95 



me to be strictly analogous to the above. If I comprehend your 

 description, you use an iodised paper in which iodide of iron is em- 

 ployed instead of iodide of potassium. 



" You may be quite right in attributing the effects to Electro- 

 lysis, but then it follows that my Calotype process, with all its 

 variations, must result from the same cause. 



" I am, Sir, 



" Your obedient Servant, 



" H. Fox Talbot. 

 "Dr. Woods." 



" Lacock Abbey, 

 ''\Sth March, 1845. 



" Sir, — I have to acknowledge the receipt of your courteous 

 letter of the 15th instant, upon which I beg leave to make a few ob- 

 servations. In my Calotype process, iodide of silver is decomposed 

 by the joint influence of light and a deoxydising agent (gallic acid). 

 Mr.Hunthas shewn that sulphate of iron may be substituted forgallic 

 acid, and he calls the process so altered Energiatype. But since tan- 

 nin and other substances may also be substituted for gallic acid, each 

 of these variations in the process would require, on the same princi- 

 ple, to have a separate name, which would, surely, be inconvenient. 

 In your method, iodide of silver is decomposed by the joint action 

 of light and iron ; the three reacting substances being tJie same as 

 in Mr. Hunt's Energiatype; and therefore, imperfect as the theories 

 of photography confessedly are, I cannot persuade myself that a 

 catalytic action can take place in your process, unless it also takes 

 place in the Energiatype and in my original Calotype process : I 

 therefore cannot help considering these three processes as variations 

 of the same, and not essentially different. I hope, however, you 

 will not consider me as detracting in the least from your valuable 

 labours : my remarks only refer to the nomenclature of the science. 



If I am not mistaken, the three methods I have named produce 

 pretty nearly identical results, though I speak from experience of 

 only two of them, Mr. Hunt's and my own. Both of these are 

 nearly certain in operation, very rapid, giving a camera picture of 

 a bright object in a second of time, and requiring no second wash 

 if enough of the deoxydising agent is employed in the first wash. 

 It is customary to make ihe positive copies on a different paper. 



