342 SCIENTIFIC AMUSEMENTS. 



photographic " camera " no reflecting arrangements are employed 

 to invert the image a second time or re-erect it, so that the 

 image formed on a ground glass plate is upside down. When 

 the camera is used for drawing purposes, or to obtain a picture of 

 surrounding objects inside a dark room (whence the name camera 

 obscura = dark chamber), an inclined mirror is employed to erect 

 the image. In the case of the photographic camera, the essential 

 points are that a perfectly achromatic combination of lenses must 

 be used, such that the visible and chemical rays are focussed 

 together at the same points (Expt. 365) ; and that the workmanship 

 should be such that after the picture is focussed on the ground 

 glass plate, and this is withdrawn and replaced by the plate carry- 

 ing the materials to be acted on by light, the latter plate may be in 

 exactly the same position as the former one, so as to produce a 

 sharp photograph. The lenses must also be so constructed and 



Fig. 188. Camera. 



proportioned as to give a " flat field," all parts of the image formed 

 on a vertical plane surface being as nearly as possible in focus, no 

 matter whether the objects in focus are all at the same distance or 

 not. A single uncorrected lens causes the image of a flat object in 

 front to be curved. 



Fig. 188 shows the use of the camera for drawing purposes ; 

 the rays of light, k, i, passing through the lens, fixed in a sliding 

 tube at the end of a box painted dull black inside, are reflected 

 upwards from an inclined mirror so as to form the image on a 

 horizontal ground glass plate. A lid, hg, working on a hinge pre- 

 vents the direct light from the object reaching the eye of the 

 observer, a result still better accomplished by throwing a thick 

 black cloth or sheet of black velvet over his head when in position, 

 as usually done by the photographer when focussing his instru- 



