Assertions from Europe that battle planes have been painted with a secret varnish making them invisible may be merely an excuse for poor marksmanshipof the defenders. Complete invisibility is impossible although an airplane may be rendered less conspicuous;by applying certain well-known principles. Mere transparency, however, is not enough, as can be seen by examining a piece of glass. Though light passes through it freely, glass is quite visible because it changes the speed and direction of light rays; yet a sheet of glass ordinarily is much less conspicuous than a similar sheet of opaque material. Some experts sdy that if an airplane were made of a glasslike substance, such as transparent plastic, it would be very difficult to see in the daytime except at low altitudes, even though the engine and many other parts wotild require metal. Giving all surfaces a mirrorlike finish, so they would reflect lots of light and appear bright as the sky, would have a somewhat similar effect. A different factor is involved in seeing planes against the dark background of the night sky, when ground observers depend on light reflected back from searchlight beams. If the lower surfaces of the fuselage and wings are painted with a flat black paint, most of the light will be absorbed, and, scarcely any reflected back to the ground.