TAKING PICTURES 131 



be taken, the shutter is opened, the largest stop being in 

 position, and the flash paper is lighted ; then after the flash 

 the shutter is closed at once. The time occupied by the sheet 

 in burning is about one second. 



Autographic camera. In the autographic camera, there is 

 a spring door on the back covering a slit. After the exposure 

 of the film, this spring door is opened and one may write on 

 the red paper back of the film any explanation of the picture 

 he wishes, using a hard-pointed stylus. This is then exposed 

 to the sky or to artificial lights for a time varying from two 

 to sixty seconds. The door is then closed. When the film 

 is developed, this writing appears on the edge of the picture. 



DEMONSTRATION 16 



Purpose. To show how to use a camera. 



In order to show the various steps in taking a picture, take 

 a group picture of the class indoors, explaining the things that 

 must be done in focusing the camera, setting the stop, and 

 making the exposure. 



Developing. After the film is exposed, if it is taken into 

 a dark room and examined by a red light, it will be found to 

 look just the same as before exposure. The changes that 

 have occurred are not of such a kind that they can be seen 

 with the eye. But there have been some very complex 

 chemical changes produced by the light that struck the film. 

 We are all familiar with the fact that light produces changes 

 in some substances. When some kinds of cloth are exposed 

 to light, they fade. Likewise some wall papers fade after a 

 few years, as do carpets and rugs. Even an ordinary news- 

 paper changes color when exposed to light for a long time. 



Some substances are much more sensitive to light than 

 others. Compounds of silver with the elements iodin and 

 bromin are very sensitive to light, and photography is based 

 on this property of these compounds. The film or glass 



