132 SCIENCE OF HOME AND COMMUNITY 



plate on which the picture is to be taken is covered with a 

 thin coating of gelatin in which these compounds are em- 

 bedded. When these are exposed to light, chemical changes 

 take place, the amount of change depending on the in- 

 tensity of the light. A white object will produce more effect 

 on the plate than a black object ; and so by this difference 

 in the action of light from different objects, changes are 

 produced which can later be made visible and reproduced 

 as a picture. 



LABORATORY EXERCISE 17 



Purpose. To show the effect of light on certain salts of silver. 



Materials. Silver nitrate, sodium chlorid, and potassium 

 bromid. 



Directions. Dissolve a little silver nitrate in water in a test 

 tube. Add an equal quantity of a solution of sodium chlorid. 

 Filter and expose the filter paper to sunlight. What change 

 takes place ? Repeat the experiment, using potassium bromid 

 in place of sodium chlorid. 



In order to make these changes in the film visible, the film 

 is treated with a certain chemical called a developer. This 

 developer acts on the film in such a way that metallic silver 

 is deposited in those portions of the film which have been 

 acted on by the light. In those parts that have been most 

 acted upon, the largest amount of silver is deposited ; and 

 where there has been the least action, the least silver is 

 deposited. Light objects, therefore, appear in the negative 

 as very dark because there is the most silver, and dark objects 

 very light, because there is the least silver. The plate at 

 this stage is called a negative .because the shadows are re- 

 versed. 



Fixing solution. If this negative were taken out into the 

 light, the silver salts which had not been acted upon would 

 be affected by the light, and the negative would be spoiled. 



