(1) Sharpness of vision, especially as tested with a Snellen chart. Normal visual acuity based on the Snellen chart is 20/20.

(2) Astronomers Ignas Snellen of Leiden Observatory in the Netherlands and colleagues have now demonstrated that such optical observations are feasible.

(3) Some children have can see only 0.5 snellen chart at exam or even worse at the time of screening. Their parents are very anxious for this, considering their children as myopia.

(4) But, says Snellen, "even if we assume that the observed brightness variation is fully due to reflected star light, we find that the planet's reflectivity is probably lower than 10%."

(5) Can you read the numbers on that line... -If a Snellen chart is not available, test visual acuity by using a special hand-held eye card.