EIGHT years have passed since Lady Thatcher returned from Beijing waving a piece of paper and declaring ''Prosperity and stability in our time''. Looking back, it seems amazing that so many of us succeeded in suspending our natural scepticism and actually believed that China might grant Hongkong a measure of autonomy.

Together, it was thought, the Joint Declaration and the pressure of world opinion would ensure that China respected its promise of ''One country, two systems''. It is surprising that Hongkong people have not taken more interest in the fate of the Tibetans, the last people to come under the rule of Beijing. For the Tibetans, too, had a treaty, the Agreement of the Central People's Government and the Local Government of Tibet on Measures For the Peaceful Liberation of Tibet, concluded on May 23, 1951.

It begins with a wholly fictitious assertion of Tibet's long history as part of China, and how its ''local government'' fell prey to imperialist intrigues and adopted ''an unpatriotic attitude to the great motherland''. The agreement then lists a numberof guarantees, for example, that the Tibetan people should exercise autonomy, that the Central Authorities should not alter the existing political system in Tibet. The agreement also provided for protection of the status and function of the Dalai Lama, freedom of religious belief, development of Tibetan agriculture and commerce, etc. It was also stipulated that there would be no compulsion on the part of the Central Authorities in effecting any reforms, and that the PLA would abide by the agreement.