434 tyIjER and pbeisendorfeb [chap. 8 



sources of error are perturbation of the light field as a result of the presence of 

 the instrument and forward scattered light within the beam (which is un- 

 wanted). Preisendorfer has estimated that, for a one-meter instrument with a 

 1-cm diameter beam of light, the error due to perturbation is 0.3% when it is 

 used in water having a = 0.402/m and ao= 1.92/m steradian. 



An a-meter of advanced design for oceanographic work is in use by the 

 Institute of Oceanology of the Academy of Sciences, U.S.S.R. (Kozlyaninov, 

 1958j. The optical system of this 1-m instrument is shown in Fig. 13. The 

 vacuum phototube, 6, monitors the flux output from the lamp, 1. Not shown 

 is the modulator which modulates the direct beam and also the comparison 

 beam. The two pulsating signals thus obtained are compared by means of a 

 balance circuit and their ratio is plotted on an EPP-09 (Russian) strip chart 

 recorder. This a-meter has calibration filters as well as color filters, 16, which 

 can be introduced into the beam from a remote control station. The instrument 

 is built to withstand depths up to 200 m and, in addition, has a sample tube 

 that fits between the windows, 9 and 10, which can be used for the measure- 

 ment of samples taken at greater depths. 



A great deal of specialized information is available in the literature on the 

 variability of the total attenuation coefficient, a, with location, depth, wave- 

 length, time and other parameters. Fig. 14 gives the wavelength variability of 

 distilled water after Dawson and Hulburt (1934), Hulburt (1945) and Curcio 

 and Petty (1951). 



5. Volume Scattering Functions and Total Scattering Coefficient 



The measurement of the volume scattering function is difficult. From 

 equation (16) it can be seen that the measurement requires volume calibration 

 as a function of angle as well as a determination of input irradiance. Because of 

 the difficulties associated with these calibrations and, perhaps, because of a lack 

 of interest in the real magnitude of the volume scattering function, there have 

 been very few instruments specifically developed for this measurement. 



Some instruments and techniques which have been described recently are 

 applicable to the problem of measuring the volume scattering function and these 

 will be mentioned briefly. 



The need for an in situ type instrument with a controlled sample volume was 

 recognized by Waldram (1945), who was interested in light scattering in the 

 atmosphere. Waldram developed a rotating stop which operated to maintain 

 a constant sample volume as the angle for (j{d) determination was changed. A 

 stop of this design has been incorporated by Tyler (1958) in a nephelometer for 

 measurements of the volume scattering function in natural waters. Tyler's 

 instrument, shown in Fig. 15, was designed for in situ measurements and has a 

 cylindrically restricted beam of detectivity as well as a cylindrically restricted 

 beam of light. Stray light is controlled by internally baffled lens shades which 

 can be seen in Fig. 15 and by the black trap which forms the background for 

 the beam of detectivity. Experimental values of directional intensity, input 



