Fig. 22.
In [fig. 21] the card is raised one inch from the paper, and here f g is about three-quarters of d e, and h k about two-fifths; therefore, in this case, the light on B would be only four-twenty-fifths, or about one-sixth of that acting on a, and about nine-sixteenths or one-half nearly on c. It is thus evident that the further away the card is, the more extended will be the gradations. Again, suppose, in the last figure, the bit of sky at g f was twice as bright as at d e, then the amount of light acting on c would be the same as that acting on a. It will thus be seen how important it is for proper gradation that the hole in the card should be exposed to an equally illuminated sky, or that some artifice should be employed to render the illumination equal. If we paste a bit of tissue paper over C D, this is accomplished, for then it becomes the source of illumination, and it is illuminated equally all over, since on every part it receives the light of the whole sky; but this is not the case if it is transparent to diffused light, and is never the case if it is exposed to direct sunlight, since a shadow of the hole is always cast on the paper beneath. If you choose to put another piece of tissue paper, (say) one inch above the hole, and extending over the whole length of the card, this difficulty is got rid of, and this last piece of tissue paper illuminates that pasted over the hole C D, and the gradations will then be nearly perfect.
Now to apply the above to forming a vignetting block.
Suppose we have a one-inch head to vignette and to show the shoulders and chest, to be of the size of a carte-de-visite, that the background is about a half-tone between black and white, and that but a trace of it shall appear above the head. To make a good vignette, the graduation from black to perfect white should lie within a limit of half an inch for a carte size portrait. The question then arises at what distance from the plate should a vignetting card be cut to help this object, and what shape should be made the hole in the card. We take it that one-fifth of the light necessary to produce a full black tone would hardly produce any effect on the sensitised paper; knowing this and the size of the aperture, we can calculate exactly what height the card could be raised. Take the breadth between the shoulders that is to be fully printed as 11/2 inches, then by constructing a figure similar to figures 18 or 19 we shall find that the necessary height is about one-third of an inch.[23]
Fig. 23.
By judiciously cutting out an aperture in the card and vignetting, defects in a background may often be entirely eliminated from the print. Proceed in this way: Take a print of the portrait, and cut out the figure in such a way as to get rid of the defective background, and then place this on a piece of thick card (we prefer a thick card, since it will not sag easily, and thus alter the gradation), and cut out an aperture corresponding to it. The outsides of most carte-de-visite frames are raised from the glass about one-third of an inch; place the card on the front so that the aperture corresponds to the figure on the negative, and tack it on to the frame. The dotted lines ([fig. 23]) show the card fastened on to the frame, and the opening left. This latter may be covered with tissue paper, and the frame placed in diffused light from the sky. In some cases it may be necessary to use a larger printing frame than the ordinary carte frame, in which case the operator should be able to make a vignetting apparatus raised at a proper height from the glass. Suppose it is required to raise the opening half an inch above the glass, and that the card is 41/4 by 31/2.