Galileo Galilei was puzzled. The Renaissance-era astronomer had noticed that planets appeared to expand with a “radiant crown” when viewed with the naked eye—but the effect was greatly diminished when viewed through a telescope.
The discrepancy led him to wonder: Was this size illusion caused by moisture on the cornea? Light scatter? Neither possibility satisfied because these effects would persist even if one used a telescope. In fact, Galileo had hit on a visual riddle that researchers are still unraveling.
One clue came from the observations of another luminary: artist and engineer Leonardo da Vinci. Just decades before Galileo's discovery of the radiant crowns, Leonardo had noted that dark objects on a light canvas seemed more defined than light objects on a dark background. Vision scientists since have found that a white shape on a black background often appears larger than an equally sized dark object on a light background. An explanation for these peculiar perceptual patterns arrived in the 19th century, when Hermann von Helmholtz, the venerable German physicist and physiologist, determined that there were at least two contributors to this effect, which he dubbed the “irradiation illusion.”
The first contributor results from the way light scatters in the eye. When we look at a very bright object, photons pass through the retina, and some are absorbed by photoreceptors, creating focused vision. Unabsorbed photons can then reflect off the back of the eye behind the retina and disperse as they reflect back through the retina, resulting in scattered, unfocused activation of a larger patch of photoreceptors. This phenomenon is called entoptic glare.
This optical effect helps to explain not only Galileo's radiant crowns but also why Jupiter, our solar system's largest planet, appears smaller than Venus. Technically, both astronomical bodies are so far away that their photons subtend an area smaller than a single photoreceptor in an Earth-bound human's retina. In fact, depending on the time of year, the planets should appear between nine and 66 arcseconds across—visual distances so small to the naked eye that a person with 20/20 vision would only perceive them as about one arcminute each. (To illustrate how small that is: if you painted your thumbnail with 60 alternating black and white vertical stripes and held it at arm's length, each stripe would be about one arcminute wide.) Because Venus is closer to the sun and to Earth, its surface reflects significantly more photons into your eyes than does Jupiter's, causing greater entoptic glare.
But a second contributor is also in play in the irradiation illusion. Glare could not explain, for example, the fact that Galilean moons appear smaller when seen as black dots against Jupiter's mass than when seen as white beacons against the night sky. The answer, von Helmholtz realized, must reside in how the brain processes light versus dark objects. Scientists are only now finding ways to elucidate these neural processes.
As the examples in this column illustrate, the interplay between light and dark is critical not only to stargazing but to our everyday vision because contrast is fundamental to how we see everything.
CONTRAST VS. DETAIL
BLACK AND WHITE AS EQUIVALENT