128 SCIENCE OF HOME AND COMMUNITY 



Loading the camera. Cameras may be loaded in three 

 ways with glass plates, with a roll of film, or with a film 

 pack. If plates are used, the loading is done in a dark room. 

 As ordinarily used, the film is done up in rolls having from 

 4. to 1 2 exposures. This is covered by a strip of black paper. 

 This film may be loaded in the daylight. It is mounted on a 

 spool, which is placed in slots at one side of the camera. It 

 is then unwound and passed around the back of the camera 

 to a spool on the other side. The camera is then closed and 

 the spool is turned by means of a key on the outside of the 

 camera till the figure i on the back of the film is seen 

 through a little window of red glass on the back of the 

 camera. 



FIG. 41. Principle of camera, 



Making the exposure. Three things need to be considered 

 in making the exposure : first, focusing the camera ; second, 

 the size of the stop to be used ; and third, the time of the 

 exposure. 



Focusing. Two methods of focusing are used, one by 

 throwing the image on a ground glass, and the other by 

 judging the distance of the object and setting the lens of the 

 camera to correspond. In the first method a piece of ground 

 glass is set in the back of the camera in the position to be 

 occupied by the plate on which the picture is to be taken. 

 The camera is pointed at the object, the shutter is opened, 

 and a door at the back is opened to expose the focusing glass. 

 The operator throws a cloth over his head to exclude the 

 light, and by looking at the image on the glass determines 



