7.A concavemirror reflects parallel light rays so that they meet at the so-called focal point. (If you try to use such a mirror to start a fire in sunlight, you place the tinder at the focal point.) The distance from the mirror to the focal point is its focal length.
8.Renaissance craftsmen, moreover, would have faced severe technical challenges in silvering and sealing a concavemirror, which required the application of hot tar and pitch on the outside of the glass.
9.I copied a reproduction of the silverpoint using a homemade epidiascope based on a small circular concavemirror and direct solar illumination, and when I deliberately “bumped” my own epidiascope, I found such a mismatch to be very conspicuous.
10.Hockney surmises, however, that this convex mirror could be turned around and used as a concave projection mirror: “If you were to reverse the silvering, and then turn it round, this would be all the optical equipment you would need for the meticulous and natural-looking detail in the picture.
