as Photographic Agents. 267 



the maximum of effect is in the red rays (42 50.). I am in- 

 clined, however, to believe, that examination of these phseno- 

 mena by a fixed spectrum will prove a shifting of the actions 

 according to the kinds of paper used; but 1 am satisfied the 

 entire action will be confined in the one instance to the green 

 and yellow rays, and in the other to the orange and red 

 rays. 



70. The very curious brightening of those parts of the pho- 

 tograph corresponding with the darks of the engraving, at- 

 tracted my attention powerfully. My first impression was, 

 that the carbonaceous matter of the ink used in printing ex- 

 erted a kind of catalytic action in directing the formation of 

 an iodide of silver. I think I have proof of this to some 

 extent. 



71. Most printed pages, unless they are very old, when 

 placed in juxtaposition in the dark with an hydriodidated 

 photographic paper, leave an impression after some hours, 

 and 1 have succeeded in partially copying some engravings 

 thus. However, the result is uncertain ; the copy, at all 

 times faint, is often very imperfect, being sometimes bleached 

 in circles, of which a letter or two form the centres ; at other 

 times the letters are copied, but all of them shaded from an 

 extension of the bleaching action. 



72. From effects I noticed from the accidental contact of 

 some carbonate of iron, I was sanguine of being enabled to 

 copy a written page. In this I was disappointed. I have not, 

 with any of the numerous kinds of writing ink which I have 

 tried, succeeded in obtaining the slightest trace of a letter on 

 the photographic paper. 



73. From the rapidity with which this effect is produced 

 when the photograph and engraving are exposed to light, it is 

 evident some other cause than the one I have just considered 

 was in active operation. A careful examination of the pho- 

 tographs formed under the before-mentioned glasses, particu- 

 larly under the red glass, convinced me that the quickening 

 agent was to be sought for in the calorific rays, which are ab- 

 sorbed and retained with greater force by the dark parts of 

 the engraving. 



74. To put this notion to the test, I placed a printed page 

 in contact with a paper wetted with an iodidated solution, 

 over which I placed a glass, and then a plate of copper, which 

 I made hot by rubbing it with a heated iron. The passage 

 of the heat through the glass was sufficient to effect as fair a 

 copy as is produced under the red glass by the influence of 

 light. 



75. These researches, which were pursued with a view 



