CH. xxxin. PHOTOGRAPHY. 317 



substances, so as to leave distinct marks upon anything 

 touched by them. 



Photography. You will see at once that this is the 

 secret of Photography. In 1802, Sir Humphry Davy and 

 Dr. Thomas Wedgwood suggested that pictures might be 

 taken in this way by the rays of the sun acting upon chloride 

 of silver, and they even succeeded in making some. But 

 they could not prevent them from fading away again, and 

 it was not until 1839 that a Frenchman named Dsguerre 

 learnt how to fix the pictures so that they would remain, and 

 Mr. Fox Talbot afterwards improved the process. We can- 

 not enter here into a complete account of photography, but 

 you may form some idea of how the rays of light produce a 

 picture. 



When you go to have your photograph taken, the glass 

 plate which is to receive your picture has been bathed in 

 nitrate of silver, with some other chemicals. When you 

 stand in front of it and it is uncovered, a luminous image 

 of your face or body, which has been brought to a focus 

 on the lens of the camera, falls upon the plate, and the che- 

 mical rays (which are chiefly those beyond the violet end of 

 the spectrum) decompose the nitrate of silver. You can 

 see nothing when the plate is taken out of the box in which 

 it was placed, but by pouring some more chemicals called 

 protosulphite of iron and pyrogallic acid upon it, the parts 

 which the light has touched all start out in different shades, 

 exactly in proportion as the chemical waves of light have 

 fallen upon it strongly or feebly. It will be exactly the 

 opposite to your real appearance, because where most light 

 has fallen, there the chemicals will be most decomposed and 

 will leave the blackest tints. 



Another fluid called hyposulphite of sodium is now 



