A world of shimmering shades of green

Another Life: Now's the one time in the year when I can be seduced by rural illumination; when even that satellite view of the world lit up in darkness can seem sparkling and festive rather than a blight of urban phosphorescence.

Here below black mountains, light spills out in the chilly glare of yard-lamps and the amber pool around the crossroads church, in rampant reindeer on gables and flickering icicles on eaves. A Christmas tree glows in a window on a lonely road and even my old misanthropic, star-gazing heart is lifted.

But I also enjoy trips across the unlit moors on winter nights, with somebody else at the wheel. That leaves me free to admire the wild theatre of it all: golden tinsel of moorgrass fluttering close at hand, silvered willows, the ruddy sweep of light across the bog. A bit of wind is good, to set the sedges dancing: little waves shivering away from our bow.

Along by Doo Lough, where a thin road hugs the lake, sheep rise to their feet from the warmth of tarmac and move aside in a gleam of reproachful, bright-emerald eyes. This spooky rite of passage was one of our early discoveries on moving to Mayo and one we have treasured as locals. If ever they come to fence the pass, as has happened all through Maam Valley, we'll have lost another edge of contact with the wild. (Are blackface ewes wild? Not strictly, but darkness returns them a measure of nervous, mountain mystery).

Last week it was the sapphire glint of a stoat's eyes as it leaped to a wall and stared back at us that sent me to the Internet. I wanted a refresher on the tapetum lucidum, the cause of night-shine some animals have in their eyes and we don't. It's a mirror-like structure behind the retina that bounces light back through the photo-sensitive cells, creating a second chance to react to the light's information - a sort of double exposure to intensify dimly-lit scenes.

You can imagine why night-hunters like cats, stoats, foxes and crocodiles would need this intensified vision, but nervous herbivores have their own forms of it, too: expanding their beautiful, depthless pupils, horses can gather light almost as well as owls. Diurnal primates - that's us - and squirrels, pigs and red kangaroos are some of the animals that manage without this "second sight". (The "red-eye" phenomenon of so many photographs of people is literally a trick of the light - a straight reflection of the flash from the retina when the pupil is expanded in a weakly-lit setting. Pets, too, can suffer its indignity, though a light-coloured dog's tapetum is as likely to shine bright blue as red).

A general erosion of the senses of the city-dwelling human, as reported recently from Germany, should scarcely surprise us where night-vision is concerned: like hearing, it is at once overwhelmed and out of use. For millions of people, darkness is almost unknown, still less the steady progression into dusk that challenges the eye to adjust. At first hint of the sun's surrender, artificial light is brought to bear at a level that would have shocked our forebears (even in my own childhood, a single 60-watt bulb seemed quite enough to light a whole room).

So it's no wonder that such night-vision as humans naturally have has been blunted for many decades: enough, perhaps, to encourage atrophy. There do seem to be remarkable exceptions. In the 1980s, an Englishwoman called Chris Ferris, a sufferer from insomnia, began to watch wildlife by night, achieving the astonishingly intimate woodland encounters with badgers and foxes she described in her book The Darkness Is Light Enough. I have envied her to the point of incredulity, finding my own night-vision on a moonless light just about adequate to keep me out of the ditch.

What, then, of night-vision binoculars, of the kind the military use? An Internet tutorial from www.howstuffworks.com has left me just a little wiser on the science of image enhancement. Electrons were never my strong-point, but there's a significant difference between image intensification that collects and amplifies what light there is, including infra-red, and thermal imaging that relies solely on infra-red energy emitted from objects. The technology of the first is in its fourth generation of perfection and pushes the cost of the latest binoculars close to $4,000 (though now would be the time to buy them).

There is a certain lure to becoming a night naturalist, exploring a wild world in shimmering shades of green. There is also a satisfaction in moving through the dark as humans were meant to, aware of every rustle and sigh, but content to spy on the details of parallel lives.