The Stereo Fly Test has long been recognized as the standard stereopsis test. Test book includes: Fly test for stereopsis, graded circle test (800 to 40 seconds of arc), and animal series for younger children (400 to 100 seconds of arc). Includes 3-D viewers. hold 16" away. Leather-like cover. Measures: 6.5" x 8.5". Comes as singles.