THE writer Richard Hughes (''High Wind in Jamaica'') was explaining how a novelist's imagination gets going. Of course, being Welsh, his fancy is easily triggered. Once when he was in Spain and all the heat and brightness suddenly seemed too much, he closed his eyes and imagined he was back among the greenness and wetness of Wales. He had painted such a convincingly refreshing scene for himself that he wasn't surprised when his mind's eye saw a man approaching. As the man grew closer, Mr. Hughes could see he was carrying something, and when he grew closer still, he could see that it was a child. And so his novel began.
While he was talking, I closed my eyes too, and saw brown trout streams rushing over gray rocks. The trees lining the banks were green with moss, gray with lichen. I was lost in memories of Wales: its high mountains, waterfalls, castles, and the texture of stone walls. Wales is rich in wild flowers - primroses, snow drops, pink campions, the little blue milkwort, dog myrtle, as well as the inevitable heather and gorse. It is one of the few places where bluebells grow in the open, not just in the woods, and some of its plants belong to the Alps or the Arctic.
In fact, Mr. Hughes had said enough to make this Englishwoman nostalgic for long-ago Welsh holidays, nostalgic enough to bring me back from the United States to see Wales again last September. Most tourists depart at the end of August, just when the weather is at its best. Besides I wanted to write about ''Britain seen from one place.''
There's a lot to be said for rushing around the UK in an automobile, but to get the feel and smell of a place into your bones, nothing beats settling down for a day or two. And not much beats the chance to get out of a suitcase for a while. I got out of mine in Dolgellau. The name is Dolgelley in English, and you will be understood if you pronounce it ''Dogethley.''
It is a gray stone town, built so it is said, before streets were invented, and was the site of the last Welsh parliament in 1405. Once it was famous for its flannel; now it is part of Snowdonia National Park and a good center for exploring the nearby countryside. Besides, I have some favorite hotels around here.
When World War II was just a nasty gleam in Hitler's eye, my family used to stay with Mr. Hall, who had three hotels and three sons - one hotel for each son. Gilbert has sold his, but the other two sons still run excellent hotels, both well known for their food. Bill keeps Bontddu (pronounced Bonthee) Hall, a sort of junior but extremely elegant stately home with magnificent views over the Barmouth estuary.
But my favorite is still the one by the side of Gwernan Lake, two miles from the town on the narrow uphill road climbers take to begin their way up the mountain (Cader Idris). Kept by Peter and Llywela (call me Boo, she says, in mercy to the non-Welsh speaker) Hall, Gwernan Lake Hotel is simpler than the other two, although the food certainly isn't - gourmet food partway up a mountain takes visitors by surprise. Peter does all the cooking.
There are bed-and-breakfast places around, too. Very near Gwernan Lake is Rhydwen Farm, straight out of a children's book. Fat and contented hens and cats (two - one jet black, one snow white) wander through an amazingly clean and tidy farmyard. The double bedroom and sitting room are clean and comfortable, the family welcoming.
The people who come to Gwernan Lake come to fish, climb mountains, walk or visit the places round about. The kind of stroll we took on the lower slopes of Cader hardly counts as mountaineering, but along with the birds and the sheep we did meet an enthusiastic young Dane striding down the mountain, his fair hair still damp. ''You swam in the pool just below the peak,'' we said, remembering the numbing water. Of course he had, but he was more impressed by its clarity than its iciness.
Serious climbers who follow his example will want to consult a guidebook and maps before taking one of the routes up Cader Idris. The highest point, Pen-y-Gader, is 2,927 feet above sea level. Both the Foxes Path and the less, but not much, strenuous Pony Track begin near Gwernan Lake.
The town itself is a center for walks (and a good place to buy ingredients for a picnic - pork pies, Caerphilly cheese, and rich Welsh butter).
The best-known walks have names: the Torrent and the Precipice. Take the Torrent Walk (three miles outside the town and about a mile long), and you find yourself walking through trees alongside a rocky stream. It's green enough, wet enough, and beautiful enough to satisfy a Richard Hughes on the hottest day. Last September we saw a trout struggling up one of the waterfalls.
The Precipice Walk (seven miles round trip from the town) is cut like a ledge out of the hillside - high and steep but perfectly safe, and the views are magnificent.
Within driving distance is the Bird Rock (Craig-yr-Aderyn) - a dramatically shaped, cliff-sided, accurately named hunk, a favorite spot for sea birds, including thousands of cormorants.
About a mile away is the village of Llanfihangel-y-Pennant, home of one of my childhood heroines, Mary Jones. For years Mary saved up to buy a copy of the Bible in Welsh. By the time she was 16, in 1800, she had got enough money, so she set off to Bala, walking barefooted 50 miles over the mountains, only to find the last copy had been sold. But the story has a happy ending. The Rev. Thomas Charles not only sold her one of his copies, but was so impressed by her zeal that he began campaigning for more Bible translations. The British and Foreign Bible Society was the result. In the village there is a monument erected by Sunday School children, ''Er cof am Mari Jones''m (''In memory of Mary Jones.'')
Wales, particularly this section of Wales, has also its miniature steam railways to boast about, many of them maintained by volunteers; most of them are over a hundred years old and all of them run through magnificent scenery. A particularly charming one starting at Abergynolwyn, began life, like so many of these tiny railways, carrying slate down from the quarries. Special trips encourage passengers to get out and sightsee.
The Festiniog Narrow-gauge Railway has acquired a more dignified (and more accurate) name since my childhood (it used to be ''The Ffestiniog Toy Railway,'' and once my sister got scolded by the driver for leaping out to pick primroses en route), but it's still a favorite way to go through the mountains of Snowdonia.
Castles - of course there are castles nearby. In fact in Wales there are castles near wherever you happen to be - one reason why l983 has been designated Wales's Year of the Castle. Twenty miles away there's one, Harlech Castle, that looks exactly the way it should: massive stone towers and massive stone walls (you can walk along them), standing guard high over the bay. The view is just what it should be, too - the town roofs and the sea below us, the peaks of Snowdonia standing out in the distance.
Of course there's more to Wales than the Dolgelley neighborhood, but I think it's a fairly reliable sample of what the countryside is like as a whole. It has its mining and industrial areas, of course, but one-fifth of the country is national park land. Whether my account of it is equally dependable is another question. Perhaps if I throw in a few criticisms, my delight will be more convincing.
So here goes: When ribbons of rain blot out the mountains and turn slate houses a dull gray, Wales can be depressing; and caravan sites (trailer parks) have mushroomed up along some of the loveliest stretches of coast and some wild and beautiful scenery has been ''made accessible'' to tourists. But these are blots than can be minimized by doing one's visiting off-season. September is my favorite month, but at any time of the year there is unspoiled beauty to spare.
Listen, for instance, to a turn-of-the-century writer describing the still-unchanged route along the side of the Mawddach estuary: ''There is no more charming drive in Great Britain than the drive from Dolgelley to Barmouth, unless it be the drive from Barmouth to Dolgelley.'' Practical information:
Visitors travelling from London to Dolgelley (about 207 miles) need a car and a good road map. Then they can plan to take in favorite landmarks like Bath, the Cotswolds, Oxford, or Stratford, or to choose a coastal road. A double room with breakfast costs about $35 at Gwernan Lake or at Bontddu Hall from $35 to $66.