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 (ec) is pointed toward a fish (v—-) 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 x2’, 
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 
Fic. 1.—Diagram illustrating the photography of objects beneath the water by acamera above shown in the fio- 
the surface. Forexplanation see the text. = 
ure. Some of 
these rays converge to the lens, and thence diverge to form on the ground glass 
the image 7 7’. 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 2’, 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 
