COPYING. This term is used in photography, when manuscripts, maps, paintings, engravings, and works of art of that class are to be reproduced.
In copying black marks upon a white ground, by the collodion pro cess, it is generally advisable to have recourse to organic matter in the bath and collodion, in order to obtain density at the expense or sensitiveness and half tone. An old and rather discoloured collo• dion should therefore be used, because it contains an organic salt of potass, and the nitrate bath should be more strongly acidified with acetic acid, and a little acetate of soda added to it. The work to be copied should then be placed either in sunshine or a strong light, and a small stop used to the lens. Care must be taken not to give so much exposure as to redden and enfeeble the blacks of the negative.
A rather strong pyrogallic developer may be used, and a little silver added towards the end of the development. A small stop is necessary in order to give sharpness to the edges of the picture, and a strong light to give density to the blacks of the negative. ' The copying of oil paintings is a matter of great difficulty, because reflected light from the varnish is likely to enter the camera and blacken some part of the negative. A lens of long focus should in all cases be employed, for reasons which will be under stood by referenoe to the fallowing diagram, which is exaggerated in its proportions, in order to render it more intelligible.
Let A B be a varnished oil painting, L the lens, and a b the image ; the lens being oue of short focus and placed near to the painting. Sup pose the painting illuminated by light falling obliquely upon it as shown by the arrows.
Then some of the rays in cident about E, will after re, flexion at the vaxnished surface pass through the lens and fall upon the sensi tive plate at e, producing a. dark patch in the negative.
Next, suppose a lens of longer focus used, and the picture placed further from it, the image being the same size as before, as shown by the dotted lines ; then, no re flected rays will fall upon the sensitive plate, but they will all pass to the left of the lens. , In copying oil paintings, it is important, therefore, to use a lens of long focus, so as to avoid introducing very oblique pencils. The following is another point of equal importance. If light objects are immediately behind the camera, the light from them will strike the painting nearly at right angles, and then be refle,cted into the camera. This will be avoided by the arrangement shown in Fig. 10, which explains itself.
Oil painting's should be plaoed in the sunshine and a stop used to the lens. There is no advantage in using a weak light in pho tography. A strong light gives a quality similar to that which is called " penetration" in microscopy ; that is to say, it brings out the feebler details of objects.
Daguerreotypes should be copied in full sunshine, the original being placed. at the end of the projecting front of an appropriate camera, and the light thrown obliquely upon it, (as in Fig. 10,) through an open lid in the top of the box, the inside of the box being blackened, or lined with black velvet.
It now remains to add a few words on the mode of copying transparent negatives, by means of a copying camera.
A good deal has been said on this head in the article on the copying camera, (aee " Copying Camera,") and it only remains for us to discuss the various methods of obtaining a luminous background to the transparent negative. As this is a point which should be thoroughly understood, we shall endeavour to make it as clear as possible with the aid of a diagram. See Fig. 11.
The great principle to be borne in mind in the optics of photo graphy is, that the object to be copied is an assemblage of bright points, each of which is the origin of a pencil of rays, and that the image formed by the lens is an assemblage of foci of these pencils. From cvery point of the object a diverging pencil of rays proceeds, which after refraction through the lens is brought to a focus on the focussing screen. Now, in copying a transparent negative, the case is evidently different from that of copying an engraving or print placed in the same position with respect to the lens. For instance, in Fig. 11 ; if AB is an engraving and C any white point of it, a pencil of light diverging from C is refracted through the lens, and brought to a focus at c ;—but if AB is a transparent negative and C a piece of transparent glass, it is evident that C cannot be the origin of a diverging pencil of light in the same way as before, unless rays fransmitted through C from a luminous background behind, can be made to intersect at C, and then pass on as if C had been an origin of light.
This being understood, let us suppose that the dotted line DE behind AB is either the sky or an illuminated white screen. Then if C be considered a minute transparent hole, the sky or screen could be seen through it by an eye placed any where between 1 and m, and, therefore, rays M Cm, LC 1, would intersect at C, and produce the effect of a pencil diverging from C. And similarly of every other transparent point of the negative. It appears, there fore, that when a transparent negative is copied with the sky or an illuminated screen as a background, the same effect is produced as if the transparent parts of it were opaque but luminous, and emitted diverging pencils of light.
In all cases, the diameter of the stop, or lens, is small compared with the distance of the lens from the negative ; therefore the angles / Am,' l Cm, 1B m, are small ; and if instead of the sky or an illuminated screen, DE were a convex lens transmitting a pencil of light DAEB, which converges with great spherical aberration towards a focus in the neighbourhood of / m, the same effect would be produced as before. A lens of this kind is called a " Condenser," (See " Condenser,") and transparent negatives may be copied by means of it, with either an artificial light placed behind it, or reflected sunshine transmitted through it.
When any object is to be copied by superposition in the pressure frame, the ordinary printing processes may be employed which are described in the article on Printing. See " Printing." ConnosrvE SUBLIMATE. Bichloride of mercury; q. P. COSMORAMA STEREOSCOPE. See