SPECTRUM, in optics, the colored image or images produced when the rays from any source of light are decom posed or dispersed by refraction through a prism. It has been proved that white ness is simply a totality of effect pro duced by the simultaneous effects of many colors falling at once upon the retina. It has been shown (see RE FRACTION) how a beam of light is de flected on meeting at any inclination the surface of a denser medium, and it is obvious that by using a prism with two inclined surfaces, the beam may be per manently deflected. It is found that each different color, representing a dif ferent length of wave, is differently re fracted by the prism, or has its own special index of refraction; hence the prism separates or spreads out, in order, according to their refrangibility, all the different colors of which the beam is composed. This appearance is the spectrum of that particular light. Solids or liquids heated to incandescence—as the particles of soot in a candle flame— always yield an unbroken band of colors shading into one another; this is called a continuance spectrum. Incandescent
gases generally yield lines or bands only, and this is a line or banded spectrum. When portions of what would have been a continuous spectrum are intercepted or cut out by an intervening medium this is called an absorption spectrum. Besides the waves of such a length as cause vis ual effects, there are many more beyond the red at one end of the spectrum and the violet at the other, which produce powerful chemical and heating effects. This portion is sometimes called the in visible spectrum, sometimes described as the ultra-red or ultra-violet spectrum. Its length greatly exceeds that of the visible spectrum, and it is found to com prise lines and bands precisely analogous to those occurring in the luminous por tion.