174 COLOE PHOTOGRAPHY. 



lation to the subject as the earlier experiments of Neipce 

 and Daguerre do to the more advanced processes of 

 photography. 



In the first place, he uses a very thin glass plate, coat- 

 ed with such a thin emulsion as to be practically trans- 

 parent. This sensitized emulsion is so compounded as 

 to be perfectly homogeneous, presenting no discon- 

 tinuance in the film. This plate is placed in a glass box 

 or camera, so constructed that the plate placed film in- 

 ward forms the front thereof. The interior is filled with 

 mercury so that the sensitized plate comes in immediate 

 contact with the mercury. 



Thus prepared there are thrown upon the outer sur- 

 face of the sensitized plate the rays of the solar spec- 

 trum. After long exposure, varying from 30 minutes to 

 2 hours, time to allow the perfect impression of the red 

 rays or waves, the picture is completed and the plate is 

 removed, developed and fixed in the ordinary manner. 



The plate thus developed dried, and viewed by trans- 

 mitted light is said to give a duplication of the solar 

 spectrum. 



There is a conflict of opinion as to the success of 

 Lippman's experiment. 



The theory of the method is this : the plate and its 

 sensitive surface being transparent, the mercury forms a 

 mirror before the film and reflects the rays of light upon 

 themselves. There is, as physicists say, an interference 

 between the incidental and the reflected rays. There 

 follows in the interior of the sensitive film a series of 

 positions of interference and each of these is marked by 

 its particular deposit of silver. The result is that the 

 sensitive film, after the photographical operations are 

 completed, is subdivided by the deposit of silver into a 

 series of thin sheets. These sheets are precisely of the 

 thickness necessary to produce by reflection the in- 

 cidental color which gave them birth ; for instance, the 



ISO 



