NO. IO WATER TRANSPARENCY — WILLIAMS, JOHNSON, DYER 5 



bined output of the two cells is adjusted by means of light irises so that 

 the meter in the control box reads ioo percent when the underwater 

 unit is in air (air is assumed to be a nonattenuating medium). Then, 

 as an attenuating medium such as water is placed between the light 

 and photocell P lt the meter will read some fraction of ioo percent. 

 Actually, since there is a light loss of about 4 percent per glass-air 

 interface owing to the different indices of refraction of glass and air 

 which does not occur when the device is submerged because of the 

 similarity of glass and water indices of refraction, the reading in air 

 should be set to 92 percent instead of 100 percent [34]. 



There is a definite temperature effect on the device, but in view of 

 the sources of error existent in the other methods of measurement 

 and the length of time required for an internal temperature change 

 to occur, it is felt that this temperature dependence is negligible. This 

 temperature effect is reported in the National Bureau of Standards 

 Text No. 43P-1/47. 



F. Hydroscope 



This instrument is essentially an underwater telescope having a 15 

 field of view with interchangeable heads for either vertical or hori- 

 zontal viewing for Secchi Disc or other visibility range readings. 

 Plate 1, figure 3, shows the device which is approximately 15 feet 

 long and uses a lens system of unit magnification. The viewing head 

 is equipped with a focusing eyepiece, a rubber face pad to exclude 

 external light, and two positioning control handles. 



In use, the hydroscope is supported in a ball-and-socket mount on 

 a platform extending from the side of the ship, with the objective 

 head of the instrument extending 5 feet below the water surface. 



II. LABORATORY TYPE INSTRUMENTS 

 A. Peraqnamcter (pi. 2, fig. 1) 



This device is very similar in principle to the illuminated letter 

 described above, except that the letter to be identified is placed in a 

 long tube (11 feet long) which is filled with the water of interest by 

 means of a pump. The observer looks into this tube and is able to 

 move the image of the letter, by means of a movable mirror, until 

 positive identification is possible. 



The peraquameter was used when visual range, using the illumi- 

 nated letter, was found to be under 22 feet. 



