RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 
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II. Of taking Photographic copies and transfers. 
22. I shall not long- detain the Society with an account of the various trials I have 
made for perfecting the multiplication of photographs. The following are the chief 
precautions it will be generally found necessary to attend to. 
23. First. An extremely close contact of the photographic paper with the ori- 
ginal to be copied, which can only be ensured by subjecting both (laid face upon 
face) to the pressure of powerful screw- clamps acting on plates of very thick glass. 
The largest pictures I have copied occupied 110 square inches of exposed surface 
(eleven inches by ten), and for these dimensions a thickness of glass equal to the one 
third of an inch was found barely sufficient, or rather too weak. This thickness is, 
however, ample for photographs whose dimensions do not exceed fifty or sixty square 
inches. The glass should of course be as colourless as possible, and rather verging 
to a blue than a yellow or pink tint, yellow media being especially active in absorb- 
ing the chemical rays, as will hereafter be shown. The glass should press the papers 
on some soft bed, such as velvet, stretched on a thick board of a truely plane surface. 
The contact should be as close as can be produced by any pressure short of what 
will break the glass, as the smallest interval is injurious by allowing dispersed light 
to spread. Even that arising from the interposition of a thin film of mica is hurtful 
to the sharpness of the impression. 
24. When only a single transfer is to be made, closeness of contact is the only pre- 
caution to be attended to ; and this being taken, the transfer takes place with perfect 
fidelity, and with a depth and sharpness fully equal to the original. But if the pho- 
tograph so obtained be used for making a re-transfer, a host of annoyances and dif- 
ficulties arise from the imperfections and unequal and uncertain texture of the paper, 
as well as from the subtle nature and want of absolute opacity in the reduced silver 
deposited under the influence of the light, and in which the design is traced, to 
which must be added the great care which must be bestowed on the fixing process 
and final washing out of every particle of silver except what actually enters into the 
design, and which is by no means easy if any but very thin paper be used. 
25. Such paper is easily procured of very even texture, but if examined in the light 
it is found penetrated with actual holes, which though of minute dimensions, suffer 
light to pass freely and are very injurious. To obviate this, and to equalize the light 
by dispersion, an additional thickness of such paper may be advantageously inter- 
posed between the glass and the photograph to be re- transferred. And, finding this 
in fact to be quite necessary to the production of a tolerably good effect, it occurred 
to me to impress this paper also with a facsimile of the original, or in other words 
to double the photograph intended to be re-transferred. This may be done by taking 
off two such photographs from the original design, which being fixed, dried and 
pressed flat, are to be applied to each other (face to back), and being first delicately 
adjusted under a magnifier, by fiducial points made with a fine needle in each, are in 
