Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XXVI.

IN this operation, as in the preceding, a very bright, sharp, clear negative is required. Transparent positives by direct contact are obtained best by dry collodion plates; they can, however, be prepared as follows: Let the negative be varnished and thoroughly dry. Place it in the plate-holder, as you would the sensitized collodion plate. Next cut out a piece of thin writing-paper of the same size as the negative, and then cut out of this an interior piece of the same shape, thus leaving a margin all round of about a quarter of an inch in width. Place the marginal rectangle upon the negative, and see that it lies in contact all round. Now prepare a collodion plate; sensitize it, and allow it to drain thoroughly; then place it also in the plate-holder, and in contact with the margin of paper, and close the slide and shutter. Previously a cylinder of thin wood, blackened with ink within, is prepared with grooves at one end for the reception of the plate-holder, and open at the other extremity for the reception of the light. Such a cylinder may be six feet in length The object in view is to obtain only direct and parallel rays of light, to counteract the effect arising from the imperfect contact between the wet plate and the negative. Direct the open end of the cylinder to a white cloud, and then drain the slide for a moment, that is, a fraction of a second, and close it again. Probably this maybe too much exposure, in which case it will be well to paste a sheet of white paper over the end of the cylinder, in order to moderate the action of light. The plate is afterward taken out, developed, blackened, and fixed, as already described.

On removing the plate from the holder, the marginal paper will probably adhere to the wet collodion; if so, remove it carefully, and lay it on a flat surface to dry. It is possible too, owing to the inequality of surface, that the negative has been wetted by the superincumbent wet plate, in which case it must be carefully washed in rain-water, and dried. Without the long cylinder, oblique rays would enter from all sides, and destroy all the sharpness of the picture by producing thick lines out of thin ores. Whereas in the man- prescribed, vertical rays alone are admitted to the bottom, and entering perpendicularly are not refracted.