SPECTRUM, in optics. When a ray of light is admitted through a small hole, and received on a white surface, it forms a luminous spot. if a dense, transparent 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 it is divided into seven different rays. The ray of light no longer forms a luminous spot, but has as an oblong shape, terminating in semicircular arches, and exhibiting seven different colours. This image is called the spectrum, 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 differ ent. Those which are nearest the middle are the least refracted, and those which are the most distant the greatest. The order of the seven rays of the spectrum is the following : 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 spec trum was divided into 360 parts, the num ber of the parts occupied by each of the colours to be the following : Red . . . 45 parts.
Orange . . . 27 Yellow . . . 48 Green . . . 60 Blue . . . . 60 Indigo . . . 40 Violet . . . 80 These different coloured rays are not subject to further division. No change is effected upon any of them by being fur ther refracted or reflected ; and as they differ in refrangibility, so also do they dif fer in the power of inflection and reflec tion. The violet rays are found to be the most reflexible and inflexible, and the red the least.
SPECULATlyE, something relatedto the theory of soffie art or science, in con tradistinction to practical.