Photography — Its History, Position, and Prospects. 155 



suffered no perceptible alteration, although the light was too 

 brilliant for endurance ; but that it was blackened in six mi- 

 nutes, when it was exposed, in a similar way, to the rays between 

 the green and the extremity of the violet. Philosophers are 

 now aware that the solar spectrum in reality consists of three 

 spectra, possessing very different properties, partially, but not 

 uniformly, superimposed — a luminous, a calorific, and an actinic, 

 photographic effects being due to the last ; and that the lumi- 

 nous consists of a yellow, a red, and a blue spectrum, similarly 

 superimposed. Whether the calorific and the actinic also are 

 compound, our present knowledge does not enable us to decide. 



The important circumstance of the action of light on a 

 salt of silver having once attracted attention, such a modifica- 

 tion of the. effect as would result in a picture was but a step, 

 though an important one. This step is due to the combined 

 ingenuity of Wedgewood, the son of the celebrated porcelain 

 manufacturer, and Sir H. Davy, the illustrious chemist. The 

 effects they produced excited admiration, but their pictures 

 gradually changed into mere blackened surfaces. The know- 

 ledge of one simple fact alone was required to give permanence 

 to their productions ; but that fact was not discovered until 

 long afterwards. These achievements of Wedgewood and 

 Davy had been, in some degree, anticipated more than two 

 centuries previously; since Pabricius, in a work on metals, 

 published in 1566, asserted that a lens produced on chloride of 

 silver an image in which the bright parts of objects formed 

 dark shadows, and their dark parts lights — that is, a " nega- 

 tive picture," the lights and shades being reversed. Such 

 were the pictures of Wedgewood and Davy, since they were 

 unable to obtain a positive by transmitting the light through 

 a negative. At this stage, the camera obscura naturally 

 suggested itself as a valuable aid to photography ; but, when 

 Wedgewood made the trial, he found that too long an exposure 

 to light was required, if it was used. The solar microscope, 

 however, was found to be available for the purpose ; but he 

 usually employed the direct rays of the sun, transmitted 

 through the engraving, or other object which it was desired 

 to copy. Wedgewood discovered that the chloride was more 

 sensitive than the nitrate of silver, and that both were more 

 sensitive in the moist than in the dry state. 



Little or no further progress was made for some time ; but 

 at length the grand difficulty was surmounted; Niepce and 

 Daguerre succeeded in arresting the action of light. Nicephorus 

 Niepce was born at Chalons-sur-Saone, in 1765. In his early 

 years he had been in the army, but in 1814 his attention was 

 accidentally turned to photography. Seeking a substitute for 

 lithographic stone, he observed that bitumen was rendered of 



