For the second time in a decade, the technology industry finds itself pressing a vision of augmented eyewear onto a public that has not asked for it. Smart glasses — spanning audio companions to heads-up displays — command enormous investment, yet fewer than one in twenty consumers actually wears them. The obstacle is not ignorance of the product, but something older and harder to engineer away: a deep, instinctive unease about being watched, and about those who might be watching.