3 io 



OUR PHYSICAL WORLD 



The lens may be moved nearer to, or farther from, the sensitive 

 plate as is required to obtain a sharp image of the object. In 

 plate cameras of this type (Fig. 157), there is a ground-glass 

 screen covering the opening on the opposite side of the box 

 from the lens. One throws a black cloth over his head and also 



FIG. 157. A plate camera on its tripod: (a) adjusts time of exposure; 

 (b) adjusts size of diaphragm opening; (c) raises or lowers the lens; (d) moves front 

 back and forth; (e) swings back on its vertical axis; (/) moves back of camera 

 forward or backward; (g) swings back of camera on its horizontal axis; (h) plate 

 holder. 



over the camera box, as in the case of the pinhole camera above, 

 and then moves the lens back and forth until the image seen on 

 the ground glass is perfectly sharp. The plate is then inserted 

 into the camera in a plate holder in the same position that the 

 ground glass occupied when the camera was focused. 



In film cameras of this type a small pointer is attached to 

 the lens board. Under this pointer lies a fixed scale. If the 



