THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 291 



aloug the whole length of the plate from lielow and on the side opposite the 

 coating. When the liquid has ceased to run, the plate is dried in the dark. The 

 coating being well dried it is to be placed in the camera obscura. The time 

 required to procure a photographic cojiy of a Jtindscape is from seven to eight 

 hours, but single monuments strongly illuminated by the sun or very bright in 

 themselves are copied in about three hours. 



When operating on glass it is necessary, in order to increase the light, to 

 place the plate upon a piece of paper, with great care that the connection is 

 perfect over every part, as otherwise confusion is i)roduced in the design by 

 imperfect reflection. 



It frequently happens that when the plate is removed from the camera there 

 is no trace of any image ui)on its surface. It is therefore necessary to use an- 

 other process to bring out the hidden design. 



To do this, provide a tin vessel larger than the tablet, having all around a 

 ledge or border 50 nmi. (2 English inches) in depth. I^et this be three- 

 fourths full of the oil of petroleum. Fix your tablet by the back to a piece of 

 wood which completely covers the vessel and place it so that the tablet, face 

 downwards, is o^-er but not touching the oil. The vapor of the petroleiun pene- 

 trates the coating of the plate in those parts on which the light has acted 

 feeltly — that is, in the portions which correspond to the shadows — imparting to 

 them a transparency as if nothing were there. On the contrary, the iwints of 

 the resinous coating on which light has acted, having been rendered impervious 

 to the vapor, remain iinchanged. 



The design. must be examined from time to time and withdrawn as soon as 

 a vigorous effect is obtained. By urging the action too far even the strongest 

 lights will be attacked by the vapor and disappear, to the destruction of the 

 l)iece. 



It may perhaps appear to some that I have needlessly given the 

 particulars of a process, now superseded by others, possessing- the 

 most infinite sensibility, producing in a few minutes a better effect 

 than was obtained by the heliographic process in several hours. 

 There are, however, so many curious facts connected Avith the action 

 of light on these resins that no treatise on photography could be 

 considered complete without some description of them: and this 

 l^rocess is now revived with a view to the production of etchings 

 directly from nature. 



M. Daguerre remarks that numerous experiments tried hy him 

 with these resinous preparations of M. Niepce prove that light can 

 not fall upon a body without leaving traces of decomposition ; and 

 they also demonstrate that these bodies possess the power of renewing 

 in darkness what has been lost by luminous action, provided total 

 decomposition has not been effected. This heliographic process must 

 be regarded as the earliest successful attempt at fixing on solid tablets 

 the images of the camera obscura and at developing a (k)rmant image. 



B. Talbot's Photogenic Dkawinijs. 



On the 31st of January, LS31), six mouths |)rioi- to the j)ublicatioii of 

 M. Daguerre's process, Mr. Fox Talbot comnninicated to the Ivoyal 

 Society his photographic discoveries, and iu February he gave to the 



