Colonel E.W. Harding notes in The Flyfisher and The Trout's Point of View, New Light On Flyfishing Theory and Practice. London, 1931.
"With the light behind them and sunk a little below the surface, both sorts of hooks are inconspicuous. The threadlike core of a bronzed hook takes up an olive tone slightly darker than the rest of the water, while the nickel-plated (Crystal)TM hook seems to reflect its surroundings completely."
"When the sun is straight ahead and low down, it is very difficult to see the nickel-plated (Crystal)TM hook at all."
"In diffused light it, a nickel-plated (Crystal)TM hook, should be nearly a perfect mirror and reflect its surroundings almost completely. It does, and so is practically invisible."
"More generally, wherever the part of the hook which the trout may see is in diffused light and not exposed to the direct rays of light, it is probably an advantage to use nickel-plated (Crystal)TM hooks. . . . Hooks should be well polished: they show up far less so than when dull or rusted." "(Crystal)TM being my addition to Colonel Harding's comments."
(Comments by Alec Jackson)
Alec Jackson's (Crystal)TM Covert Nymph Hooks are available in 9, 11, 13, 15 and 17.