On Vision and Colors are, however, nonessential properties that differ according to the type of light-refracting substance that is used. The top of the upper image is violet. Below the violet is blue. The bottom of the lower image is orange. Above the orange is yellow. In this way, along with the white disk and the black background, four prismatic colors appear: violet, blue, yellow, and orange. This is in disagreement with Newton's claim that there are seven prismatic colors. As the upper image overlaps black, it is seen as violet. Where it overlaps white, it is seen as blue. As the lower image
Full-spectrum light phenomenon called metamerism. Where light sources use an electric discharge through low pressure gas, the light spectrum may be quite discontinuous, with some wavelengths of light missing or at very low amplitude. Such light sources have a strong tint, such as low pressure sodium lamps, or neon lamps. These lamps are used more for their color effects than for general illumination. Some scientific instruments use discharge tubes to produce light that has only a few wavelengths in it,a so-called "line spectrum". A laser is a single-wavelength source, which would produce light of a very pure color. Ideally, during the day,