NO. 10 WATER TRANSPARENCY — WILLIAMS, JOHNSON, DYER 3 



ure the medium in its natural environment and (2) laboratory-type 

 instruments in which a water sample is removed from the medium 

 and examined in the shipboard laboratory. The first type is usually 

 considered the more reliable when dealing with natural waters, since 

 the transparency properties seem to change rather markedly when a 

 sample is taken out of its natural environment, and therefore this type 

 is discussed first. 



I. IN SITU INSTRUMENTS 

 A. Secchi Disc (pi. 1, fig. 4) 



The Secchi Disc, owing to its ruggedness and ease of use, was the 

 most often used of any of the devices to be listed. The disc used 

 was j\ inches in diameter and was painted a flat white, having a 

 reflectance coefficient of about 0.8. It was obtained from the Oceano- 

 graphic Institution at Woods Hole, Mass. A specially designed hydro- 

 scope (pi. 1, fig. 3) was occasionally used in conjunction with the 

 Secchi Disc to eliminate water-surface effects. Generally the Secchi 

 Disc was observed by means of a glass-bottom bucket. Readings were 

 made from the sunny side of the ship, except where otherwise noted 

 in the data tables, and the recorded value is the distance from the 

 bottom of the hydroscope to the disc, i.e., the distance traveled by the 

 reflected light from the disc surface through the medium in which it 

 is suspended. 



B. Point Source Light 



On a number of occasions the transparency of water was measured 

 by observing the distance at which a point source of light can be seen. 

 This method of measurement may be seen to be similar to that of the 

 Secchi Disc. 



Although a true point source of light is well-nigh physically impos- 

 sible, the tungsten filament of a 1,000-watt diver's lamp approximated 

 this well enough for the range of transparency encountered in the 

 near coastal and inland waters. It unfortunately fails badly in the 

 ultraclear sections of the open ocean, where it diminishes in size and 

 eludes the observer before reaching extinction through absorption. 



In turbid waters the point source shows up as an incandescent spot 

 surrounded by scattered light having the appearance of luminescence 

 in which the visual range is the point at which it disappears into the 

 background of scattered light. In clearer water, on the other hand, 

 the background of scattered light, if it can be seen at all, is seen only 

 when the point source is close to the observer and disappears while 



