STUDYING THE HABITS OF FISHES. 



1 1 23 



glass for the purpose of rendering the surface of the water smooth enough for a 

 photograph to be made through it. Figure 2, plate cxiv, shows such a combi- 

 nation of water glass and screen as used for photographing the lampreys of 

 figure 6. The broad white bands in figure 6 are the edges of the water glass. 

 It is to be noted that within these bands the picture is clear while outside 

 them it is not clear. The lack of clearness in that part which lies outside the 

 borders of the water glass is due to the running water, which is there much 

 disturbed, as shown in figure 2, plate cxiv. 



(b) The second method of photographing objects beneath water in their 

 natural environment is to inclose the camera in a water-tight box and immerse 

 this, and for this purpose a reflect- 

 ing camera should be used. The 

 principle of siich an instrument is 

 illustrated in the accompanying fig- 

 ure, which shows it in longitudinal 

 section. The light entering the 

 camera is reflected by a mirror, m, 

 to a ground glass, gl, in the top 

 of the camera and the image formed 

 by it is viewed by the photogra- 

 pher as he looks down through the 

 hood, /;/;', on top of the camera. 

 This image is of full size, and owing 

 to the action of the mirror is erect. 

 While looking at it the operator may 

 focus the camera and thus keep the 

 image always sharp, no matter how 

 much the object may move. The 

 mirror is hinged at its upper edge at 

 X, and the operator can, by pressing 

 a release button, cause it to swing 

 upward until it reaches the position 

 7n'. It then covers the lower surface of the ground glass in such a way as 

 to prevent the entrance of light through it. The light, hitherto reflected upward 

 from the mirror, now passes backward and may form an image on the sensitive 

 plate, p. In front of the sensitive plate there is a shutter, s, of the focal-plane 

 type. This is essentially a roller shade of black cloth which may be wound on an 

 upper roller, r, and made, by the action of a spring, to unwind very rapidly from 

 the upper roller and wind on a lower roller, r'. In passing from one roller to the 

 other the curtain passes in front of the plate. In one place in the curtain is a trans- 

 verse slot and through this the light falls upon the sensitive plate as the slot, si, 



Fig. 2. — A reflecting camera shown in section, with magazine 

 plate holder attached. &/, Ground glass; h h' , hood. /. lens; 

 ni , mirror in position during focusing: vi', mirror, showing 

 position during exposure; p. sensitive plate; r and r', rollers 

 of focal plane shutter; s. the shutter; .«/, slot in shutter; x, 

 hinge on which mirror turns; y y y. ray of light traversing 

 the lens and reflecting from the mirror to the ground glass. 



