681 



struction of the photogenic effect, the plate is perfectly restored to 

 its former sensitiveness to \yhite light. 



After exposing a plate to the daylight, and then submitting it to 

 the destructive action of red, orange or yellow rays, it will be found 

 to be again sensitive to the same white light. It appears from the 

 author's observations, that a plate may be exposed to these two 

 actions alternately, for any number of times, without altering the 

 final property of the surface, which will be invariably sensitive to 

 the vapours of mercury, if its last exposure has been to the action 

 of white light ; whilst it will be deprived of that sensitiveness if it 

 has been exposed lastly to the action of the red, orange or yellow 

 rays. It results from the restoring action of the red, orange and 

 yellow rays, that Daguerreotype plates may be prepared in open 

 daylight; and that in order to give sensitiveness, it is necessary 

 only to place the plate for some minutes under red glass before 

 putting it into the camera obscura. The knowledge of this will be 

 advantageous to persons wishing to take viev.'s in places where it is 

 difficult to find dark rooms in which to prepare the plates. 



Besides the destructive actions of the red, orange and yellow 

 glasses, these same radiations are endowed with a photogenic action 

 of their own ; that is to say, they have, like the blue and violet rays, 

 the power of causing the fixation of mercurial vapours. Those 

 radiations, therefore, are endowed with two actions of a contrary 

 nature; one destructive of the effect of the photogenic light, and 

 the other producing an effect analogous to that light. 



The photogenic action of the red rays is 5000 times, that of the 

 orange 500 times, and that of the yellow 100 times slower than white 

 light in producing an equal amount of effect. The destructive action 

 of the red rays is 100 times slower than that of the white light, the 

 orange 50 times, and the yellow only 10 times. When a plate has 

 been exposed to the destructive action of any particular ray, it can- 

 not be affected photogenically by the radiation which has destroyed 

 the first effect ; it is sensitive only to the other radiations. The 

 photogenic action of any radiation cannot be continued by another. 



The solar spectrum is therefore endowed with three different pho- 

 togenic actions, and three different destroying actions, correspond- 

 ing with the red, the yellow and the blue rays. The three photo- 

 genic actions of the spectrum thus distinguished have distinct cha- 

 racters ; each of these radiations is endowed with a photogenic 

 power peculiar to itself, and which gives to the Daguerreotype plate 

 an affinity for mercurial vapours ; nevertheless these three actions 

 are so different, that we cannot, by mixing them artificially, make 

 one assist the other; for they are antagonistic. The effect pro- 

 duced by the blue rays is destroyed by the red and yellow ; the red 

 and yellow mutually destroy each other, and the effect of either is 

 destroyed by the blue. The alternate changes of the surface of the 

 plate by these three kinds of radiation seem to prove that the chemi- 

 cal compound remains always the same under these different in- 

 fluences, and that there is no separation or disengagement of the 

 constituent elements. 



