6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 139 



B. Scattering Meter (pi. 2, fig. 2) 



To measure light scattering due to suspended particles in natural 

 waters, Dyer developed a device which essentially consisted of a light 

 source that sent a beam of light through the sample. At right angles 

 to the beam, a photocell was placed, and the amount of scattering was 

 then a function of the output of this photocell. 



The sample cell used was first a 2,\" x 2^" x 1" rectangular glass 

 container, but this was later changed to a 3" x 3" x 2" plastic cell to 

 handle a larger sample and at the same time defeat the problem of 

 condensation on the outside of the cell due to cold-water samples. 



The electrical circuit was so designed that the output current of the 

 photronic tube affected the grid current of an amplifier tube, thus 

 causing changes in the plate current of the amplifier for small changes 

 in the output of the photocell. A microammeter with scale ranging 

 from o to 100 was selected as an indicator of the degree of scattering 

 and was connected in the plate circuit of the amplifier. The circuit 

 was adjusted so that the output current could be zeroed for any given 

 beam intensity with the sample cell empty. For operating con- 

 venience, a reflecting rod was so mounted that it could be swung into 

 a fixed position in the light beam in order that a check could be main- 

 tained on the source light output by means of its effect on the output 

 of the photronic cell. The entire unit, including batteries, was mounted 

 in a glass-fronted metal case for convenience. 



As finally evolved, the device proved capable of covering the entire 

 range of turbidity from Delaware River water to the finest obtainable 

 grade of triple-distilled pharmaceutical water. 



METHODS OF DATA ANALYSIS 



For the sake of uniformity it seemed desirable to convert all the 

 hydrophotometer readings to "Equivalent Secchi Disc Readings," as 

 defined in a previous section. To do this required some relationship 

 between actual Secchi Disc readings and hydrophotometer readings, 

 which was not readily available. Williams, however, has developed an 

 expression involving the extinction coefficient as a function of the 

 Secchi Disc reading, and since the hydrophotometer transparency 

 measurement is similar to the extinction coefficient measured under 

 ideal conditions, it was decided to use this approach. 



Let: 



B 3 = Illumination at the sea surface. 



Bo =■ Brightness of the Secchi Disc as seen by the eye. 



