PHOTOGRAPHY OF AQUATIC ANIMALS. 61 
such a camera in longitudinal section. The ground glass (gl) is placed, not at the 
back of the camera, as is usual, but in the top. The operator, holding the camera in 
front of him, looks in the direction indicated by the upper arrow, at the ground glass 
through the hood (h kh’), which takes the place of a focusing cloth. The interior of 
the camera contains a mirror (m), which extends from beneath the back edge of the 
cround glass downward and forward at an angle of 45°. The mirror is hinged at « 
to the top of the camera. When it is in the position shown at m in the figure the 
space between the back of the mirror and the back of the camera is quite dark. 
Light entering through the lens is reflected by the mirror and strikes the ground 
glass, as shown by the line y y y’.. The image as seen on the ground glass by the 
operator looking down through the hood is, on account of the action of the mirror, 
an erect image, not an inverted image such as one sees on the ground glass in the 
back of an ordinary camera. It is also an image of the full size permitted by the 
plate and the lens, not a reduced image such as one sees in a finder. The shutter 
(s) is a focal plane shutter situated at the back of the camera just in front of the 
plate (p). Such a shutter is essentially a roller curtain of black cloth with a slot (s/) 
across it at one point. The width of the slot may be regulated. The shutter is 
wound upon an upper roller (7) until the slot is upon the roller. The exposure is 
made by causing the curtain to unwind from the upper roller (7) and wind upon the 
lower roller (r’) so that the slot passes very rapidly across the face of the plate. 
The length of the exposure depends on the width of the slot and the rate at which it 
moves. The rate may be varied by changing the tension of the spring which 
actuates the lower roller. The operator holds the camera in front of him with both 
hands while he looks down at the ground glass through the opening in the hood. 
With one hand he focuses. When the object appears in sharp focus and in the 
desired position on the ground glass, he presses a button with the other hand. 
This causes the mirror to swing on its hinge to the position shown by the dotted out- 
line m’ beneath the ground glass. In this position the mirror excludes light which 
might otherwise enter the camera through the ground glass. At the same time the 
change in position of the mirror permits the light, which was before reflected to 
the ground glass, to fall upon the plate. The adjustment is such that an image 
which is in sharp focus on the ground glass will be in sharp focus on the plate when 
the mirror changes position. The image does not actually strike the plate so long 
as the shutter is wound upon either roller. Before the instrument is to be used the 
shutter is wound on the upper roller. When the mirror in swinging upward reaches 
the position m’ the shutter is released from the upper roller and taken up on the 
lower roller. As the slot passes across the plate from above downward, the image 
falls through the slot onto the plate in successive strips corresponding to the width 
of the slot. 
The advantages of this form of camera are the following: 
1. The operator sees a full sized, erect image on the ground glass, while at the 
same time the sensitive plate is in position for exposure. 
2. He is able to focus and to regulate the position of the image on the ground 
glass up to the instant of exposure. 
3. Much more rapid exposures may be made with the focal plane shutter than 
with the ordinary diaphragm shutter. The diaphragm shutter occupies a considerable 
