To complete the stereopticon an oval chimney is to be made, and fitted with a flange so that it may be held to the top of the iron hood with four little iron buttons or lap-hooks riveted to the top of the hood. This chimney should be four inches long by two inches wide at the bottom, and one and a half inches square at the top, and eight or ten inches high. At the top arrange a light-cap about as shown in Fig. 16, so that the light from the lamp or gas-jet will not throw rays up against the wall and weaken the picture on the sheet.
Precaution must be taken to stop out all surplus light other than that projected through the lenses, since the useless rays tend to weaken the round disk of light on the screen, and render the picture weak and neutral in tone, instead of sharp and vigorous. The wood-work should be given two or three coats of shellac, and the iron parts should be blackened.
Dissolving views cannot be shown in a single lantern, but if two lanterns are fitted with the same kind of lenses, and placed side by side, so that the disks on the sheet are uniform in diameter, it will then be possible to obtain some very beautiful dissolving effects.
Lantern Slides by Contact-printing
Many amateurs have an idea that it requires a great deal of skill to make lantern slides, but any one who can produce a good negative can soon learn how to make a good lantern slide. The simplest way is by contact-printing.
Select a negative free from spots, scratches, or pinholes. It must have fine detail in the shadows, and no harsh contrasts of light and shade. The regulation size of a lantern slide is 3¼ × 4 inches, so choose a negative which will still make a good picture if all but the portion included in these dimensions is blocked out. Cover the part of the negative which is to be blocked out with black needle-paper, or paint it with non-actinic paint, applying it to the glass side of the negative. The negative is placed in a printing-frame, and then by means of a red light the slide-plate is placed over the part to be printed from, the film side towards the negative.
If one has a lantern, the light of which is suitable for printing slides, cover the negative, open the door of the lantern, and then, holding the printing-frame about fifteen inches from the light, expose from five to twenty seconds, according to the density of the plate. A plate that prints quickly will need but five or eight seconds, but a denser plate will require a much longer exposure, often as long as thirty seconds. Cover the plate as soon as it is printed, close the lantern, remove the slide from the frame, and place it face up in the developing-tray. Turn the developer over it quickly, taking care that the whole surface of the plate is covered immediately. Any developer that makes good negatives will make good lantern slides. A weak developer is to be preferred to one which brings out the image quickly. Develop till the detail is well out; wash and fix same as a negative.
As every imperfection in a plate is magnified many times when thrown upon the screen, great care must be taken in the developing, fixing, washing, and drying. When the slides are washed, take a piece of clean surgeon’s cotton and wipe the film very gently, then place to dry where no dust will settle on the surface.
If there are any spots on the plate after washing and before drying, they may be removed with ferricyanide of potassium in solution. Tie a small piece of surgeon’s cotton to the end of a glass rod, dip it into the solution, and touch the spot very lightly. Rinse the plate at once, and if the spot has not entirely disappeared repeat the operation. The ferricyanide works very quickly, and must be rinsed off as soon as applied.