The National Geographic specials now have a contender for most popular show on Public Broadcasting. Last month's premiere of the ''Smithsonian World'' series of specials proved to be a major PBS debut with the show garnering high enough audience numbers to place it in the select category of most-watched PBS programs. Now, the second in the series is being offered and it is even better than the first: Crossing the Distance (Tuesday, Feb. 15, 8-9 p.m., check local listings for premiere and repeats).
Once again, PBS viewers are being taken on an exciting voyage of discovery, using the Smithsonian Institution's wide-ranging activities as the jumping off point for a series of segments expertly linked together to emphasize the relationship of various forms of knowledge to each other. The program seems to revel in jolting viewers with stimulating little shocks of recognition and intellectual revelation.
This second show begins with a visit to the Panama Canal, then investigates the work being done at a research outpost on a tropical-rain forest island in the lake formed when the canal was first built. You'd never believe that the relationship between bats and katydids, and the possibility of using iguana tails as a food source, could be so fascinating.
But the piece de resistance, the segment that adds human proportion and depth to this stimulating hour of offbeat travel tales, concerns Anne Morrow Lindbergh. One of the original pathfinders in aviation, her past is investigated in newsreel footage and snapshots. Then, she is interviewed by host-narrator David McCullough and confirms the impression her books have made that here is a complex, meditative woman.
''Publishing a book or having a child'' can be as important as crossing the Atlantic alone, she says. ''Different things give you a sense of finding your place in the world. One wants to find where one is useful or where one can contribute. You must make a contribution to your world.''
Mr. McCullough, executive producer Martin Carr, writer Michael Winship, together with the Smithsonian Institution and the James S. McDonnell Foundation, which funded the program, can certainly feel proud of the contribution they are making with ''Smithsonian World.''