Chapter 3, page 41, paragraph 2:

Having refracted sunlight with a prism, he found that he could pass the colors through a lens and make them recombine. The lens undid what the prism had done, leaving a colorless circle of light on the wall.

Chapter 1, page 15, paragraph 1:

Once Galileo had established his law, Drake believed, he showed it to others in an easier, less precise manner: by marking the track beforehand -- 1, 4, 9, 16, 25, 36, 49, 64 -- and then using a water clock to confirm the timing.