity 



