PRACTICAL ENLARGING 765. Fixing the Sensitive Paper in Position. With vertical enlargers the placing of the paper is usually done by metal squares or strips which act both as masks with variable openings and as pressure plates, holding the paper fiat. Failing these, the paper may be held down by small weights of cast iron or lead placed at the edges and serving also to keep down any mask or strips of opaque paper which may be used to limit the surface to be exposed. The same methods of fixing are applicable to plates or films.
Fixing the paper to a vertical easel' by means of drawing pins 2 is practical only if the easel is fixed to the wall or hinged so that it can be turned down into a horizontal position on a rigid support and then returned to the same position that it occupied during focussing. Otherwise there is a risk of shifting the easel when pushing in the drawing-pins. A facing of the easel with cork lino partly removes this objection. It is often preterred to keep the paper flat under a thin sheet of glass in a kind of simplified printing frame, in spite of the fact that the glass sometimes causes a slight diffusion of the light, or a double outline in the images (especially where two very contrasted portions of the picture meet), even when the glass is without flaws and perfectly clean. It is of service to have squares ruled, or a scale in inches, or rectangles corresponding with the various usual sizes, on the easel, or on the board closing the paper frame (or on a sheet of white card used to cover either when focussing).
Sheets of paper of small or medium sizes are held sufficiently by a masking frame, i.e. a frame with a cardboard rebate in which a mask of black paper is placed, and the sensitive paper then kept against the rebate by a flap of card board hinged to the frame. This frame is slipped under strips of wood or under the heads of one or two screws partially driven into the enlarging easel.
The sensitive paper could also be fixed to a sheet of metal coated with an adhesive varnish (§ All these devices may be used for holding films. When enlarging on glass plates it is customary to use a printing frame or a dark slide, either of which is attached to the easel by any convenient means.
766. Testing Parallelism of the Negative and Sensitive Paper. In the case of a vertical en larger the parallelism of the planes of the nega tive and easel can be readily tested by means of a spirit level. The instrument is first placed so that the easel is horizontal, and then the light box is removed and tests are made to see that a glass plate put in the negative carrier remains horizontal in all the positions in which the carrier can be placed.
When using an enlarging lantern, the parall elism is tested by sights on a mirror successively applied against the negative carrier and the enlarging easel as described in § 736 for adjusting a copying camera.
767. Placing the Negative in Position. Glass negatives are usually held in a frame with re bates with spring turn-buttons to keep them firm. When several negatives of different sizes have to be enlarged one after the other, one may use either a set of adapters nesting one within the other, or separate negative carriers ; the latter arc better for holding the negative in It is well to cut down to a minimum the amount of stray light due to successive reflec tions, both in the lens (§ 57) and in any glasses used to hold a film negative. This is done by masking all the superfluous parts of the negative. In this way much cleaner high-lights and margins are obtained in the enlargements.
When a film negative is being enlarged it is usually sandwiched flat between two glasses, which introduces four additional surfaces for stray reflections, and thus tends to veil more or less the enlarged picture, especially in the absence of a mask stopping all light around the part of the negative being enlarged. The number of marks due to dust or to imperfect cleaning are also The results are much im proved if the number of free surfaces is reduced to two by wetting the film with glycerine and then placing it between the glasses (all air bubbles being removed), as the whole may be considered as one optical medium ; at the same time the effect of scratches on either side of the film is In cases where film negatives have often to be enlarged (commercial developing and printing for amateurs and cinematograph film studios), the same result can be obtained more simply by means of a glass trough filled with a volatile fluid which renders unnecessary the washing and drying needed with glycerine. Carbon tetrachloride (a non-inflammable liquid) is gener ally used ; its refractive index is nearly that of the mean refractive index of gelatine, celluloid, and glass (K. C. D. Hickman, 1926). When using a vertical enlarger, a trough with a glass bottom is placed in the negative carrier. This trough contains a small quantity of carbon tetrachloride, and the film is pressed against the bottom by a block of glass with plane surfaces which acts like a paper weight. The under surface of this block carries a mask of tinfoil affixed with gelatine. When using an ordinary enlarging lantern, a vertical glass trough with plane-parallel sides is employed ; in this the film is pressed against one of the sides by a piece of glass held by wooden In the customary case of enlargement on bromide paper, so as to obtain a positive image to be kept as such, the emulsion side of the negative must be turned towards the lens ; the negative must be turned the other way round if the enlargement is to be used to make a Bromoil transfer (Chapter XLIII).