MEASURING THE PRODUCTIVITY OF THE SEA 



63 



The photometer used on the ex- 

 pedition. The photo-electric cell 

 is enclosed in the three-armed 

 container. 



fluence of light on plants. Plant production is entirely dependent on the 

 visible rays and a little of the ultra-violet part of the spectrum. Within 

 this range the influence of the rays varies. The maximum energy in sun- 

 light is provided by the green-yellow part of the spectrum. 



Some of the light which strikes the surface is lost by reflection. The 

 amount depends on the sun's altitude; and it is only when the sun is 

 very low on the horizon that a considerable proportion of the light is 

 reflected. At an altitude of over 30° the percentage of reflected light is 

 small. The altitude of the sun is also important in another respect: the 

 rays penetrate deepest when they are vertical. 



The penetration of light varies also with the wavelength; that is to 

 say, with the colour. Here, however, there are wide regional differences, 

 so that we must also consider the ways in which light is absorbed. The 

 intensity of light as it penetrates the sea is reduced by the following fac- 

 tors : absorption by the water itself and by matter dissolved in it ; scattering 

 by the molecules of the water; absorption by organisms and particles; 

 scattering by organisms and particles. Particles may be either organic or 

 inorganic. 



These factors have a somewhat different effect on light of different 

 wavelengths. Pure water rapidly absorbs the red part of the spectrum, 

 the green part much more slowly, and the blue part most slowly of all. 

 The rays at both ends of the visible spectrum are absorbed with great 

 intensity. Scattering by the molecules of the water is greatest in the blue 



