Magic Lantern

Take a box roughly 250 mm long and line the inside matt black. Cut a 60 mm round port in one end for the lens and a chimney hole in the top.

Cut a 50 mm vent in the top above the lamp position, with a baffle over it so no light escapes upward. A sealed box with a flame in it is a fire, not a lantern.

Stand a spirit lamp on a non-combustible tile at the rear third of the box. Keep the flame at least 60 mm from any cardboard surface.

Mount a concave mirror behind the flame, facing forward. Light heading backwards is otherwise wasted — the mirror turns it round and roughly doubles the usable output.

Fit a large convex lens between lamp and slide slot, close to the lamp. This is the condenser: its job is to collect the diverging light and push it evenly through the slide, not to make a picture.

Cut a vertical slot just in front of the condenser, sized for your glass slides. It should hold a slide upright with a little friction so it stays where you put it.

Mount a second convex lens in a sliding tube at the front port. This one forms the image, and sliding it in and out is the focus control.

Paint a simple bold design on a glass slide with translucent colour. Fine detail disappears at projection scale — strong shapes and clear colour read best.

Put the slide in inverted. A single projection lens flips the image, exactly as in the camera obscura — so the slide goes in upside down to land the right way up.

Darken the room, light the lamp, and slide the projection lens until the image sharpens on the wall. Moving the lantern further back gives a bigger but dimmer picture.

Take the condenser out and look. The image survives but goes dim and unevenly lit — bright centre, dark corners. That is proof the condenser's job is light distribution, not image formation.

Measure image height at 1 m, 2 m and 3 m from the wall, refocusing each time. Plot height against distance — the relationship is linear, and the slope is set by your projection lens.

Huygens, 1659. The magic lantern was first described by the Dutch physicist Christiaan Huygens in 1659, and he had a working lantern by that November. He drew the working principles and — notably — already demonstrated the illusion of movement using glass slides. Athanasius Kircher had described a projection system close to a magic lantern in 1646, and credit is genuinely debated, but Huygens is generally regarded as the most probable creator of the device as such.

Two lenses, two jobs. This is the idea the build teaches. The condenser sits near the lamp and gathers widely-diverging light into a beam that passes evenly through the slide — it improves brightness and uniformity and contributes nothing to sharpness. The projection lens takes the illuminated slide and forms a real, magnified, inverted image on the wall. Every projector since, including the one in a cinema, keeps this division.

Why it inverts. Same reason as the camera obscura: rays cross as they pass through the lens, so top maps to bottom. Loading the slide upside down is the fix, and every lantern operator did it automatically.

What it was used for. From the start the lantern served entertainment, education, research and children's play. Travelling showmen carried them across Europe projecting biblical scenes, ghost stories and scientific demonstrations. The eighteenth-century phantasmagoria shows used hidden lanterns on rails, moving toward the screen to make projected ghosts appear to rush at the audience — the first special effect built on the fact that image size scales with distance, which is the thing you measure in step 12.

The line to cinema. Add a light source bright enough, a mechanism to change slides fast, and film in place of glass, and you have a film projector. The optical layout — lamp, reflector, condenser, image plane, projection lens — did not need to change. When a modern projector is described as having a "lamp house" and a "lens", those are magic lantern words.