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. [Illustration] The dotted line is drawn perpendicularly to the surface, and it is seen that the ray of light on entering is bent towards this line. On emerging, on the other hand, it is bent to an equal extent _away from the perpendicular_, so that it proceeds in a course parallel to, but not coincident with, its original direction. If we suppose the new medium, in place of being more dense than the old, to be _less dense_, then the conditions are exactly reversed,--the ray is bent away from the perpendicular on entering, and towards it on leaving. It must be observed that the laws of refraction apply only to rays of light which fall upon the medium _at an angle:_ if they enter perpendicularly--in the direction of the dotted lines in the last figure--they pass straight through without suffering refraction. Notice also, that it is _at the surfaces of bodies_ that the deflecting power acts. The ray is bent on entering, and bent again on leaving; but whilst within the medium it continues in a straight line. Hence it is evident that by variously modifying the surfaces of refractive media the rays of light may be diverted almost at pleasure. This will be rendered clear by a few simple diagrams.