<p><strong>SF. Fig. 1.7.</strong>&nbsp; Images from the Ishihara color perception test. People with normal color vision will see numbers in each circle. The numbers are, clockwise from top left, 2, 12, 42, and 6.</p>

SF. Fig. 1.7.  Images from the Ishihara color perception test. People with normal color vision will see numbers in each circle. The numbers are, clockwise from top left, 2, 12, 42, and 6.

Image adapted from Wikipedia