Standard color blindness tests and box of colored yarns.

Standard color blindness tests and box of colored yarns. The box of colored yarns is from a rather dated test for color blindness (Dr. Holmgren's Worsted Test for Color Blindness.) The yarns are colored red to violet, spanning the visible spectrum. The Book of Pseudo-Isochromatic Plates has 40 plates. Each plate is composed of grey or green background dots and foreground numbers of red or purple. People with some forms of color blindness can not make out the numbers. The yarns or plates while viewed under the yellow sodium light appear radically different than in white light.