I agree with the commenter that the gesture is the "reverse V sign" or a middle finger-like gesture to the heavens, which Huxley describes as containing [infinite] mystery.

The significance of the gesture being that the lights and buildings represent the technology/scientific and industrial progress, which was prompted by mankind's desire to understand the mysterious universe. The product of that desire and skepticism is "developed" society [the gesture], in the book, now used to sedate man from mystery and discovery and skepticism. (imo)