FARALLONS PHOTOGRAPHS -- At the Museum of Natural History through September 21.

CAPTION: Picture, A pair of Western gulls, from the show of Tupper Ansel Blakes's photographs of the Farallon Islands.

Twenty-five miles west of San Francisco, under constant assault by the sea, lie seven islets almost as craggy and barren as the mountains of the moon.

From a passing ship the Farallon Islands seem forbidding and lifeless, but they long have been a nursery for vast flocks of birds and great heards of seals.

Nineteenth-centruy seal hunters wrought much slaughter there until the seals were all but gone. The discovery of gold on the mainland nearly doomed many of the nesting flocks, particularly the common murre, whose misfortune it is to lay eggs as tasty as those of chickens. Flush '49ers would pay a dollar apiece for them, and for nearly half a century eggers took about 25,000 a year. The total was some 1.2 million eggs; it's testimony to the tenacity of the life force that the common murre still is common.

In 1897 California put an end to the raids, and in 1909 the Farallons became a national wildlife refuge. Protected from everybody but nosy naturalists, the seals and birds have rebounded to something like their old numbers.

It has been a spectacular success, as shown by the exhibition of photographs by Tupper Ansel Blake on the third-floor rotunda of the Smithsonian Natural History Building. The standards of nature photography have risen so high it takes stunning pictures to engage the eye. Blake has produced them, and his work is complemented by a section of photographs from the bad old days.

Color photographs tend to idealize a subject by enhancing or washing out shades and tones. Blake's uncompromising composition and precise color control show the subtle beauty of the wildlife without vivifying the stark landscape or the bleak ocesn.

The exhibition is also outstanding for the quality of the captions, which manage to gracefully compress a great deal of information within the iron discipline of brevity. It was surprising to learn they had been through a reviewing process involving half a dozen state and federal agencies, because the captions have none of the clumsiness usually associated with committee work.