SPECTRUM. A luminous spot formed by a ray of light on a white surface when admit ted through a small hole. If a dense transpa rent body be interposed, the light will be re fracted in proportion to the density of the medium ; but if a triangular glass prism be interposed, the light is not merely refracted, but divided into seven different rays. The ray of light no longer forms a luminous spot, but has assumed an oblong shape, terminating in semicircular arches, and exhibiting seven dif ferent colours. This image is called the spec trum, and, from being produced by the prism, the prismatic spectrum. These different coloured rays appearing in different places of the spectrum, show that their refractive power is different. Those which are nearest the mid dles are the least refracted, and those which are the most distant, the greatest. The order of the seven rays of the spectrum is the follow ing: red, orange, yellow, green, blue, indigo, violet. The red, which is at one end of the
spectrum is the least, and the violet, which is at the other end, is the most refracted. Sir Isaac Newton found, if the whole spectrum was di vided into 360 parts, the number of the parts occupied by each of the colours to he the fol lowing; red, 45 parts ; orange, 27; yellow, 48 ; green, 60; blue, 60; indigo, 40 ; and violet, 80. These different coloured rays are not subject to farther division. No change is effected upon any of them by being farther re fracted or reflected; and as they differ in re frangibility, so also do they differ in the power if inflection and reflection. The violet rays are found to be the most reflexible and inflexible, and the red the least.