332 NINETEENTH CENTURY. PT. IIL 



they could not prevent them from fading away again, and 

 it was not until 1839 that a Frenchman named Daguerre 

 learnt how to fix the pictures so that they would remain. 



Photography is now chiefly worked with dry plates, but 

 the old wet process will best illustrate how the rays of light 

 produce a picture. In this process the glass plate which 

 is to receive the picture has been first covered with a thin 

 film of collodion (that is gun cotton dissolved in ether and 

 alcohol) mixed with finely powdered iodide of potassium. 

 The plate is then bathed in a solution of nitrate of silver, 

 and a chemical reaction takes place which leaves a film of 

 iodide of silver on the surface. When the rays of sunlight 

 reflected from your face are brought to a focus on the lens of 

 the camera, and fall upon this plate, the chemical rays (which 

 are chiefly those beyond the violet end of the spectrum) de- 

 compose the iodide of silver; and after some more chemicals 

 called protosulphate of iron and pyrogallic acid have been 

 poured 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 them strongly or feebly. The 

 picture 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 next 

 poured upon the plate to dissolve away any of the iodide 

 of silver which remains undecomposed by the light, so that 

 when the sun next falls upon it it may not blacken the rest 

 of the plate and destroy the picture. Then the glass plate 

 is again placed in the sun with paper under it which has 

 been sensitized, that is, impregnated with chloride of 

 silver, by being bathed in solutions of nitrate of silver and 

 chloride of sodium. Upon this paper the shades will be 

 reversed, for under the dark parts of the plate the sun wilJ 



