4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 139 



the incandescent spot is still plainly visible. The energy from this 

 spot is so reduced by attenuation that the structural shape of the 

 filament can be clearly seen. The visual range is then taken to be the 

 distance to that point at which the filament completely disappears. 



Most of these observations were made horizontally with the lamp 

 and the objective of the hydroscope both placed 5 feet below the 

 water surface. For the sake of completeness, observations were made 

 both during the day and at night. Plate 1, figure I, shows the point 

 source of light being observed through the hydroscope. 



C. Illuminated Letter 



This observation method involved the use of a low-powered lamp 

 enclosed in a small housing with an opal glass window, in front of 

 which was mounted a rotatable disc which had a series of cutout let- 

 ters. The whole rig was mounted on a pole which could be extended 

 approximately 5 feet below the surface and was observed by means of 

 the hydroscope. The procedure adopted consisted of bringing the illu- 

 minated letter toward the hydroscope in a horizontal direction until 

 the observer could make a positive identification of the nature of the 

 letter. 



D. Underwater Objects 



To obtain some idea of the horizontal visibility available at various 

 stations, black and white balls approximately 6 inches in diameter 

 were lowered about 5 feet below the surface of the water and observed 

 with the hydroscope. The horizontal distance at which the balls dis- 

 appeared from view was recorded. 



E. U.S. Navy Hydrophotometer Mk. II (pi. 1, fig. 2) 



To obtain a measure of the variation in transparency with depth, 

 standard U.S. Navy hydrophotometers were used quite extensively. 

 They consisted of two principal parts; a control box and an under- 

 water unit connected by an electrical cable. The underwater unit may 

 be lowered to any desired depth and the transparency at that depth 

 is indicated at the control box. It is very similar in its operation to 

 a number of earlier instruments [6, 29, 33]. 



The underwater unit consists of two heads separated by a fixed 

 distance of 0.5 meter, one head containing a photocell, P lf and the 

 other containing a collimated light source and another photocell, P 2 

 which is connected so that its output is in opposition to the output of 

 cell P 1 . In operation the light shines both on P x and P 2 and the com- 



