42:3 



action is exceeded, the plate comes out of the camera with a visible 

 imago, which the vapours of mercury scarcely affect. This species 

 of images is well known to experimentalists, and they have not 

 hitherto been applied to any useful purpose. 



" Now, when any one of the above states of the image has taken 

 place, wc may always, and easily, cause the image to retrograde on 

 the plate, and arrest it at any anterior state wo please. For ex- 

 ample, I leave the iodised plate 300 minutes and more in the 

 camera, whereas 10 minutes are sufficient to produce an ordinary Da- 

 guerreotype picture. When this plate is withdrawn from the camera, 

 it presents a distinct negative image. I then expose the plate du- 

 ring 22 seconds to the vapours of iodine, and then to those of mer- 

 cury/, and the result of this is the production of a splendid Daguerreo- 

 type picture. The vapour of the iodine has, therefore, caused the 

 action of the light to retrograde. I have repeated and varied these 

 experiments in every way, and always with the same success, taking 

 care not to allow the vapours of iodine to act too long, otherwise they 

 would completely destroy the action of the light. 



" There are many other bodies which, in their gaseous state, pro- 

 duce the same negative actions, when applied to an iodised plate, 

 such as chlorine, bromine, the hydro-fiuoric add, &c. &c. ; but I shal 

 not enlarge on this point, because I have to mention to you a gas 

 much more intei-esting, viz. oxygen, which exercises the same nega- 

 tive action. 



" Having left the iodised plate from/owr to six times the period ne- 

 cessary for an ordinary picture in the camera, and exposed it to pure 

 oxygen during a few seconds, the mercurial vapours produce the or- 

 dinary image. The oxygen in atmospheric air acts also upon the 

 plate, and requires only that the action be pi-olonged. If we expose 

 a plate with an image that would become negative by mercurial va- 

 pour, during one or two minutes, to a current of air from a pair of 

 bellows that would raise a column of water four or five inches high, the 

 image will be presently brought back to its ordinary state ; or if the 

 blast is continued longer, to any other anterior state, the primordial 

 state not excepted. xVtniospheric air, indeed, quite calm, exercises 

 also these negative actions, provided its action is continued lor some 

 hours. 



" I may also add, that among the gases which compose atmospheric 

 air, oxygen alone produces these negative effects. Nitrogen, car- 

 bonic acid, and the vapour of water, do not produce them ; the latter, 

 however, produces the effects which I have described in my Memoir 



