44 



BULLETIN OF THE BUREAU OF FISHERIES. 



viewed through air alone. In the latter case the photographer may set up his 

 camera and find that he can get a perfect image on the ground glass; but if he 

 then exposes a plate with the expectation of getting a good negative he will be 

 sorely disappointed. Only rarely does he get any image at all of what lies beneath 

 the water's surface. Usually the negative shows only the surface itself, and that 

 appears as opaque as though the camera had been pointed into a lake of tar. The 

 writer has often attempted such photographs, to find on his negative no visible 

 impression of the fish which showed so clear on his ground glass. This is doubtless 

 a common experience. Why is it? 



Explanation will be clearer by referring to the diagrammatic figure 1, where 

 the camera (c) is pointed toward a fish (x-y) beneath the surface (a-b) of the 



water. The fish 



^^ is illuminated 



chiefly by rays of 

 light which enter 

 the water almost 

 vertically. The 

 rays of light x x', 

 y y', reflected 

 from the fish, 

 which strike the 

 surface of the 

 water from below 

 at an angle less 

 than 48 35' with 

 the vertical, 

 emerge into the 

 air, while at the 

 same time they 

 are bent from 

 their course, as 

 shown in the fig- 

 ure. Some of 



these rays converge to the lens, and thence diverge to form on the ground glass 

 the image i i'. When the photographic plate is exposed these rays, with the rays 

 from other submerged objects, form an image on it, as they do on the ground 

 glass. Yet this image does not appear in the negative, for if the surface of the 

 water is smooth it acts as a single great mirror which, although it permits a 

 part of the light to penetrate, yet reflects another part, greater the more obliquely 

 the light strikes the surface. For this reason the images of sky and trees 

 and other distant objects are often seen mirrored on the surface of smooth water. 

 Some of these reflected rays (z z', w w'), after leaving the surface of the water, 

 enter the lens of the camera and form an image. The fish is a near object and if 

 the camera is focused upon it the image of the fish is sharp on the photographic 

 plate. The reflected rays from the water's surface come usually from distant objects, 

 commonly sky or clouds. When the camera is focused on the fish a sharp image 



FIG. 1. Diagram illustrating the photography of objects beneath the water by a camera above 

 the surface. For explanation see the text. 



