In February 2015 Cecilia Bleasdale photographed a lace dress at the Cheshire Oaks Designer Outlet in England and sent it to her daughter Grace, who was getting married on the Scottish island of Colonsay. Grace saw white and gold. Cecilia, who had just seen the dress in person, saw blue and black, which it was. After the wedding, singer Caitlin McNeill posted the photo on Tumblr, a BuzzFeed poll took off, and the page hit 14,000 views a second and a record 673,000 concurrent visitors, with 4.4 million tweets in a day and Taylor Swift admitting she felt confused and scared.

The science centers on color constancy. The brain must separate an object’s color from the color of its light, and the cropped, overexposed photo gave it no clues. Bevil Conway tied the split to daylight shifting from warm to blue, and Pascal Wallisch found that early risers, with more lifetime exposure to natural daylight, leaned toward white and gold while night owls saw blue and black. Brain scans and philosopher Barry C. Smith’s comparison with the rabbit-duck drawing explain why almost no one could switch.

• A Current Biology study of 1,400 people found 57 percent saw blue and black, 30 percent white and gold, and 11 percent blue and brown
• Women and older people disproportionately saw white and gold
• White and gold viewers showed more frontal and parietal activity, a sign of top-down processing
• Roman Originals sold out in 30 minutes, and Ellen DeGeneres gave the family a honeymoon trip and $10,000
• The Salvation Army in South Africa used the image for a domestic violence campaign asking why it is so hard to see black and blue