178 CHEMICAL AGENCY 



admiration in. Paris, in producing pictures in a few 

 minutes, they found often that an exposure of an hour 

 was insufficient under the bright and glowing illumina- 

 tion of a southern sky. 



Experiments with the spectrum have been made in 

 different latitudes, and it is found, that, as we proceed 

 towards the equator, a band which is always left 

 unchanged, corresponding exactly with the rays of 

 greatest illuminating power, regularly enlarges in size, 

 thus proving the increase of light over actinism and 

 the interfering power of the former. 



By increasing the sensibility of the photographic pre- 

 paration, this difficulty is overcome, and particularly 

 "when any organic compound enters into the preparation. 

 So that we are now enabled to copy nature in all her 

 varying moods, whether we employ our photographic 

 tablets in temperate Europe, or in tropical Africa. 



The degree of sensibility which has been attained is 

 remarkable. Mr. Fox Talbot, by uniting a process 

 devised by Dr. Woods, of Parsonstown, and another 

 which was first introduced by the author of this volume, 

 and combining them with an ether, obtains a most 

 unstable compound, which he thus employs. A glass 

 plate is covered with albumen united with the above 

 solution, and then with nitrate of silver : this forms the 

 sensitive surface. The plate being placed in the dark, 

 in a camera, it is so adjusted that the image of a 

 printed bill fixed upon a wheel may fall upon it when 

 uncovered, and the wheel illuminated. The wheel is 

 made to revolve with the utmost rapidity, in a perfectly 

 dark room, and the sensitive plate uncovered. Then 

 the whirling bill is illuminated for an inappreciably short 

 space of time by the discharge of a Leyden jar. 

 Notwithstanding the rapid rate at which the printed 

 paper is moving, and the instantaneous nature of the 

 illumination a miniature flash of lightning the bill is 

 found to be copied with unfailing fidelity upon the 



