AFTER the pro-democracy demonstrations in China and the subsequent backlash in Hongkong, the United States acted quickly to prop up the territory's ailing stability by changing its immigration laws to allow more immigrant visas to Hongkong people employed by American firms. It went further. It permitted certain classes of visas issued to Hongkong people seeking to immigrate to the United States to be valid for 10 years. ''Normally, immigrant visas have to be made use of within four months,'' said Richard Williams, chief of the consular section of the US Consulate General in Hongkong. This long extension had a sobering effect on those who sought to leave the territory at the earliest opportunity. This was not a concession granted to everyone who held an immigrant visa, but to special categories whose continued stay would have been considered necessary for Hongkong's stability. ''Any change in the issuing of visas depends on change in the law. The change in the immigration law in 1990 was designed to aid the stability of Hongkong,'' said Mr Williams. It was understandable that local employees of American companies in the territory would be anxious about their future and sought some assurance that they would be provided with an immigration safety net. Hence, the US Government's decision to increase the number of immigrant visas granted to such employees. The number of immigrant visas issued over the last few years has been fairly steady. Mr Williams said that last year the Consulate General issued 16,000 visas to prospective immigrants from Hongkong. He expected the figure to be slightly lower this year. He said that, in common with Canada and Australia, the US had not seen an increase in immigrant visa applications recently. Meanwhile, the US mission issues about 150,000 non-immigrant visas each year. This number increases slightly every year. These are the visas issued to students, tourists and those having employment in the US. The employment visas are generally issued to young people from hi-tech industries.