THE CAMERA OBSCURA. 477 



a b e d efg (in the figure the side a b c d is partly cut 

 away so as to allow the internal arrangement to be seen) 

 has in the front side a tube r, in which another tube 

 with the lens may be moved to and fro with moderate 

 friction. In the back part of the box there is the in- 



c 



FIG. 274 (an. proj., | real size). 



clined mirror a g h i the part a g k I of the top of the 

 box is formed of a plate of glass which is either ground 

 or covered with semi-transparent paper. Without 

 the mirror tbe image produced by the rays which 

 pass through the lens and tube would be at B l ; the 

 mirror reflects the rays so that the image appears at 

 -Z> 2 upon the plate of glass. It may be seen conveniently 

 by standing behind the camera, bending the head forward 

 over the glass plate, throwing a thick cloth over head, 

 shoulders, and camera (except the tube), and moving 

 the tube with the lens to and fro until a distinct image 

 appears. 



For our lens of 28 cm focal length the box should be made 23 cm 

 long, and 15 cm wide and high. The glass plate (a piece of flat 

 window-glass) should be 15 cm long and wide ; the portion of the 

 upper side which is not transparent will thus be 8 cm long. The 

 mirror which makes an angle of 45 with the back of the box is 

 15 cm wide and 21 em< 2 long ; it is kept in position by four little strips 



