By resolving white light into its constituent rays, we find that these properties are associated each one with certain of the elementary colours.
The yellow is decidedly the most luminous ray. On examining the Solar Spectrum, it is seen that the brightest part is that occupied by the yellow, and that the light diminishes rapidly on either side. So again, rooms glazed with yellow glass always appear abundantly illuminated, whilst the effect of red or blue glass is dark and sombre. The yellow colour therefore constitutes that portion of white light by which surrounding objects are rendered visible; it is essentially the visual ray.
The heating properties of the sunlight reside principally in the red ray, as is shown by the expansion of a mercurial thermometer placed in that part of the spectrum.
The chemical action of light corresponds more to the indigo and violet rays, and is wanting, as regards its influence upon Iodide of Silver, both in the red and yellow. Strictly speaking however it cannot be localized in either of the coloured spaces, as will be more fully shown in the Fourth Section of this Chapter, to which the reader is referred.
SECTION II.
The Refraction of Light.
A ray of light, in its passage through any transparent medium, travels in a straight line as long as the density of the medium continues unchanged. But if the density varies, becoming either greater or less, then the ray is refracted, or bent out of the course which it originally pursued. The degree to which the refraction or bending takes place depends upon the nature of the new medium, and in particular upon its density as compared with that of the medium which the ray had previously traversed. Hence Water refracts light more powerfully than Air, and Glass more so than Water.
The following diagram illustrates the refraction of a ray of light.
