One interesting section of the Act said that the owner could enlarge the window, but the enlarged section itself would also have to remain unblocked for 20 years before it attained the same Ancient Light rights. In the meantime the neighbours could erect a screen to block off light to the enlarged section but not the existing Ancient Lights portion in order to prevent granting of the new rights!
The American Legal System took a more pragmatic approach, which might explain the proliferation of towering skyscrapers on Manhattan
The doctrine of ancient lights has not been adopted in the United States since it would greatly hinder commercial and residential growth and the expansion of towns and cities. - West's Encyclopedia of American Law