152 



SCIENTIFIC RECREATIONS. 



people to draw by following the focussed picture with a crayon, but he 

 could not conquer the aerial perspective. 



So the camera languished till 1820, when Wedgwood and Sir 

 Humphrey Davy attempted to obtain some views with nitrate Oi f silver, 

 but they became obliterated when exposed to the daylight. 



As early as 1814, however, M. Niepce had made a series of experi- 

 ments in photography, and subsequently having heard that M. Daguerre 

 was turning his attention to the same subject, he communicated with him. 



Fig. 163. The Camera. 



In 1827 a paper was read before the Royal Society, and in 1829 a part- 

 nership deed was drawn up between Daguerre and Niepce for " copying 

 engravings by photography." Daguerre worked hard, and at length suc- 

 ceeded in obtaining a picture by a long process, to which, perhaps, some of 

 our readers are indebted for their likenesses forty years ago. By means of 

 iodine evaporated on a metal plate covered with " gold-yellow," and exposing 

 the plate then in a second box to mercurial vapour, he marked the image 



Fig. 164. 



Arrangement of lenses 



Fig. 165. 



in the camera, and then he immersed the plate in hyposulphate of soda, 

 and was able to expose the image obtained to daylight. 



But the mode now in use is the " collodion " process. We have all 

 seen the photographer pouring the iodized collodion on the plate, and letting 

 the superfluous liquid drain from a corner of the glass. When it is dry the 

 glass-plate is dipped into a solution of nitrate of silver, and then in a few 

 minutes the glass is ready. The focus is then arranged, and the prepared 

 plate conveyed- in a special slide to keep it from the light to the camera. 

 When the " patient " is ready, the covering of the lens is removed, and the 

 light works the image into the sensitive plate. The impression is then 



