This is usually done by means of a lantern, as shown in Fig. 50. The system consists of a lantern, A, for the illuminant, a condenser, B, a carrier for the negative at C, and a camera-like arrangement, D, with the lens, E. The illuminant may be ordinary paraffin or incandescent gas, In either case, the rays must be concentrated by means of a condenser. This consists of two plano-convex lenses mounted in a brass cylinder, as indicated by the dotted lines in the condenser chamber, B. It collects and passes the rays to the negative. True centring of the light must be obtained, indicated by a bright even disc on the easel, otherwise an uneven exposure will result. The paper is carried on the easel, H, in the ordinary way. If the lens is provided with a yellow glass cap, focussing may be done direct upon the bromide paper when in position. The cap will then merely require taking off to make the exposure.
Experimental exposures should be made, and when the, correct one is found it should be noted, to facilitate work on any future occasion, because, as the illuminant is under control, it is always possible to obtain certain results. Grouping the negatives in their various densities, as thin, medium and thick, will materially assist in the estimation of the necessary exposure.
Enlarging by Artificial Light must be done in a darkroom. The illuminant should be lighted, and the lantern allowed to remain open to warm and for any condensation moisture to pass off, or wiped from the condenser. In the meantime the negative is placed - film side facing into D - in the carrier, and this pushed home through the opening in C. The image will appear more or less blurred upon the paper on the easel, H. It is roughly focussed by means of the milled-screw, F, and the final fine adjustment of focus made by the lens screw, G. The daylight-enlarging apparatus, Fig. 49, may be used for artificial light with magnesium ribbon. The ribbon is burned in a reflector made from metal bent to a half-circle. The inside of the reflector is polished and the ends turned to screen all light, except that reflected from the polished surface. The light must be diffused by being made to pass through a piece of white tissue paper or ground-glass placed between it and the negative. ' After the exposure has been made, the development of the paper is carried out in the same way as with other bromide paper of small sizes. Enamelled metal dishes may be used for the purpose. The paper must be well soaked before pouring on the developer, and the appearance and growth of the image closely watched, as the large sheets of bromide paper are somewhat expensive. A tuft of cotton-wool lightly passed over the surface of the paper, after pouring on the solution, will break up and remove any air-bells that have formed. After development the print must be thoroughly fixed, washed and dried.
Enlarged negatives may be made in two ways, either by making a small transparency (or positive on glass) and enlarging up directly from it on the large plate, or by making a large transparency from the small negative, and from the transparency a large negative by contact.
For the small transparency a plate should be used which has a fine grain; such plates are specially prepared for the purpose, and these should be obtained. It is laid upon the negative, placed in a printing-frame and exposed to a lamp or gaslight. If to the latter, only about one second exposure will be required at a foot. The development must be carefully carried out with the idea of obtaining an image full of detail without any heaviness. When the transparency has been fixed, washed and dried, it is placed in the enlarging apparatus and the enlarged negative made therefrom. The enlarged negative having been obtained, any printing medium may be used. Negative glass is usually thin, and the precaution should be taken of having a piece of good ordinary glass placed in the printing-frame first and the negative laid upon it during printing.
The Transparencies may be used for decorative purposes if they are bound up in contact with a piece of finely-ground glass, by means of lantern-slide binding material. It is then hung that light may pass through it.
Negative paper or even bromide paper may be used for enlarged negatives in place of plates. These have to be treated with something such as vaseline to render them more transparent before being used for printing.