When the Sun goes down in August, and lingering twilight fades, look about halfway up the northwestern sky to the Big Dipper. Use the finder chart on this page to find Mizar, the second star from the end of the Dipper’s handle. Take a good look at this star. Do you see anything peculiar? If you have normal vision, you should see that Mizar has a faint visual companion, named Alcor, very close to it. While seeing Alcor is no exceptional feat of visual acuity, the star’s visibility can teach us something about how the eye perceives light.