Here, upon close examination, one can see the crossed layers of cloth, which acted as a "photographic veil", through which even retention of ink and continuous tonality were achieved. Talbot explained this modification in a letter, dated 25 April 1853, to the Athenaeum:
...–suppose the object to be the opaque leaf of a plant, of irregular outline, first, I cover the prepared plate with two oblique folds of black crape or gauze, and place it in the sunshine for two or three minutes. The effect of this is, to cover the plate with a complicated image of lines passing in all directions. Then the leaf is substituted for the crape, and the plate is replaced in the sunshine for two or three minutes more. The leaf being then removed from the plate, it will be seen that the sun has obliterated all the lines that were visible on the parts of the plate exterior to the leaf, converting all those parts to a uniform brown. But the image of the leaf itself is still covered with a network of innumerable lines. Now, let this be etched in the way already described, and let the resulting etching be printed off. The result is an engraving of the leaf which when beheld by the eye at a certain distance appears uniformly shaded, but when examined closely is found to be covered with lines very much resembling those produced by an engraver’s tool, so much so that even a practical engraver would probably be deceived by the appearance. This crape arrangement I call a photographic veil:– and as I think it likely that the idea will prove useful....1
1 L.J. Schaaf, The Correspondence of William Henry Fox Talbot, University of Glasgow, http://www.foxtalbot.arts.gla.ac.uk, document 06762.