164 

 The Relative Merits of Transparency Instruments 



The antiq-uityi, simplicity^ and obvious inherent errors in 

 the use of the Secchi disc for the determination of the trans- 

 parency of water has lead to mnch skepticism as to the validity 

 of data obtained from the instr'ramento Many conditions other 

 than water t?irbidity effect the depth to which an observer can 

 see a Secchi disc The amount of cloud cover, the state of 

 the sea? the seasosii of the year, in addition to the experience 

 of the observer are certainly among the most important. Even 

 sOd the compilation of data gathered by the use of the Secchi 

 disc by astute observers has yielded remarkably correlative 

 information (Emery, 1954), Obviously^ the readings cannot be 

 considered absolutes, but the relative values have been so 

 frequently exhibited in the past three years in the nearshore 

 areas of the Los Angeles region^ that they cannot be considered 

 to be completely aberrant. This coupled with the simplicity 

 of operation and the ready mader standing of the results by 

 the non-scientist make it seem that the instrument may still 

 be of some value in any marine program „ 



The hydrophotometer, because of the theoretical precision 

 of its measurements!, has been considered to be a much more 

 accurate method of determining the transparency of sea water. 

 The instrument can be calibrated, the calibration standardized, 

 and the intensity of the light source set so that a constant 

 reproducibility in readings may be obtained. The instrument 

 may be lowered to any depth so that the transparency within a 

 narrow horizontal layer can be measured with accuracy at any 

 interval desired and reproduced under any set of sea or sky 



