At the bottom of the lantern, and secured to the feet, is a tray-holder of sheet-iron; in this slides the bottom tray, measuring 3 1/2 in. by 8 1/2 in., and shown at Fig. 66. Halfway up is another tray to work with the second or upper system of lenses, and this is carried by another holder, which should be riveted, not soldered, to the metal lining. In work, the lower jet would so heat the bottom of the upper holder, and, in consequence, the tray, that damage would result. To obviate this use a flame guard or deflector. This is a piece of sheet iron, shown at Fig. 65, also riveted to the lining and tin below the bottom of the tray holder, the air space between being found to sufficiently protect the upper jet and tray from damage. Farther up the lining is turned over on a wire, so as to terminate about 1 in. below the mahogany work* By this means the hot air passing up the air space escapes into the upper chamber of the lantern, and so out by the hood. On the right hand side of the lantern should be the doors; they are 6 in. square, with a hole l 1/4 in. diameter through the centre (see Fig. 67). The doors, as well as the body itself, should have a tin or sheet-iron lining, kept off 1/2 in., as described. In the centre of the doors should be a circular hole 1 1/4 in. diameter.
The use of this is to be filled with a piece of dark blue glass (the tin lining being cut away to match) through which the operator can examine his limelight jet, without being exposed to its dazzling brilliancy or having to open the door.
Next comes the back (see Fig. 63); here we must cut away the wood and lining, so as to obtain an aperture through which the tray and limelight jet pass. At the front, Fig. 64, two circular holes must be cut where the condensers pass through, and should be 1/2 - 3/4 in. larger than their diameter.
As to the arrangement of front, Stewart, Middleton, and many other well-known makers fit the entire lens system to a brass plate or stage, Fig. 68, which is hinged to the lantern body, and adjusted, jut described; the condenser being carried by a short tube projecting into the lantern, whilst the power or focus lens is carried, as usual, at the end of the brass part. Wood, and several other manufacturers, on the other hand, lit, as it were, a mahogany door hinged to a cross bar running across the centre of the lantern front, and carrying the condenser on the inner side, and the brass front and focus lens as before described. In this form, Fig. 69, the slide-holder and stage are of mahogany, and on this plan (which is superior in many points) all our highest class bi-aud tri-unial lanterns are constructed.
Parts of Bfunlal lantern.
This plan, though more costly, has advantages in appearance and facility of adjustment - the coincidence of the discs being secured with milled-head screws passing through the ends of a brass strap running across the doors. This has one great advantage: it dispenses with springs of any kind. The wood and cap for the top of the lantern are shown at Fig. 70. The condensers should be either 3 1/2 in. or 4 in. diameter. The focus lenses, which should be of the double achromatic form, with rack and pinion, are now made to give long and short focus.
The utility of this is in working with a screen of given size, say, for instance, 10 ft., 12 ft. 15 ft. or 20 ft. Assuming we have an achromatic power of which the short focus is 4 1/2 in., and the long 9 in., we can utilise the concave of the back combination, as also the crossed biconvex lens, to either further prolong or shorten the focus. Taking the example of a screen 20 ft., and supposing we have to work in a room 25 ft. long, our double combination just covers it nicely. But, supposing the room was 50 ft. long, we should be just in the middle of the audience. We will use the long focus lens, and, placing ourselves at the further end of the room, we still throw the 20 ft. disc By the same mode, if we have a room larger still, by using the concave back we can still further lengthen the focus. By this means an expert operator can place himself at any convenient distance, and still maintain his given diameter of view on the screen. The front or tubes of this form of lantern are so constructed as to slide one within the other, and by this means we are able to lengthen our tubular front when using long-focus lenses. (Amateur Work,)
To print transparencies for the lantern by contact, when the negative is of a larger size than the. picture required, necessitates some special kind of printing frame, if the negative is to be kept free from scratches. The following may be simply made, and will be found- a great convenience where a number of pictures are required alike: - Take an ordinary printing frame, say a 12 by 10, of the kind made to use without a plate-glass in front, and in the rebate where the negative is usually placed fasten, with strips of paper all round the edges, a piece of very flat glass; turn the frame over, and on the other side of the glass fasten a mask of paper or cardboard having an opening 3 1/4- in.by 3 1/4 in. exactly in the centre. Now, in place of the ordinary hinged back, make a frame of the same size and thickness, with an opening in the centre about 6 by 4 in., and cover all over one side, with the exception of the opening, with a piece of velvet. This frame, when placed in position, will be held by the springs that originally held the hinge back. To complete the arrangement, cut out a piece of dry mahogany 1 in. thick, and exactly 6 by 4 in. to accurately fit the opening in velvet-covered board, and on this block draw a square 3 1/4 in. by 3 1/4 in. exactly central.
At one end of this square glue down a very thin slip of hard wood - that is, rather thinner than the glass plates to be used - and at the other end cut a mortice 3 1/4 in. long, and about 1 in. wide, right through the block, beginning just within the 3 1/4 in. line, say 1/16 in. less. Into this mortice tit a piece of wood 3 1/4 in. by 1 in. by 1/2 in. On to one side of this piece glue a similar strip of hard wood to that placed on the end of the block only, projecting 1/4 in. each end, and on the other side screw a similar piece so that it can be removed. Place the piece in the mortice and screw on the back slip of wood; there will then be left a space of 1/2 in. at the end, just room to put a piece of bent steel clock spring sufficiently strong to clip the 3 1/4 in. plate in position. The sides of the block may be rebated down 1/2 in. at the 3 1/4 in. line to allow the thumb and finger to adjust the plate in exact position. To complete the frame, place the block in the opening of velvet board, and arrange an ordinary brass pressure frame spring to keep it in position. For use, take the frame, remove the board, and adjust the part of negative required over the 3| in. spening, then place over the velvet-covered board, and fasten down the porings.