Page:The American Cyclopædia (1879) Volume X.djvu/446

 440 LIGHT contain. (See MIBAGE.) The phenomena of reflection and refraction, such as the formation of images, the production of caustics by means of lenses, and mirrors, are treated in the arti- cle OPTICS. Dispersion. Thus far light has been considered as homogeneous, that is, com- posed of rays having the same wave length; but most of the light with which we are ac- quainted is compound, consisting of innumer- able rays of different degrees of refrangibility, a discovery which we owe to Newton. If a beam of solar light is received into a darkened chamber through a small circular aperture at D, fig. 7, it will produce a luminous spot upon a screen at F, and the diameter of the image will be equal to that of the aperture. If the light is passed through a prism, ABO, placed horizontally, the rays will all be bent toward the base of the prism, but not in an equal de-. gree. Upon the screen M N there will be de- picted an elongated spectrum, of a width equal to that of the diameter of the original beam, and composed of innumerable rays of differ- ent degrees of refrangibility, and of an infini- ty of tints, of which seven principal ones are capable of being distinguished by th human FIG. 7. Solar Spectrum. eye, viz., violet, indigo, blue, green, yellow, orange, and red, the violet being refracted the most and the red the least. They do not all occupy an equal space, the violet having the greatest and the orange the least extent. New- ton proved that white light is composed of these various colors, not only by decomposing it, but also by recombining the colored rays and reproducing white light. He also showed that the rays in each portion of the spectrum always retain their characteristic color when isolated and passed singly through prisms, and that they will not be dispersed as in the case of white light. According to the theory of Young, which has lately been ably supported by Helmholtz, color results from the impres- sion made by rays of different refrangibility upon three kinds of nerve elements in the reti- na, one of which alone is impressed by red, another by green, and another by violet light. When these nerves are simultaneously im- pressed by varying quantities of rays of differ- ent degrees of refrangibility, the sensation of a variety of tints is the result. According to the wave theory, the intensity of light depends upon the amplitude of the vibrations, that is to say, upon the distance the ether particles travel and which is in a direction perpendicular to the time of propagation. The color of a ray, which varies with its refrangibility, depends upon the wave length, that is, upon the distance of the wave crests from each other, in the direction of the line of propagation. The solar beam also contains invisible rays of different wave lengths. When the rays in the beam are dispersed by a prism, these invisible rays are also dispersed, those which are the most refrangible being found beyond the violet, and those which are the least refrangible beyond the red. The most refrangible invisible rays are called ac- tinic, and the least refrangible calorific or heat rays. That portion of the spectrum which is visible is also illuminated in different degrees in different parts, the greatest proportion of light being in that part corresponding to the yellow rays. According to Fraunhofer, the amount of light contained in each part of the spectrum is as follows : red, 94 ; orange, 640 ; yellow, 1,000; green, 480; blue, 168; indigo, 31 ; violet, 6. All the rays possess the prop- erty of more or less affecting the tempera- ture of bodies on which they impinge, but those of a certain degree of refrangibility pos- sess it in a far greater degree than the others. When a diathermanous substance, as rock salt, is used as the dispersing medium, by far the greatest amount of heat is contained in that part of the spectrum which lies beyond the red. The violet rays have the power of ex- citing a faint degree of heat, and the actinic rays are not totally devoid of it ; and all the rays possess more or less actinic power, but the least refrangible only in an extremely small degree. The rays which produce vision, how- ever, have a limited degree of refrangibili- ty, and in the spectrum lie within certain bounds. They have relation to a vital func- tion, and are therefore confined to those whose wave lengths are capable of exciting the nerves of vision. Newton supposed the spectrum to be continuous, but it has been found on careful examination to be interrupted by certain dark bands or lines representing va- cant spaces, which have fixed positions. A prism of low dispersive power does not ex- hibit these lines, because the colors are super- imposed ; but when a succession of prisms is used, the spectrum is lengthened out, so that they are easily seen and their places noted. When the refracting substance is varied, the dark lines may have their positions with regard to one another changed; but with regard to the colors they are not changed, the refracting substance having the same effect upon the extent of the colors that it has upon the posi- tion of the lines. A diffraction grating, as will be seen further on, may be advantageous- ly substituted for a prism for the purpose of showing the position and counting the number of the lines. The vacant spaces or lines are caused by the absorption of rays given out by the incandescent body in some part of their passage to the eye. It has been found that the