And there, staring back, is another human eye. So thin is the divide, our lashes almost touch. In the fading light of dusk, it is an unsettling moment of intimacy.
This person, I know, is waiting only for me to move on. Then he will choose the right instant to vault the fence and, like a rabbit bolting its hole, plunge into the floodlit landscape of brush and small canyons that lies between here and the first streets of San Ysidro, the southernmost community of San Diego.
The distance is only a few hundred yards, but since the completion of the fence and the installation of the lights earlier this spring, that final dash has become more hazardous. And there are more US Border Patrol agents lying in wait than there used to be. Of the one or two thousand migrants - or 'customers' as the agents teasingly term them - who attempt this crossing from Mexico nightly, normally about half are caught and sent back.
But half is not enough. The political mood in California, and indeed in most of America's southern border states, demands that not one is allowed through. In an important election year, Californians are displaying, not for the first time, a deepening antipathy towards those entering their domain illegally. This mood is linked to the state's continuing economic troubles and perhaps to the rhetorical encouragement from its leaders. There seemed little surprise recently when a candidate for the California state assembly called for landmines to be laid alongside the fence.
Pete Wilson, California's Republican Governor, who is fighting for re-election in November, has not gone that far. However, he has made immigration a central issue of his campaign, proposing, for instance, that children of 'illegals' born in this country should no longer be given automatic US citizenship and therefore be denied welfare assistance. His most powerful television advertisement features stark black-and-white news footage of desperate Mexicans bursting through the customs checkpoints at San Ysidro and sprinting up the freeway. 'They keep coming,' warns the governor's voice.
This tactic appears to be working. Mr Wilson's poll ratings have risen markedly, and surveys pinpoint illegal immigration as an area of increasing concern. He has filed two lawsuits against the federal government, the most recent last week, demanding that it reimburse Sacramento for money it must spend on illegal aliens, either to educate their children, afford them hospital treatment or imprison those caught committing crimes. It is an approach, incidentally, mimicked in recent weeks by Arizona, Texas and Florida.
Most Draconian, however, is a popular petition launched by a former Reagan administration official for inclusion on the November ballot. Called SOS, Save Our State, it would bar illegal aliens from receiving welfare support and almost all medical treatment. It would also prohibit state public schools from accepting children even 'suspected' of living in the US illegally.
Judging by opinion polls, the SOS initiative is likely to be approved.
'The levels of immigration, both legal and illegal, are far too high for any country to successfully absorb,' argues Ira Mehlman, California spokesman for Fair, the Federation for American Immigration Reform, a group calling for a virtual moratorium on all immigration.
The US receives a little more than a million new immigrants annually, of which about one-third are thought to be illegal. Unsurprisingly, Fair is a strong supporter of the SOS movement.
'What is happening is the cumulative effect of year after year of the stresses and strains that this has put on many parts of the country, particularly California,' Ms Mehlman says.
For a glimpse of what passage of SOS could mean, however, you might visit the Sherman School in Sherman Heights, San Diego. Squashed into one city block, the school has more than 1,000 pupils aged 4-12. Almost all are from families on welfare.
The principal, Cecilia Estrada, guesses that about one in four are illegals. New pupils show up every day asking for places, which she is obliged to provide. Most used to arrive from northern Mexico - and were often further ahead in their schooling than their American counterparts - but recently they have been coming from ranch country in central Mexico, with no education.
Mrs Estrada can barely contemplate what the SOS initiative asks: that she should in effect become an agent of the government and report undocumented children. 'I don't want to think about it,' she says. 'If we are going to turn the children away, what are they going to do? They are not going to go back to Mexico, because their parents come here for jobs. They'll be on the streets. Is that what we want?'
Only gradually are voices of protest being raised. Last weekend, more than 20,000 Hispanics, mostly Mexican-Americans, marched in Los Angeles to voice opposition to Mr Wilson and the supporters of SOS. Although many American Latinos may support a clampdown on illegal immigration, they fear that behind the SOS proposals lies the germ of a new racism against all non-whites.
Roberto Martinez, a human rights lawyer in San Diego, sees it already. 'I can't remember it ever being bad like this,' he said. 'It is creating division and fear and xenophobia in California against all immigrants.'
Equally concerned is Bob Filner, the US Congressman for the district, which includes the San Ysidro border area and through which half of all the illegal aliens from Mexico traditionally flow. 'The polarisation that is going to occur is just incredible,' he warned. 'You don't know who is illegal anyway, except by appearance and how they speak English. That means every Mexican-American citizen is in danger.'
Mr Filner, a Democrat, says it is hypocritical to call for an end to illegal immigration when it is an open secret that almost every sector of the state's economy - from the construction trade down to most middle- class households - depends on undocumented aliens for cheap labour. 'The Republicans are demagoguing this issue. But once they get re-elected, they won't do anything about it.'
The Congressman accuses Mr Wilson of trying to place on immigrants the blame for the continuing economic ills of the state, including an unemployment rate that remains two points above the national average. 'It's just incredible. The game is find a scapegoat, rather than attacking the real issues - why we don't have an industrial base in California any more? How are we handling defence conversion? Rather than answer those questions, they are just going to say, let's cut off immigration and let's cut off these services.'
No California politician, however, dare ignore the issue entirely. 'I'm not going to get sucked in,' he protests. 'But as Democrats we have to recognise there is a problem, otherwise we're in trouble politically.' Consequently, Mr Filner, with other California Democrats, has produced proposals, which include a clampdown at the border, and demands for reimbursement from Washington. They also want, however, to speed up naturalisation of those Mexicans in America legally and awaiting citizenship.
All the while, on the Mexican side of the border, there is little evidence of the debate in California discouraging the would-be invaders. Night has now fallen in earnest, and all along this side of the fence, there are knots of people, mostly young men, preparing to chance their luck. Some are taking a last cup of tea from one of scores of rickety refreshment wagons; others are placing plastic bags over their shoes in preparation for wading across the Tijuana River, which flows mostly with human sewage.
'The border don't mean nothing to me, man,' boasts Alfredo, a frequent traveller to the US. 'They never catch me, man, never. It's real easy for me.' Maybe Alfredo and the owner of the eye behind the chink are in LA by now. But, then, maybe they are back here, stuck against the fence.
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