It's apparently not a numeral, but a wiggly line. I'm not sure I can guess where it is. https://www.truecourses.com/documents/Forms/IshiharaInstructions.pdf Cheers, Seb On Saturday, 22 October 2016, Bill Gosper <billgosper@gmail.com> wrote:

claims to be a red-green deficiency "trick question". You only see the numeral if you're colorblind. But after pasting the image into Mathematica, Rohan and I were unable to find a mapping, e.g., Image[(ImageData@%702) /. {r_, g_, b_, a_} :> {r', g',b',a'}] to reveal the hidden numeral. What's the trick? --rwg _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com <javascript:;> https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun