Snow Goggles

Measure the distance between pupil centres, the width across the temples, and the height from brow to cheekbone. The goggles are fitted to one face; a generic pair leaks light at the edges.

Take a piece of close-grained wood, antler or bone about 140 x 45 x 20 mm. Traditional makers used driftwood, antler, bone and ivory — whatever the region gave them.

Carve the front into a gentle curve following the line of the face. This is the shape that lets the ends sit against the temples with no gap.

Hollow the inside to clear the eye sockets and bridge the nose, testing repeatedly against your own face. The hollow is what makes them wearable — a flat back presses on the brow and is unusable within minutes.

Cut a notch in the lower centre so the goggles straddle the bridge of the nose. This one cut carries most of the weight and sets how the whole piece sits.

Mark two horizontal slits centred on your measured pupil positions. Off-centre slits force the eyes to work at an angle and cause headaches within the hour.

Cut each slit about 1-2 mm high and 25-30 mm long. Narrow is the entire design: a thin slit blocks the overwhelming majority of reflected light while still giving a usable field of view.

Widen each slit toward the outside so the opening is narrow at the eye and flared at the front. This keeps the light-blocking of a narrow slit while widening what you can see through it.

Rub soot or charcoal into the hollowed back. Traditional goggles were sooted for exactly this reason: it stops the light that does get in from bouncing around inside and washing out the view.

Drill a hole at each end and fit a sinew or leather cord that holds the goggles firmly against the face. Loose goggles admit light around the edges and defeat the whole object.

Wear them and check what you can see: the horizontal band should be usable, the vertical view will be very restricted. Note how much you have to move your head — that head movement is how they are actually used.

If you are short-sighted, look at distant text without your glasses, then again through the slit. It gets sharper. That is the small-aperture effect increasing depth of field — the same physics as a pinhole camera, worn on the face.

Try them on snow or bright water — never by looking at the sun. Compare with and without: the difference is not subtle, and it is the whole reason the object exists.

Finish with oil or fat so the wood does not take up moisture and warp. A pair that warps stops sitting flat and starts leaking light at the temples.

Thousands of years old. Inuit and Yupik people of Alaska and northern Canada carved snow goggles from ivory, antler, bone and wood, hollowed to fit an individual face and cut with very narrow slits. They are among the oldest surviving eye protection anywhere, and they were made for a specific medical problem: photokeratitis, snow blindness, caused by ultraviolet light reflected off snow and ice burning the surface of the eye.

Two effects, one slit. Narrowing the aperture cuts the light reaching the eye dramatically — a thin slit blocks the great majority of reflected light. It also reduces the size of the blur circle on the retina and increases depth of field, so vision through the slit is sharper than without. The maker gets glare protection and improved acuity from a single design decision, which is unusually elegant for any piece of equipment.

Sooting the inside. Blackening the hollow is not decoration. Any light entering the slit that then reflects off a pale interior surface reaches the eye from the wrong direction and reduces contrast. Modern optical instruments blacken their interiors for precisely the same reason.

What they gave up. Field of view. These goggles restrict vision to a narrow horizontal band, and the wearer scans by moving their head. That is a deliberate trade — on open ice the alternative is being unable to see at all within a day — and it is a good example of a design that is only optimal once you know the environment it was made for.

Not a curiosity. Glacier travellers and mountaineers used slit goggles well into the modern era, and the pinhole-aperture effect is still used in optometry: the pinhole occluder held in front of an eye during a sight test works on exactly the same principle. The object is ancient; the optics are current.