86 FRESH-WATER BIOLOGY 



Under-water Photography 



If the water is clear and the surface unruffled, near objects may 

 be seen almost as clearly in natural waters as in air. If the 

 camera be pointed at them, the resulting picture rarely shows more 

 than the surface of the water, as opaque as that of milk and with as 

 little visible beneath it. It is as though the camera has been pointed 

 at the blue sky. This result is due to the light of the sky and other 

 distant objects retlected from the surface of the water into the 

 camera. This strong light, which the eye neglects, obscures in the 

 negative the effects of the weaker light from objects beneath the 



Fig. 26. The sirr: -, v. u>e Inr photM-r.ipliiii^ wl.jcii. un i. , .1 ,..r description see text 



(From an original photograph.) 



surface of the water; if it be cut off by a screen these objects ma^ 

 be photographed. 



This is shown (Fig. 26) in a photograph of the nest of a black bas: 

 in about eight inches of water. Little can be seen beneath th( 

 water, except within the reflected image of the screen. Withii 

 this image the reflected sky light is cut off, although the sun shine: 

 from the left full upon the nest of clean stones. What is clear ii 

 the photograph lies not within the shadow of the screen but withii 

 its image. A longer exposure would have given a clear picture o 

 what Hes within the narrow shadow at the bottom of the screen 

 In field practice a serviceable and portable screen may be made b; 

 tying a square of black, opaque cloth to two poles stuck slantinj 



