Mr. Jackson, who was in Rome for a meeting of the conservation union's Species Survival Commission last month, brought an example of the trick that fooled the tiger: an inexpensive, rubber mask of a pale-faced human with a thin mustache. He said the Indian Forestry Service has issued more than 2,500 masks to workers who are among the 8,000 who get permits to go into the Sundarban Tiger Reserve. Stopping to Pray

No one lives in the reserve of mangrove forests, cut by rivers and creeks on the border of Bangladesh and India, said Mr. Jackson, who knows the area well. But people on both sides go in to collect fish, wild honey and wood in the tigers' habitat.

Often they first stop to pray for protection at little shrines that rim the area because the large Bengal tigers are unusually fierce. While tigers elsewhere often ignore humans, the Sundarban big cat may attack on sight. Local people tell stories of how the tigers even swim out and sneak up on fishermen in their boats.

Since 1973, when the reserve was formed, scientists and forestry workers have tried to find ways to coexist peacefully with the 500 or so tigers and to stop them from thinking of humans as easy prey. They have put up human-shaped dummies of bamboo and mud, dressed in clothes with human scent and attached to electric wires, for example. Fences have been wired, and the tigers have been heard to scream from the electric shocks.

But it was a student at the Science Club of Calcutta who came up with the idea of using a human mask. The reasoning, Mr. Jackson explained, was that many species use a similar technique to fool predators. ''Butterflies, beetles, caterpillars have developed patterns that look like big eyes,'' he said. ''We know that this is a deterrent.''