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 Ji'), which takes the place of a focusing cloth. The ulterior of 

 the camera contains a mirror (m), which extends from beneath the back edge of the 

 ground glass downward and forward at an angle of 45. The mirror is hinged at x 

 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 (si) 

 across it at one point. The width of the slot may be regulated. The shutter is 

 wound upon an upper roller (r) until the slot is upon the roller. The exposure is 

 made by causing the curtain to unwind from the upper roller (r) and wind upon the 

 lower roller (/) 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 hi 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 folio wing : 



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 



