C&F CFT-10 Fish Eye employs the critical angle of refraction theory to inspect a fly from a fish's point of view.
You can easily observe the inside/outside of the fly's fish window as seen from the fish's eyes on the tying desk with a mirror and water.
It is ideal for checking the fly you have tied yourself and for tying demonstrations
Fish Eye is a product that uses the principle of the critical angle of refraction of water and air.