I'm hesitant to give this as an answer, because I don't have any good references and my own memory is pretty hazy, but it could be The Moonlanders, by W. C. Chalk. There's no lens in that story, but there is a method of traveling between Earth and the moon using the gravity of specific planetary alignments.
In the story, the heroes discover a place in South America, near the famous Nazca lines, where the gravity of Earth and the moon cancel each other out at certain times, and they take advantage of this to travel to the moon. There they meet someone - a humanoid alien, possibly? - who wears a bright white suit and inflates a balloon on his back to allow him to 'fly'. It's concluded that these beings were the source of 'angel' stories.
A sequel told the story of a military expedition using the same effect to travel to, and attempt to conquer, Mars.